PACS-SONET
Annual Report 2003
Prepared
by Michael Douglas of the National Severe Storms Laboratory (NSSL) and Javier
Murillo, John Mejia and Jose Galvez of the Cooperative Institute for Mesoscale
Meteorological Studies (CIMMS) and the University of Oklahoma
Contents
Summary
1. Brief history of the
project
2. Current status of
the PACS-SONET
3. Scientific Rationale
for each observing site
4. Educational
activities and annual meetings
5. Next Year’s
Activities
6.
Some aspects of the
PACS-SONET data that suggest avenues of future research
Summary
This is the first
official annual report of the PACS-SONET project, after some 7 years of
operation. This surprising statement is
a consequence of our using the web site of the project as the source of primary
information for the activity, rather than printed material. However, there are advantages to printed
documents over web sites and these advantages, together with the need to
satisfy a now-official requirement, are the impetus for this report.
The PACS-SONET
project evolved in response to the perceived lack of in-situ atmospheric
measurements over the inter-American region to adequately support climate
research studies. The relatively recent
availability of the global NCEP reanalyses products had stimulated a large
number of studies of climate and climate variability and it was becoming
apparent that ground truth measurements were going to be needed to validate
these studies. To depend on the
individual efforts of each country in the region to enhance their own sounding
networks was not viewed as a proactive solution to the lack of sounding data,
and in 1997 the OGP supported an initial effort to establish a temporary
network to monitor the windfield in central and part of northern South America. This has, over the succeeding 6 years,
become what is now called the PACS-SONET.
At the present time
the SONET involves pilot balloon observations at some 21 sites in 8 countries
in Latin America. Seven sites are
operated in Mexico, six in Bolivia, followed by two sites each in Paraguay,
Venezuela and Peru, and one each in Ecuador, Colombia and Nicaragua. About 500 observations per month continue to
be made. Each site has a specific
contribution to the overall scientific objectives of the project, which have in
the past several years focused on supporting aspects of the two monsoon
experiments, SALLJEX (January 2003) and NAME (summer 2004) supported by
OGP. Observations are made daily,
near 1200 UTC, except in Mexico, where twice-daily observations are made, and
in Piura, Peru where frequent morning cloudiness forces routine afternoon
soundings.
The PACS-SONET
continually strives to maintain the network operations against a number of
factors that tend to reduce the efficiency of the activity. This year, delays in the availability of
funds from NOAA, due to government-wide budget delays, had a major adverse
impact on the SONET, with one station ceasing operations for the past 9
months. Changes in the procedures involved
in making funds transfers to participating countries also delayed the reception
of funds to maintain the network operations.
These procedural difficulties are now slowly being sorted out, but were
a source of considerable consternation and delay during 2003.
During the past year
the first meeting involving representatives from all institutions involved in
the SONET took place in Paraguay. The
summary of this 6-day workshop is described in an appendix to this annual
report. The objective of the workshop
was to seek input on the direction that the SONET should take during its 3-
year current extension. Issues covered
included what stations should be operated, which new sites might be valuable,
how to maintain the budget of the activity, what educational activities could
stimulate use of the data, including research activities. Finally, the use of
radiosonde observations was evaluated as a possible component of a future SONET
design.
Although a number of
presentations related to SONET were given during the past year, no publications
appeared. In lieu of this, we are
presenting, as part of this report, key aspects of the SONET data set that are
potential subjects for near-term research.
These topics include the suitability of the SONET data to describe
interannual variations of 1) the cross-equatorial flow along the Peruvian
coast, 2) the warm season trade wind flow across Central America that is
associated with the mid-summer dry season, 3) the gap flow intensity across the
Istmus of Tehuantepec, and 4) the intensity of the monsoonal flow up the gulf
of California during the warm season.
Each of these is discussed as an example of subjects that are now
becoming feasible to study with multi-year records from the PACS-SONET.
Finally, a summary
of the directions in which the project intends to advance completes the report.
1. Brief history of the
PACS-SONET
The Pan American Climate Studies Program - Sounding Network
(PACS-SONET) is a network of pilot balloon stations in Latin America funded by
NOAA to help monitor the climate variability through measurement of
lower-middle tropospheric winds in data sparse regions. The observations,
transmitted in near real-time, can also be used for weather forecasting
activities.
The original PACS-SONET configuration consisted of 12 pilot
balloon sites (Fig. 1.1). The
stations were established during April and May of 1997 in Mexico, Nicaragua,
Costa Rica, Panama, Colombia, Ecuador and Peru, together with a combined pilot
balloon and radiosonde station on Cocos Island in the eastern Pacific Ocean.
These observations, made twice-daily, were to describe the conditions
associated with wet and dry spells over Central America and to help evaluate
the quality of the NCEP reanalyses windfields over the region - analyses that
were (and continue to be) a mainstay of tropical climate research activities.
The intended duration of the observations was 6 months, ideally extending
through October, and close to the end of the rainy season in much of Central
America.
Fig.
1.1. PACS-SONET initial configuration.
Due to the strong El Niño of 1997-98, the observational
plan was modified to extend the period of observations at many stations through
the mid-year 1998 to establish additional pilot balloon stations in Ecuador and
Peru to better sample conditions in the region of anticipated heavy rainfall.
This allowed an unprecedented data set to be obtained to describe the wind
field along the Ecuadorian and northern Peruvian coasts, associated with the
warm event in the eastern Pacific.
Pilot balloon observations were also made at Santa Cruz,
Bolivia, as part of the special El Niño campaign. Although these observations
were only made for 3 months, they showed a strong and variable low-level jet at
Santa Cruz, which had not been well described from previous observations. These
observations indirectly stimulated further observations from a more extensive
network in Bolivia during 1999.
Fig.
1.2 PACS-SONET extensions due to El Niño 1997-98 (white dots), SALLJEX
pilot balloon stations (blue dots) and
current PACS-SONET sites (red dots).
In late 1999 support was obtained for extension of the
PACS-SONET for a 3-year period. In addition to an extension in time of the
program, an expansion of the network took place. In terms of geographical
extension, new pilot balloon sites were established in Paraguay (2) and Bolivia
(6). The primary objective of these additional observations was to describe the
variability of the low-level flow east of the Andes and the circulation over
the Bolivian altiplano. In Mexico, three additional sites were established in
March 2000 to bring to 6 the number operating (one more was added in April
2002). The stations in northwestern Mexico, were designed to improve the
description of the low-level flow along the Gulf of California. In March 2001 one station was established in
Venezuela at San Fernando de Apure, a location in the very flat llanos north of
the Orinoco river. In March 2002, a second Venezuelan station was established
in Ciudad Bolivar, a city of the eastern llanos on the Orinoco river basin. Figure
1.2 shows the current PACS-SONET configuration and other stations that have
been operational with financial or logistic support from the project.
