Boundary Layer forcing
mechanisms of the low-level jet events during SALLJEX
John Mejia
School of Meteorology, University of Oklahoma
Boundary Layer Meteorology class project
Abstract
A Low
Level Jet (LLJ) frequently occurs to the east of the Andean mountains
throughout the year. A field campaign called SALLJEX (South America Low Level
Jet EXperiment) took place during the austral summer 2002-2003 to measure and
investigate the thermodynamical and dynamical environmental conditions that
lead to this LLJ. Pilot Balloon observations were used to produce mean wind
profiles early in the morning and late in the afternoon along the jet region.
NOAA P-3 aircraft data were used in this project between Jan11-Feb8 when
several LLJs occured. Nevertheless, in this study, only the LLJ event that
occurred on February 6 and 7 were analyzed using one second data of
temperature, dew point and wind vectors. These data were used to examine these
LLJs events and to limn the Planetary Boundary Layer (PBL) forcing mechanisms
that drove such LLJs. During both flights, a relatively narrow low level wind
maximum was found along the mountain barrier and in the vicinity of the
so-called Andean elbow (20º S), where the orientation of the Andes changes. The
presence of a boundary layer inversion has been found to be vital part of the
forming wind profiles in both flights. These kinds of profiles are
representative of the atmospheric wind flow within low-level jets. In this
particular study, not only was the LLJ studied under this PBL environment, but
also other elements that can be argued to influence the development of these
LLJ events. On the one hand, PBL mixing processes were suggested explaining the
wind profiles found in the vicinity of the wind maximum and all along the jet
region. On the other hand, the formation of the LLJ in conspicuous narrow
maximum areas and its particular spatial location were mostly suggested based
upon the topographic effect which likely produces this particular flow
structure. At the end of this work, there were mentioned some other questions
regarding the future work toward the ABL processes involve in this LLJ are
mentioned.
1
Introduction
According
to different works (Blackadar 1957; Stensrud, 1996; Hartman, 1999) a Low Level
Jet (LLJ) is defined roughly by examining the vertical profile of the
horizontal wind to determine whether or not a low-level wind speed maximum
occurs. To that end, a LLJ frequently occurs to the east of the Andean mountain
throughout the year, here called the South America LLJ (SALLJ).
The SALLJ is a northerly jet that develops along the eastern slopes of the
Andes. It has been suggested that it is produced by either thermally driven
flow, a local circulation, generated by the large scale topographical structure
or a large/synoptic scale circulation enhanced by the topographical barrier and
with a significant influence of the intrusion of the Atlantic Subtropical High
and enhanced Chaco atmospheric low (Saulo et al., 2002). Therefore, the SALLJ
shows a large variability on daily, intraseasonal and interannual time scales
as well as an important role in the genesis of severe weather systems
(Stensrud, 1996; Saulo et al., 2002; Campetella and Vera, 2002; Berbery et al.,
2002; Nogués-Paegle et al., 1998) and
organized convective systems (Velasco and Fritsch, 1987).
This
phenomena is of great interest in the Planetary Boundary Layer (PBL) processes
as well as in the practical sense because of the turbulence mass exchange in
this atmospheric layer. These low-level wind speed maxima events are important
for both the horizontal and vertical fluxes of temperature and moisture and
have been found to be associated with the development and evolution of deep
convection in the Northern part of Argentina and Paraguay (Stensrud, 1996; Fu
and Wang, 2002; Saulo et al., 2002; Salio et al., 2002).
There
are several physical mechanisms that have been shown to explain many aspects of
the development and evolution of the LLJs in a wide variety of environments.
First, the Inertial oscillation due to the frictional decoupling of the evening
flow (Blackadar, 1957), which accounts for both the daily oscillation in the
jet intensity and for the significantly supergeostropic velocities observed during
the nocturnal phase, this theory cannot account for the observed amplitude and
shape of the LLJ. Secondly, The shallow baroclinicity (Stensrud, 1996), which
takes effect where significant changes occur in surface characteristics,
sloping terrain can also form shallow baroclinicity (inclined boundary layers),
large scale baroclinicity features creates mechanisms that can produce LLJs
relatively constant throughout the day (this mechanism makes no appeal to
variation in turbulent mixing and has the advantage of explaining why the LLJ
tends to be located east of the Andean mountains, which Blackadar theory does
not address). Onother aspect is the terrain effect (Zhong et al., 1996;
Stensrud, 1996). Terrain blocking and other terrain features (e.g.: gap flow,
corner effect) create boundary currents with consequence flow acceleration and
jet-like profiles. Although PBL processes, which are also taken into account
the Blackadar theory via Diurnal variation in the eddy viscosity in the PBL,
produces retardation of the flow near the surface jet-like profiles.
The
formation of the nocturnal inversion and related boundary layer processes have
been found to be associated with the Low-Level wind maximum in a SALLJ case. In
this particular study, not only the Boundary layer processes are analyzed, but
also other vital elements that influence the development of two particular LLJ
events.
From
the above discussion it is apparent that a number of different mechanisms can
be used to explain the formation of LLJ in this particular zone. Typically,
none of these mechanisms alone can explain the observations as found by
Blackadar (1957), who illustrated that diurnally varying eddy viscosity alone
cannot give a satisfactory explanation of the diurnal wind structure. Stensrud
(1996) highlighted the importance of terrain effects the structure of the LLJ,
the diurnal cycle of sensible and latent fluxes, large-scale forcing, and the
PBL evolution in producing LLJ. Walters and Winkler (2001) and Walters (2001)
organized the LLJ event in twelve different spatial configurations of southerly
warm season low-level wind maxima in the Great Plains (USA). They found that
only 1 of 12 configuration types appeared to be purely boundary layer-driven
wind maximum. For the remaining types, the relative influence of synoptic
forcing and boundary layer forcing varied. To that end, the SALLJ case is not
expected to be driven for clearly defined mechanisms. Nevertheless, this work
attempts to describe via observations whether or not there is an influence of
the PBL structure in the development of the SALLJ as well as to show the
importance of the terrain effect in the spatial distribution of the wind
maximum.