In addition to more observing sites, a major effort has
been made to make the network a real-time data collection and distribution activity.
This required efforts to upgrade communications at many sites, and the
development of procedures to ensure the flow of data not only to research
institutions, but to all interested operational forecasting institutions. The
latter is accomplished mainly through the development of the project’s web
page, in which the observations are made available in near real time, in the
form of both raw data and plotted maps at select levels (See http://www.nssl.noaa.gov/projects/pacs/).
More recently, the PACS-SONET stations in Bolivia and
Paraguay served as the ramp-up network for the upper-air component of the South
American Low Level Jet Experiment (SALLJEX) that started on November 15, 2002.
Fig.
1.3. Pictures from two special observing campaigns conducted in Bolivia
during SALLJEX
around Lake Titicaca (left) and in the Salar de Uyuni (right).
In
2003 a second extension of the PACS-SONET was approved for an additional 3
years. Additional changes in the network configuration and mode of operation
are expected.
2.
Current status of the network
2.1
Introduction.
Several budget-related issues affected the network’s performance during the past 12 months. The transition from the end of the previously funded period towards the new 3-year extension presented a challenge to the project. During this transition several stations were left unfunded for a lapse of 3-4 months, due to the delay in transferring funds from Washington to NSSL. The reason for this, it appeared, was a financial cycle change for all NOAA-funded projects. Finally, the problem that impacted the most our attempt at a smooth transition was related to changes in payment procedures and delays in money transfers to the foreign countries. These are now being made through the NOAA MASC Office in Boulder, instead of the Washington Finance Office previously. The problem is so serious that at the time this report was written we were still struggling to complete a large number of transfers. For reasons that remain unclear to us, MASC refused to make electronic transfers to a number of countries and decided to send checks instead. This method of payment wasn't acceptable to some of the institutions participating in PACS-SONET, and this has introduced delays in the availability of funds to many network sites.
Despite the long
delays in transferring funds to the countries, most of the stations managed to
operate almost regularly and most interruptions were ultimately related to
problems particular to each country, as is explained in the following
paragraphs. The stations in Peru, Bolivia and Paraguay were part of the SALLJEX
upper-air network and were operated during the first quarter of the year
primarily from SALLJEX funds.
2.2 Summary of station operation and performance by country.
Mexico
There are 7 stations
currently operating in Mexico. The most recent addition is the site operating
from the Naval Academy in Anton Lizardo, Veracruz. The Mexican Navy (Secretaria
de Marina Armada de Mexico, SEMAR) requested the station to be established
primarily for educational purposes (all officers in the Mexican Navy are
trained there), but observations are also made there routinely and the data is
transmitted in real time from the site.
At least 5 of the Mexican stations rely on PACS-SONET to cover
communications and other expenses; due to the lack of funds to repair a
computer, one station (Salina Cruz) has not been able to process and transmit
data in real time in the last two months.
Other stations had problems with intermittent gas supply caused by
delays in transfer of funds for gas purchase from SEMAR to the Naval bases that
operate the sites. Most of the gas
problems were rapidly solved after a letter was sent to SEMAR explaining the
relevance of the observations for NAME, specially those made in northwestern
Mexico.
The observations in
Mexico are made twice-daily, contrasting with most PACS-SONET sites. The reason for this is that the Naval
personnel at these sites are dedicated to this activity and it was suggested by
the Mexican Navy senior staff that these individuals be more fully
utilized. The Navy covers the cost of
personnel and hydrogen gas for all sites, SONET covers the balloons and costs
associated with data communications (PC's at each site, internet connectivity
and some phone line costs). Although
the twice-daily observations are an additional cost, the afternoon observations
provide information on the diurnal cycle that is missing from most other SONET
sites.
Nicaragua
Nicaragua had been
operating one station in the airport of Managua since 1997. This site was
reliable, and has generated a very good data set, especially during the period
2000-2002. However, Managua is not
currently operating because SONET has not been able to transfer funds to
INETER, the institution in charge of making the observations. This institution is very strict in regards
to payment, and will not make any observations if money is not available from
the project to pay for the observers. The forecasters at the airport recognize
the value of the observations for the forecast, but this interest has not been
able to change the attitudes of senior INETER personnel. This lack of interest on the part of the
institution responsible for geophysical observations in Nicaragua is not new,
and is motivation for a planned trip to Central America early next year. This travel will seek additional sources of
support for observations in the region and provide lectures to broader the
awareness of the SONET (and related activities) in the Central American region.
Venezuela
Venezuela has been
operating two pilot balloon stations, at San Fernando de Apure and in Ciudad
Bolivar. These sites are operated by
the Venezuelan Air Force Weather Service, which is the main weather forecasting
institution in the country. Both sites
are in the flat llanos region along the Orinoco River drainage. San Fernando is reliable and provides good
quality data. On the other hand,
unreliable gas supply and communication problems at Ciudad Bolivar are causes
of this station’s poorer performance.
Several motivated individuals from Venezuela participated in the PACS-SONET
short course in Panama in 2001 and good communication between the project and
the country has existed ever since. The
person in charge of the stations attended the PACS-SONET coordination meeting
in Paraguay. The willingness and
collaboration is highlighted by the relatively inexpensive operations in this
country, with labor being provided at no cost to the project and transport of
balloons effected through Venezuelan Air Force flights from Miami to
Venezuela. Hydrogen gas for balloon
inflation is also inexpensive in the country.
In addition to the
high level of interest shown by the Venezuelan Air Force in the SONET
activities, faculty members at the Universidad de Los Andes in Merida,
Venezuela have shown interest in SONET activities. Future activities may involve a joint field program and
associated follow-on research related to the mesoscale meteorology around Lake
Maracaibo. PACS-SONET might contribute
expertise in the design of the activity and procedures for analyzing the data.
One possible
complication related to the Venezuelan SONET activities is that Venezuela is
currently undergoing a major modernization activity, that eventually will include
the establishment of a number of radiosonde stations. One site will be at Cd. Bolivar, which may obviate the need for
the pilot balloon observations there.
As these sites will be sustained by an international loan, the long term
sustainability of this network is not assured, complicating decisions about the
potential long-term data to be obtained from the sites. We are following this
activity, as it may suggest revisions to at least one (Cd. Bolivar) of the
Venezuelan station's operations.
Colombia
The Colombian Navy
has been operating one station in Cartagena since 1997. However, this station
has had a poor performance in the last two years, with an average of only 5
observations per month. The apparent reason for this is not only the lack of
personnel at this research facility (CIOH), but also the perceived value of the
SONET observations, that keeps them from assigning more people to this
activity. We are planning travel to
Colombia in early 2004, where we expect to meet with senior-level Navy officials,
and to provide a short course related to the meteorology and climate of the
region, and the project's objectives.
Details of possible observations (Malpelo and Serranilla) on the islands
administered by Colombia and staffed by Colombian Navy personnel will also be
conducted.
During the past few
months contacts have been initiated with IDEAM, the Colombian institution in
charge of Environmental affairs (and the official weather and climate
service). As part of planned travel to
Colombia we intend to visit the IDEAM office in Bogota with the objective on
ensuring collaboration with the establishment of observations (including
raingauge) on the islands mentioned above.
The IDEAM has asked for help during this past year in maintaining its
upper-air network (under budgetary pressure at the moment) and we will provide
ideas on what might be feasible (short course possibility).
Ecuador
San
Cristobal, in the Galapagos Islands, is the only site in operation in Ecuador.
The site at ESPOL in Guayaquil was stopped in May 2002 due to the low number of
observations caused by frequent cloudiness at the site (not a problem with the
observers or institution). The San
Cristobal site performed well during a
5 month period in 2003, but very few observations have been received in
the last two months. We have been
informed that the reason is lack of personnel in Quito to process and transmit
the observations.
Because of
the difficulty in maintaining a reliable data stream from the Galapagos site,
despite the availability of observers, gas, and communications, we plan a visit
to Quito to discuss the feasibility of maintaining this site. It is currently the most expensive site to
operate, and if we do not receive strong assurances that it will operate
effectively, we are likely to see other institutions on the islands to make the
observations. Since our perception is
that Ecuador, like Nicaragua, has a weak meteorological infrastructure and lack
of appreciation for meteorological and climate information, we are anticipating
giving a short course in Quito early in 2004, as part of travel to northwestern
South America.
Peru
Two
different institutions currently operate pilot balloon stations in Peru.
The longest times series, and indeed the best SONET time series of any
stations, is from the site at the University of Piura. This site, established
in 1997, continues to operate at a high level of efficiency. It is one of the few sites that makes
afternoon observations - a fact
required by the relatively high frequency of morning low clouds along the coastal
plain of northern Peru. Some analyses of these observations areshown
in Section 6.
The National Meteorological Service in Peru (Servicio Nacional de Meteorologia e Hidrologia, SENAMHI) operated four stations during the SALLJEX at Pucallpa, Ica, Arequipa and Puno. These sites were established in November 2002, and were reliable during SALLJEX. The site at Arequipa became part of the PACS-SONET in July 2003, and has operated very reliably since. SENAMHI has agreed to operate pilot balloon stations also at Pucallpa, Iquitos, and Puerto Maldonado. The latter two sites are currently special El Niño radiosonde sites that make soundings every other day. Arrangements are being made to make pilot balloon observations at these sites; the exact protocol (number of observations per week etc.) has yet to be worked out. The Pucallpa and Iquitos sites are potentially valuable as they describe the currently poorly-monitored conditions upstream of the low-level jet axis over eastern Bolivia.
SENAMHI has also
expressed interest in obtaining information on the costs and detailed
specifications of recoverable radiosonde systems that were partially
demonstrated during the recent workshop in Paraguay. This information is being forwarded to them.
Paraguay
The pilot
balloon site at Asuncion has a good record of observations, spanning a 4-year
period. The second Paraguayan site,
Mariscal Estigarribia is at a location far (500 km) northwest Asuncion with poorly motivated individuals
and little supervision. The two sites in Paraguay performed well during
SALLJEX, when the Mariscal Estigarribia
site was staffed by additional personnel.
M. Estigarribia stopped making observations shortly after SALLJEX ended,
and has just resumed operations (October 1st) with motivated and more capable
observers. Efforts to modernize the
meteorological infrastructure and emergency management capabilities in Paraguay
have led to the acquisition of a radiosonde system that has been installed in
Mariscal Estigarribia. However, shortly
after its installation the hydrogen generator failed and, for this reason,
radiosondes have not been launched since the equipment was put in place more
than one year ago. Now, with closer
supervision by DINAC senior staff, we expect the Paraguayan PACS-SONET
observations to continue without significant problems for the foreseeable
future.
Bolivia
Gas
availability and gas costs continue to be the main issues in Bolivia. The gas
company (Praxair) in Bolivia has been unable to satisfy the demand of gas
despite of the high prices they charge for it.
Gas is imported from Brazil and the supply and transport has not been
reliable. Also, gas is delivered only
at two stations (La Paz and Santa Cruz) and AASANA has to arrange for
transportation to the other 4 relatively remote sites. Bad road conditions in
Bolivia complicate transportation and recycling of the gas cylinders. The
station at Robore has not been making observations for the last 2 months
because they ran out of gas. AASANA has
been unable to send them gas because of new regulations that prohibit
transportation of gas by train. Civil
unrest also complicates transport throughout the country; strikes and road
blockages are currently common.
We will
attempt to have lower-cost gas (either hydrogen or helium) imported from Peru
during the coming year. Currently we
are using gas purchased as part of SALLJEX activities but which was not
delivered until after the experiment ended.
On the
positive side, the AASANA observers and other personnel remain committed to the
activity and continue interested in spite of some of the difficulties.
2.3 Performance statistics over the past year.
The overall performance of the network appears to be stable in terms of the number of observations per month (Figure 2.1). However, the average number of observations per month is below 500 for the months following SALLJEX; this average is somewhat lower than that in the previous 3 years. On the other hand, the average is higher than expected given the problems with the transfers of funds.
Figure 2.1. Pilot balloon
soundings per month (PACS-SONET and
SALLJEX combined) from April 1997 to September 2003.
To date, PACS-SONET has generated more
than 30,000 observations - more than
5,000 per year in the period 2000-2003 (Fig. 4.2). This calendar year a total
of 8,000 soundings are expected, including ~2,000 soundings made at Argentine
and Brazilian stations that operated only during SALLJEX months.
Figure 2.2. Pilot balloon
observations per year (PACS-SONET and SALLJEX combined)
in the period
1997-2003. For year 2003, only 9 months are shown (Jan-Sep).
2.4
Concerns related to the network
2.4.1 Hardware
The theodolites in
use are old Warren Knight units, which were obtained from the US NWS warehouse
in Kansas City over the past 6 years.
This source of theodolites has now been exhausted, and the remnants of
the theodolites all reside at NSSL and in the countries where they are in
use. Data loss has occurred because of
faulty theodolites, and the inability to repair them rapidly in the field. Our SONET web site now has detailed
information on the use of the theodolites, but still lack some information of maintenance, though individuals
in each institution have been trained in basic maintenance procedures.
At some point in the
not distant future replacement theodolites will be needed, especially since
many were used during SALLJEX and are expected to be used during NAME. To some degree it may be possible to use
theodolites that exist in Peru and Venezuela; those in Peru are of the same
type (Warren-Knight) as used in SONET, while those of Venezuela are a different
type, but still usable.
2.4.2.
Sustainability, gas supplies
The major topic of the Paraguayan
workshop was the sustainability of the network and means to reduce the costs of
operations. One means was to use
hydrogen gas in some of the countries where helium is currently being used. The importation of less expensive helium or
hydrogen into Bolivia in particular could save thousands of dollars per year.
3. Scientific Rationale for Each Observing Site
While all SONET
observing sites contribute to filling gaps in the upper-air sounding network in
Latin America, each observing site has one or more specific reasons to justify
its operation. In this section we
summarize these objectives. Reference
to the various networks map (Fig. 3.1) would be useful.
Mexico
Puerto Peñasco. This site is optimally suited, at the northern
end of the Gulf of California, to monitor the variability of the Gulf of
California low-level jet. This shallow
jet is associated with moisture transport into the southwestern deserts of the
US and variations of the flow ("surges") are a feature of interest in
the 2004 North American Monsoon Experiment (NAME).
Topolobampo. This site, halfway between the radiosonde
sites of Guaymas and Mazatlan on the east side of the Gulf of California, is
designed to measure the up-Gulf seasonal variation of the wind at low-levels
associated with the monsoonal circulation.
Also, by operating two sites along the Gulf of California the intensity
and latitudinal extent of gulf surges can be more accurately estimated. Both Puerto Peñasco and Topolobampo make
twice-daily measurements, unlike Guaymas, so the data also provide better
estimates of the mean flow, averaged over the diurnal cycle.
Tampico. This site is located halfway between the
radiosonde sites of Brownsville, Texas and Veracruz, Mexico, a large gap in the
sounding network on the east coast of Mexico.
The original motive of the site's selection was to describe the
variability of the recurving trade-wind flow that undergoes strong seasonal
variations. Unexpectedly high
cloudiness has somewhat hampered the effectiveness of the site.
Anton Lizardo.
This site was established only recently (2003), and it is primarily to
serve as training for the cadets so that they can supervise the rest of the
network operations. The observations
provide a check on the radiosonde-winds from a nearby (~20 km away) radiosonde
site of the Mexican National Weather Service.
Cd. del Carmen. This site, moved from the nearby Naval
facility at Frontera in 2000, provides winds that describe the intensity of the
trade-wind flow across the southern part of the Yucatan peninsula. The observations have revealed a strong
diurnal variation, with an early morning low-level jet. The observations will primarily help to
identify tropical wave passage across this part of southern Mexico during the
summer months, and provide additional estimates of variations (intraseasonal
and interannual) of the trade wind flow.
Salina Cruz. The strong and variable flow across the gap
in the topography at the Istmus of Tehuantepec is described by measurements at
Salina Cruz. Under normal conditions
(northerly winds) the skies are relatively cloud free and this site provides
profiles to high altitudes (provided the winds are not too strong). Variations in the gap flow, due to tropical
wave passages in summer or cold frontal passages in winter can be readily
identified at this site.
Puerto Madero. This site was established to monitor
tropical wave activity and tropical cyclogenesis in the Gulf of
Tehuantepec. The wind regime is vastly
different, due to the blockage of the trade wind flow by a 2-3 km high mountain
range, which is located about 40 km to the east of the site, across a flat
coastal plain.
Figure 2.3 Current radiosonde
network (blue circles), current (black dots) and proposed (stars) PACS-SONET
sites.
Nicaragua
Managua. The Managua observations, of high quality
and generally high altitudinal extent (due to the lack of frequent low
cloudiness), were intended to monitor the travel wind variability (on all time
scales beyond daily) across Central America.
The low terrain over Nicaragua is ideal to avoid the very local effects
of high terrain common to other countries in Central America, and the latitude
of Nicaragua is near the latitude where the trades are strongest.
Colombia
Cartagena. This site was one of the original 12 sites
established in 1997. The justification
for the site remains the same - to improve the description of the low pressure
that is anchored to the topography in this part of South America and which
forms part of the intertropical convergence zone. With the highly intermittent nature of the radiosonde sites in
Rio Hacha (Colombia) and Balboa (Panama), the Cartagena site becomes even more
important, as it is the only sounding site between Curacao and San Jose, Costa Rica,
a distance of 1500 km.
Isla Malpelo
(proposed). The observations proposed
for Malpelo would improve specification of the cross-equatorial flow closer to
the coast of Central America than provided by the site in the Galapagos. In addition, as it is a very small island
there will be almost no diurnal cycle and little topographic on the airflow, so
the observations should be ideal for describing synoptic and seasonal
variations here. The site would also
fill an important gap in the sounding network, as it would be on the equatorial
side of the mean ITCZ position.
Rainfall observations are also proposed for this site, since it lies in
the region of greatest discrepancy between satellite estimation techniques for
precipitation near the ITCZ.
Banco Serrannilla
(proposed). This site would provide
undisturbed trade wind measurements
just north of the core of the maximum winds. The main value of this site is that it is flat and small, so it
should be free of island-induced diurnal or topographic effects. The site is located almost equidistant
between the radiosonde sites of Kingston (Jamaica), San Andres (Colombia) and
Grand Cayman.
Figure 2.4 Zoom on the northern
end of the Panamerican radiosonde network (blue circles), current (black dots) and proposed (stars) PACS-SONET
sites.
Venezuela
San Fernando de
Apure. This site continues to be
important to describe the low-level flow over the wide, flat llanos of
Venezuela. The morning observations
show a strong, but shallow low-level jet during the dry season. The original intent of establishing the site
was to monitor not only the annual cycle of the trade winds across northern
South America, but to describe tropical wave propagation during the warm season
as well.
Ciudad Bolivar. This site was established after San
Fernando, and provides redundancy in the observations, especially during the
rainy season, when cloudiness at both sites can frequently prevent high
soundings.
Isla de Aves
(proposed). We are proposing to
establish a site at Isla de Aves, located 220 km west-southwest of Guadeloupe
Island in the Lesser Antilles. This
site, flat and extremely small, will be ideal for wind profiles in the trade
wind flow for comparison with the radiosonde observations at various locations in the Lesser
Antilles. The Venezuelan Navy and
Environment Ministry has staff permanently on the island to assure sovereignty
and monitor bird and turtle nesting.
Ecuador
San
Cristobal, Galapagos Islands. This site
was originally established to monitor the cross-equatorial flow, especially for
variations associated with the ENSO cycle.
Peru
Piura. This site,
about 60 km inland from the coast, on a flat coastal plain, is useful for
monitoring variations of the flow near the coast associated with ENSO variations. It has become more useful with time, as the
site has the most complete record for interannual variability studies. The site is almost 7 degrees north of the
only coastal radiosonde site, Lima. A
radiosonde site has been established at Piura as part of ENSO monitoring
activities, but this site is not operating every day and the observations are
not routinely transmitted.
Arequipa. This site was established in November 2002
as part of SALLJEX. Because of its
drier and more cloud-free climate, this site routinely provides wind profiles
to higher altitudes than does La Paz.
The observations should be useful for monitoring flow over the northern
altiplano and the subtle variability of the Bolivian high. Peruvian motivation for the site's operation
also is due to the desire to monitor winds for the prediction of ash or fumes
from nearby Volcano Misti (Arequipa is Peru's second largest city).
Pucallpa
(proposed). The Pucallpa site proved
very useful during the SALLJEX, and provided a surprising number of relatively
high soundings. This site is feasible
to maintain, as good logistical support exists, and the observations provide
data well upstream of the Bolivian site at Cobija for monitoring LLJ
variability.
Iquitos
(proposed). This site currently makes
radiosonde observations every other day, and pilot balloon observations are
proposed for the site to provide more frequent wind profiles. This site will
extend monitoring of low-level flow east of the Andes from Paraguay to nearly
the Equator. The observations should be
useful for describing the equatorward extent of cold surges during the cool
season and the longer-period variations associated with the seasonal
cycle. The observations at Iquitos will
compliment those at Leticia (Colombia) located some 400 km to the east. The latter observations have been erratic.
Puerto Maldonado
(proposed). This site is currently a
ENSO-monitoring radiosonde site and pilot balloon observations are proposed to
increase the frequency of the wind observations. The site is partially redundant with Cobija, but the future
status of Cobija is not known with certainty, and the Puerto Maldonado
observations will insure observations between Pucallpa (~8˚S) and
Trinidad, Bolivia (~15˚S).
Fundamentally, the site will contribute to monitoring the variability of
the East Andean low-level jet.
Bolivia
La Paz. This site, and the site at Uyuni provide
altiplano-level measurements for more precisely describing the flow over the
Bolivian and Peruvian altiplano. The
position of the Bolivian high typically is found between these two sites and
subtle shifts in the position can be quantified more accurately through the use
of these station's data. There is
typically a considerable N-S gradient in the zonal wind during the winter between
these two sites.
Uyuni. See comments above. The Uyuni site is more cloud-free than La
Paz, thus helping to provide measurements over the altiplano more often that
would be possible from only one site.
Cobija. This site was intended to measure wind
conditions somewhat upstream (~11˚S) from the region of maximum winds
(~18˚S) associated with the LLJ.
Trinidad. This site, located in a very flat region
more than 100 km form the Andes, provides less-cloud contaminated soundings
near the core of the LLJ over eastern Bolivia.
Robore. This site describes the LLJ flow at
considerable distance from the Andes, providing information on the horizontal
extent of the LLJ flow. It is also
useful for describing the zonal extent of the flow during cold surges, which
typically is more closely confined to the topography.
Santa Cruz. This site, at Viru-Viru International
Airport, is near the region of strongest winds associated with LLJ east of the
Andes. However, it is a relatively
cloudy site.
Paraguay
Mariscal
Estigarribia. This site is located very
close to the mean position of the low level jet over the Chaco, and as such it
frequently reports the strongest winds of the low-level jet SONET sites. The primary objective of this site is to
provide estimates of the wind speed and vertical structure of the jet near its
core. This site is much less cloudy
than Santa Cruz, Bolivia and offers better possibilities of frequent soundings
to high altitudes.
Asuncion. This site provides measurements near the
exit region of the East Andean low-level jet.
Chile
San Felix Island
(proposed). We are proposing this
island (26˚S, 80˚W), about 1000 km west of the coast of Chile, as a
future SONET site because of its unique position to describe the flow over the
stratocumulus region of this area.
Currently there are no soundings made from this island, though VEPIC
enhancements are being considered. Wind
soundings from the island would allow estimates of the divergence to be made
(monthly mean at least), using sites on the coast of Chile and southern
Peru. These observations, if
successful, could be eventually developed into inexpensive radiosonde
observations, or powersonde observations.
Figure 2.5 Zoom on the southern
end of the Panamerican radiosonde network (blue circles), current (black dots) and proposed (stars) PACS-SONET
sites.
4.
Educational activities and annual meetings
PACS-SONET has had,
as part of its activities, educational activities throughout its
existence. During the first two years 8
visitors participated in 1-3 month visits to NSSL, and carried out applied
research activities with the recently started SONET data. Prior to the SALLJEX activities, 6 visitors
from South America participated in similar visits to NSSL during the summer of
2002.
In addition
to these activities, SONET has supported, or helped support a series of
multi-week workshops in Bolivia, Panama, and shorter series of lectures in
Mexico, Peru, and most recently Paraguay.
During the recent
SONET workshop in Paraguay the concept of SONET-supported educational
activities was discussed at length. The
principal concern was not that these activities were not a good activity, but
rather that the cost involved consumed a significant percentage of the
available SONET funds that might otherwise be used for observations. A major workshop or course, involving
international participants, has typically incurred an expense of approximately
$25K. This is about 20% of the funds
available for observations. Much
discussion centered on what was the best way to invest the available funds in
educational activities. The options
discussed included 1) multi-month visits to NSSL, short (several week) courses,
sponsoring students at selected educational institutions, funding SONET
research activities in certain countries, and having NSSL personnel provide
short courses in selected countries (just for members of the particular
country). Some of these activities are
not feasible, while others will require careful evaluation.
The participants in the SONET workshop prepared a document,
which summarizes the discussions that took place during the meetings and
presents the participants’ views and suggestions about different aspects of the
project. Much emphasis was put in the network’s design and in the planning of future educational
activities. Although many of the changes proposed may not be feasible or lack
sound scientific justification, those highlight the level of motivation of the
PACS-SONET coordinators. Their input is much appreciated and we will continue
to encourage this kind of participation. The document will be made available in
the project’s web page through the following URL:
http://www.nssl.noaa.gov/projects/pacs/laquinta/
5.
Next year's activities
a) Quality-control
of the historical PACS-SONET data
Much of the pilot
balloon data from the SONET has not been fully quality controlled. The observations are often sent from the
stations in near-real time without a careful screening of the data for
errors. This is despite the fact that
the procedure to calculate the winds actually requires passing through the
quality-control stage where the data is graphically displayed. (One can shorten the "processing
time" by hitting the return key instead of actually correcting erroneous
data. There is not time for an
individual at NSSL to correct all of the incoming data, and a significant
fraction of the data appears in paper format only after months or when the hard
copy original reports are sent to NSSL.
Although many of the
stations have quite acceptable quality observations and many others have been
checked over the years for errors we continue to find problems with different
stations in different years. Even a few
bad observations per month can affect the computation of a monthly mean wind,
and this impacts studies of interannual variability with the data.
To systematically
quality control all of the data we have hired several undergraduate students to
carry out this task. During the summer
of 2003 they processed all of the SALLJEX soundings and begun on the earlier
SONET data base. This activity will
continue through the end of FY 2004, when we will be able to
"release" a fully quality-controlled data base covering the period
1997-2003. Although late, these
observations eventually will feed into future versions on NCEP reanalyses (and
regional reanalyses), so the work must be completed for the observations to
have a full impact in these analyses.
b) Expansion of the
network
Given the difficulty
in maintaining the current SONET network of about 20 stations, it might be hard
to imagine expanding the current network.
However, some sites are being considered for elimination, depending on either
establishment of radiosonde sites are these, or nearby localities, or because
of continuing difficulties in operating sites that are providing non-critical
data. For example, we are likely to
establish new sites in Peru where the National Weather Service (SENAMHI) has
indicated that personnel and communications exist, as the sites are currently
radiosonde sites established as part of a World Bank-sponsored activity. However, there are insufficient funds for
daily radiosonde launches and the SENAMHI would like, as a minimum, to provide
daily soundings as part of the SONET network.
The Paraguayan SONET site at Mariscal Estigarribia and a Venezuelan site
at Cd. Bolivar are being equipped to make radiosonde observations. Once
reliable observations begin at these sites it may be possible to stop pilot
balloon observations at these sites - but only after careful consideration to
the sustainability of the radiosonde observations.
Most of the
motivation for establishing additional observing sites for SONET is to take
advantage of opportunities that have arise for better communications with the
institutions that are responsible for sites that would be especially beneficial
for climate monitoring. Many factors
determine a site's value for long-term monitoring. The feasibility of operating a site over many years (potentially)
requires a stable institution and interest in the observations at the local
level. But physical factors are also
crucial, such as the frequency of low clouds or the proximity to the local
effects of topography or land-water interfaces that might induce large diurnal
wind oscillations. If the site's
observations are perturbed by very local conditions it will be difficult to
relate the observations to larger-scale analysis products, such as the NCEP
reanalyses, which cannot resolve very local effects. PACS-SONET continues to seek locations that not only satisfy
logistical considerations, but also improve the networks spatial coverage and
provide "higher quality" observations. A "high quality" observing site would be one that
would show little effect of topography and a small diurnal cycle. Usually these are hard to find, as even
sites over flat land and far from nearby mountains will show a diurnal cycle in
the wind profile due to boundary changes due to daytime heating and nighttime
cooling. The best sites are small, flat
islands that should have neither significant diurnal cycles or show the effect
of local topography. Observations from
these sites should be most comparable to larger-scale analysis output.
Four sites
have been identified as being potentially of particularly high quality. These are Malpelo and San Felix Islands in
the Pacific and Serranilla Bank and Aves Island in the Caribbean Sea (Fig. 5.2.) All of these sites are inhabited constantly by military personnel
from either Colombia, Chile, or Venezuela.
Serranilla is a flat sand island of very small size, and without any
island-induced disturbance of the airflow (unlike most Caribbean islands). Malpelo is a large rockck
with a maximum altitude of
about 300 m, but of small horizontal extent (~ 2 km long). Malpelo is situated in a critical data void
west of Colombia, and has been the site of previous (but unsuccessful) efforts
to maintain surface meteorological stations and tide gauges.
A slightly different
justification exists for establishing observations on Aves Island, belonging to
Venezuela. This well-staffed facility
is ideal, being a flat (less than 2 m high) sand island less than 1 km long and
100m wide (Fig. 5.1). It is about
200 km downwind of the largest (~40km wide) and highest (~1500m peak) island in
the Lesser Antilles, Guadeloupe (Fig. 5.2), which is the site of a radiosonde
station with a long historical record. We will seek to make trial observations
on Aves Island with the aim of comparing the wind profiles obtained from Aves
with other Antillean radiosonde sites that might be affected by both the
diurnal cycle and the effect of topography induced by the larger islands.
Finally, we have
begun the process of communicating with the Chilean Navy regarding a proposed
installation of a pilot balloon site on San Felix Island, which has been passed
through the US Embassy in Chile. San
Felix is a small, relatively flat island
that is ideally suited for measuring conditions in support of possible
VEPIC-type activities in the future.
Our longer term goals are to actually operate a powersonde system from
the island, since the island maintains a large paved runway, and there are no
air-traffic concerns.
Initially, we plan
to seek approval for trial observations on all of these islands to evaluate the
feasibility of more permanent observations being made. Since personnel rotations are frequent on
these islands, it may be feasible to only make one month of observations
initially, unless training procedures are established adequately for new
crews. In any case, surface
observations, which will be part of all of these sites, should be easier to
maintain for longer durations, and these observations will be valuable (and
unique) in themselves.
c) Implementation of
radiosonde sites as part of PACS-SONET
Implementing a
routine, if not daily, radiosonde launch at San Cristobal, Ecuador is a goal of
the radiosonde component of SONET. This
would be carried out with older RS-80 Vaisala omegasondes that can be very
inexpensively obtained. Other
possibilities exist, including evaluating the quality of these sensors by
interspersing launchings of the omegasondes with launches of newer RS-80
radiosondes. This was suggested by the
Peruvian representatives to the Paraguayan workshop.
The challenge in
implementing routine radiosonde observation in the Galapagos is in ensuring
that the institution that maintains the San Cristobal site in the Galapagos
Islands (INAMHI) responsibly undertakes the activity. SONET has had mixed results with this institution in the past and
a visit to evaluate their responsibility with a new director will be undertaken
early in 2004. A site visit to the
island may also be carried out, or this may be done in coordination with NOAA
Aeronomy lab personnel who also periodically visit the island.
Another possibility
involves the establishment of recoverable radiosonde (either glidersonde or
powersonde) technology at one or more site in the region, but this option
depends heavily on development work that has not been concluded.
d) NAME
modifications
During the coming
year the planned North American Monsoon Experiment (NAME) may put a strain on
the operations of the SONET, both in terms of personnel and equipment. One component proposed for NAME is a network
of pilot balloon stations; if this is approved then as many as 18 pilot balloon
sites will be temporarily established in Mexico during the summer of 2004. This will strain the available resources - there
are currently not 18 theodolites available as part of SONET. In addition, personnel currently overseeing
SONET activities will be partly detailed to NAME activities.
e) Educational
Activities
Despite the
expressed interest in the Paraguayan workshop in educational activities and the
need for these to continue, it appears infeasible within the current budget to
support major courses every year.
During the coming year we intend to carry out two special SONET-related
trips, to strengthen the SONET activities in these countries and provide
information to a much larger audience that can previous courses. The first of these trips will be to Central
America, and include multi-day visits
to Nicaragua, Costa Rica, El Salvador and Honduras. The latter 3 countries are not currently part of SONET but either
were in the past (Costa Rica), have expressed interest (El Salvador), or
control valuable sites for SONET observations (Honduras). During our visits we intend to communicate
with institutions other than the NMS's; the educational institutions and other
ministries that may also have interest in climate-related studies. A common weak link is the lack of
communication between research carried out in national universities and the
NMS's. We are preparing a 3-day course
that will be given in each country to a broad audience.
The second planned
trip will be to Venezuela, Colombia, and Ecuador. Although Venezuela has carried out SONET observations reliably,
modernization activities now underway will require some modifications to the
SONET network. These modifications,
plus the possibility of observations from Aves Island, will require travel to
Venezuela early in FY2004. This travel
will be followed by travel to Colombia to (hopefully) arrange observations on
Isla Malpelo and Serranilla Bank with Colombian Navy personnel, and explain our
activities to the IDEAM staff in Bogota.
The final component of this travel will be to Ecuador, where we have to
resolve the problems with the Galapagos pilot balloon site with the Director of
INAMHI. Continuing inefficiencies at
this site require a re-evaluation of the site, especially if radiosonde
observations are to be considered for the island.
While in Quito, we
intend to offer our 3-day short course to not only INAMHI staff, but also
personnel from local universities and other institutions. In our view INAMHI is perhaps in greatest
need of such educational materials.
Figure 5.1. Approaching Aves Island, the inhabited platform (and the island) are visible. The island is only about 100m wide.
Figure 5.2. Location
of Aves Island
Figure 6.1. The mean annual cycle for the afternoon Piura meridional winds.
Figure 6.2. Afternoon monthly evolution of the anomalies of the zonal (up) and meridional (bottom) winds at Piura.
Figure 6.3. Mean zonal (left) and meridional (right) winds for February and March and June and July.
Figure 6.4. Afternoon vertical profiles of the zonal (upper panel row), meridional winds (mid panel row) and number of observation taken into account (lower panel row) at Piura during each of the years. Averaged over the months of January to April, June to August and September to November on the left, center and right panel columns, respectively.
The
difference in meridional wind profiles is apparent from Fig. 6.3, with shallower (~1000m) and stronger (~ 6m/s max)
northward flow during the cool months and deeper (~1500m) and weaker (~4 m/s
max) flow during the months of highest SST.
A summary
of the vertical profiles of the zonal and meridional winds at Piura during each
of the years, averaged over months when the profiles appeared similar, is shown
in Fig. 6.4. The essence of this complex figure is that
the interannual variations are quite small, compared with the mean profiles
(not plotted, but evident by "eyeballing" the mean value of the
individual profiles. The largest
variability appears during the warm season and the least difference in the
profiles appears to be during the June to August period, though may vary with
the level considered.
6.2 Low-level jet crossing the Yucatan peninsula
The
observations at Frontera, and more recently at Ciudad del Carmen (~100 km east
of Frontera on the flat coastal plain), on the southern part of the Yucatan
peninsula, have shown a strong, diurnally oscillating low-level jet. This jet is most common during the spring
months, under pre-frontal synoptic conditions but is also clearly present in
monthly mean profiles. The strongest
easterly flow occurs during July, similar to Managua farther south, with the
zonal wind anomaly extending throughout the lower-middle troposphere (Fig. 6.5). As with Managua, the NCEP reanalyses at 850 mb (Fig. 6.6) underestimate the strength of
the flow; the observations suggest a zonal wind near 7.5 m/s while the analysis
indicates about 5.5 m/s.
The July
mean u and v profiles (Fig. 6.7)
show that the diurnal variation is considerably larger than the interannual
variability, and also show that the diurnal variations are large only below
about 1700 m. Above this level, despite
there being fewer observations, the difference in the mean morning and mean
afternoon profiles is quite small - at least up to 3 km.
Figure 6.5. Annual anomalies for the morning (AM) and afternoon (PM) zonal winds at Ciudad del Carmen.
Figure 6.7. AM and PM vertical profiles of the zonal (upper panel row), meridional winds (mid panel row) and number of observation taken into account (lower panel row) at Ciudad del Carmen during each of the years. Averaged over the month of July.
6.3 Gap flows at Salina Cruz
The
cross-gap flow at the Isthmus of Tehuantepec is a well-known feature, and is
now being routinely documented by scatterometer observations. However, the only direct measurements of the
vertical structure of the flow come the PACS-SONET observations at Salina Cruz,
started in 1997 there to monitor gap flow and its variations associated with tropical
wave passages in the summer and cold frontal surges during the winter. Figure
6.8 shows the profiles of the morning and afternoon wind components,
averaged over the months of July and August, for three years of particularly
complete data. Figure 6.8 also
showing the mean (AM + PM) of these years.
The most significant aspect of these figures is that the interannual
variability of the monthly mean flow is quite small - compared with the mean
profiles. The diurnal variability of
the meridional wind profiles is detectable - about 1.5 m/s difference near the
wind maximum at 700 m asl, but is small compared with the meridional wind (~10
m/s). The range of the diurnal
variation of the zonal wind is larger, about 3 m/s. There is a clear vertical reversal of this diurnal variation
(also evident in meridional wind), associated with the sea-land breeze. In general, the implication of these
observations is that the interannual variability is small compared with the amplitude
of the diurnal cycle. This implies that
care must be taken in order to compare the observations to larger-scale
analyses that do not resolve the diurnal cycle well.
Figure 6.8. Am and PM vertical profiles of the zonal (upper panel row), meridional winds (mid panel row) at Salina Cruz during each of the years. Averaged over the months of July to August.
6.4 Topolobambo.
Inflow into the lower Gulf of California
This site,
near the lower end of the Gulf of California, has the most complete record of
the two PACS-SONET sites along the Gulf.
Figure 6.9 shows the annual
cycle of the u and v components. The
main feature is that, below 1 km altitude, the meridional wind undergoes a
prominent annual oscillation between northerly winds from October to April and
southerly winds present during the period June through September. Strongest northerly winds occur from
December to February, while the strongest southerly winds are found during June
and July. Variations in the zonal wind
component are much smaller near the surface, but increase with height and are
larger than meridional variations by 3 km asl.
Figure 6.10a shows the
difference in the mean zonal flow for different years, and highlights the fact
that the diurnal cycle is far larger than the interannual variability of this
component of the wind. However, there
is a larger interannual variability of the meridional wind (Fig. 10b). The mean of the morning and afternoon
soundings, again averaged over the period June-September, is shown in Fig. 6.10c-d. In all years there is up-Gulf flow, though the depth and
intensity can vary. The northward flow vanishes at about 1500 m, in the
mean.
Figure 6.9. Composites of the mean annual cycle for the AM and PM soundings of the zonal (up) and meridional (bottom) winds at Topolobambo.
The
importance of these simple observations from Topolobampo is apparent when one
examines the results of studies such as those involving Eta-model diagnostics
(e.g. Berbery 2001 - JCL) that show
flow down the Gulf at this location.
Over this region, the eta analyses are clearly wrong.
6.5 Flow at the northern end of the Gulf of
California. Puerto Peñasco
The
observations at Puerto Peñasco were intended to monitor the strength of the
flow over the northern Gulf of California, and as such, the intensity of the
summer north American monsoon circulation.
This site has reported stronger low-level winds than any other sounding
site in the region, and displays a clear wind direction shift of the monthly
mean winds between winter and summer (not shown). As with most sites, the diurnal variation is larger than the
interannual variation (Fig. 11a-b),
especially in the zonal wind component below 1000m. However, the interannual variability is significant, especially
above 1 km asl. The near-constant
difference between the morning and afternoon mean meridional wind profiles
(above the surface layer - where it is very small) is curious, since it cannot
be ascribed to fewer observations with height.
It should
be noted that the Puerto Peñasco (and other site's) soundings have not been
fully quality-controlled and it is expected that the results will be smoother
when this is completed. This site is
one of two of the 7 Mexican sites that are not located at Oceanographic offices
of the Mexican Navy and unfortunately this has led to problems with supervision
of the observers and quality control of the observations.
The most
significant result from the Puerto Peñasco observations is the fact that every
year, when observations are averaged over morning and afternoon and over the
months of June-August, shows a clear
up-Gulf flow throughout the lower few km (Fig.
11c-d). The meridional wind, nearly
equal to the total wind since the zonal component is quite small, shows a maximum of about 4.5 m/s near 300 m asl. The interannual variability is smallest near
the surface and increases with height.
Comparison
with the results for Topolobampo shows that the meridional (and total wind) is
about twice as strong in Puerto Peñasco than at Topolobampo. This agrees with earlier findings based on
SWAMP-1990 and EMVER-1993 experiment data of much more limited duration.
Figure 6.10. AM and PM interannual vertical profiles of the zonal (a) and meridional (b) winds from June to September during each of the years at Topolobambo and average for the AM and PM vertical profiles for the zonal (c) and meridional (d) winds.
Figure 6.11. The same as figure 6.10 but at Puerto Peñasco.
6.6 Gap flow through Central America at Managua
Observations
at Managua, with the exception of the current year, have been generally
reliable and of high quality. These are
sufficient to describe the annual cycle; the zonal wind section is shown in Fig. 6.12. The monthly mean zonal wind shows two peaks during the year.
The first, which occurs in July-August, is of about 15 m/s. This peak abruptly weakens to near 6-7 m/s
during September and thereafter starts to increase, reaching about 18 m/s in
February. Both peaks in zonal wind
occur near 1.5 m asl. However, the
anomaly cross section of the zonal wind (Fig.
6.13) shows that the amplitude of the anomaly is vertically extensive, with
the maximum anomaly near 3 km in July-August, but extending to above 6 km with
nearly the same amplitude.
The
monthly mean zonal winds at Managua are considerably larger than those based on
the NCEP reanalyses (Fig. 6.14),
which indicate for July-August mean winds, a broad maximum in the easterly flow
between the 850 and 700mb levels, with speeds over Nicaragua of between 7.5 - 8
m/s (at 850 mb). Curiously, the NCEP
reanalyses show the winter maximum in the easterly flow to be slightly weaker
than the summer maximum.
The
comparison of the pilot balloon-based mean zonal wind at Managua with the
radiosonde station observations at San Andres island is of interest. The island is about 500 km east of Managua,
along the axis of maximum winds in the trades over the Caribbean Sea. While there is good agreement between the
zonal winds at 5-6 km from both Managua (see Fig. 6.12) and San Andres (Fig.
6.15), the difference between 1-2 km is quite large - about 5 m/s for the
July mean zonal wind (~ 9 m/s for San Andres and ~15 m/s for Managua). The difference is even larger during the
January - March winter peak in zonal winds, where Managua shows a broad maximum
of winds above 14 m/s with a peak of 18 m/s, while San Andres shows values of
7-8 m/s respectively. The vertical shear
above the Managua jet is much larger than at San Andres, though the San Andres
data did not always include the full-resolution radiosonde data and may thus be
smoother than the actual soundings.
The
stronger winds found at Managua can be explained (away?) as a gap-induced effect, but this is hardly a narrow gap,
and the strong winds found at Managua are quite high (~1.5 km) rather than
closer to the surface. The NCEP
reanalyses do not have the spatial resolution to show this gap wind
structure (see Fig. 6.6. ).
Figure 6.12. The mean annual cycle for the morning Managua zonal winds.
Figure 6.13. Morning monthly evolution of the anomalies of the zonal winds at Managua.
Figure 6.14. Annual cycle for the zonal winds at Managua (closest grid point) using NCEP/NCAR Reanalysis data
http://www.cdc.noaa.gov/cdc/reanalysis/reanalysis.shtml
Figure 6.15. The mean annual cycle for the morning San Andres zonal winds. The average was performed using the same year available for Managua. San Andres wind soundings ware obtained from the Forecast Systems Laboratory (FSL) web page
http://raob.fsl.noaa.gov/
.
6.7 San Fernando de Apure and the trade wind flow
across the Venezuelan Llanos
The annual
cycle at San Fernando de Apure, in the Venezuelan llanos (a very flat region)
shows a dry season low-level jet with maximum zonal winds of 15 m/s in
February the jet is quite low - about
600 m agl (Fig. 6.16). The anomaly plot of the zonal wind annual
cycle (Fig. 6.17) shows a relatively
symmetric pattern, with a 5 m/s positive anomaly in August and a 5 m/s negative
anomaly in February.
A
comparison with the NCEP reanalyses (Fig.
6.18) shows that the low-level strong winds are not captured in the NCEP
analyses. However, some similarities
are present, including the June-July maximum in easterly winds near 4 km
(~600mb) and the secondary east wind maximum in winter at low levels.
Figure 6.16. The same as figure 6.12 but at San Fernando, Venezuela.
Figure 6.17. The same as figure 6.13 but at San Fernando, Venezuela.