This
paper proceeds as follow. Section 2
gives a brief description of the climatoly of the phenomena. Section 3 presents
the characteristics of the data sets used in the analysis. Section 4 includes
analysis related the diurnal cycle of the LLJ during SALLJEX . Section 5 shows
the NOAA P-3 data and discusses different PBL features influencing the LLJ.
Section 6 contains some remarkable aspect about the terrain effect. Comments
are included in the final section.
2
Climatology And Regional
Description
Figure 1 shows, based upon the long term meridional wind
values, it is observed that on average the North America Low Level Jet (NALLJ)
events over the eastern slope of the Rocky Mountains occur mainly during
summer. In contrast, SALLJ has been identified as a key part of the monsoon
system, but unlike other LLJs, and in particular the NALLJ, it occurs
throughout the year (Fu and Wang, 2002). SALLJ plays a major role in the
moisture transport from the Amazon Basin to northern Argentina, Paraguay and
Southern Brazil during the summer (Figure
1c) with both moisture and wind enhancing the develop
of precipitation (Berbery et al., 2002).
The Andes and
the Rockies are major mountain ranges that deflect the prevailing atmospheric
flow producing low level jets. Both mountain ranges extend from the tropics to
high latitudes, and effectively block the low-level circulation, particularly
in summer. There are however, important differences in topography. The Andean
peaks are nearly twice as high as the Rocky Mountains, and they rise abruptly
from low elevations, without substantial intervening zones of gently sloping
terrain, see Figure
2a. Although the Andes extend from high latitudes to
equatorial latitudes, they exhibit mesoscale dimensions of the order of 500 km
or less in the east-west direction. Possibly because of this, the seasonal
cycle of the east Andean LLJ may not be as pronounced as that of its North
American counterpart (Nogués-Paegle et al., 1998).
Elevated
orography is also found around southeast Brazil, near the Atlantic coast.
Although these mountains are much lower than the Andes, they are more extensive
than the Appalachian mountains of eastern North America, and play a prominent
role in low SALLJ (Nogues-Paegle and Paegle, 2000). The bend of the Andean
mountain (the so-called Andean elbow 20º
S) is other topographic structure to notice.
Campetella and Vera (2002) made reference to this feature where LLJ wind
maximum episodes have been observed with some frequency.
3
Data
Twenty
Pilot Balloon (PPBB) stations were established over the SALLJEX region to
measure vertical wind profiles, see spatial distribution in Figure 2b. Although the profiles were measured every 3-Hours,
only 1200UTC and 2100UTC profiles were used to compute the early morning (AM)
and late afternoon (PM) averages. Data set range from Dec/2002 to Feb/2003
(here called the Austral Summer period).
Using these data, mean profiles were composed extracting the days with
strong Northerly or Northwesterly low level wind events. In addition, the NOAA
P-3 aircraft was used during the experiment to measure different boundary layer
processes related with SALLJ including the measurement of lower troposphere
moisture fluxes and the description of favorable conditions for organized
convective systems, mentioned above. From 13 flights completed during the period
of the SALLJEX (Jan11-Feb8/2003) only two LLJ cases were chosen for practical
purposes: February 6 flight (11:55 - 19:49 UTC) and February 7 (11:56 - 19:50
UTC). In both flights, the basic flight plan was an horizontal
"Z"-shaped pattern followed by a vertical sawtooth pattern (between
950 and 700 mb) across eastern the Andean mountain; this pattern was made in
order to sample both along-jet and vertical structure of the jet as well as its
boundary layer configuration.
The
mean kinematic evolution of the wind profiles is depicted by the pilot balloon
observations. Summer average (Dec-Feb)
wind speed profiles at AM and PM were estimated for 16 stations. Only days with
episodes of LLJ were taken into account calculate the mean values. For practical
purposes, only three stations are shown in the Figure 3; see their exact location in Figure 2b. From Figure 3a to Figure
3c, the soundings were found representative of the
regions upstream of the LLJ, the elbow of the Andean mountains, and downstream
of the LLJ, respectively. In the AM profiles, a narrow jet-like structure develops
across the LLJ region, which extends from the eastern part of Brazil to the
central part of Argentina, this feature was observed all along the east side of
the Andean Mountains. The characteristic LLJ signature is weaker in the PM
average soundings.
The
physics behind these shallow AM maxima are fairly clear. After sunset, a
surface based inversion layer developed, and the stable stratification reduces vertical
mixing, so that, friction was no longer effective up to such heights as during
the day time. As the growing surface inversion increasingly shields the jet
from the surface friction, winds above the surface inversion layer release of
all frictional constraint and then accelerate developing (eventually) a low
level wind maximum. The sharp vertical wind shear and the height of the jet
maximum between 800 and 1000 AGL is observed all along the jet region.
Nevertheless, along the jet and at the level of the wind maximum, the mean
boundary layer flow is significantly accelerated (almost three times the mean
flow of the station upstream and two times the station downstream). This region
of vertical and horizontal wind maxima is colocated with recognized topographic
structures likely leading to such acceleration, further details about this
aspects will be describe in the foregoing section.
The PM profiles show a broad, higher,
and relatively smooth maximum. During the
afternoon actual winds are subgeostrophic due to turbulent mixing in the deep
daytime boundary layer. Friction
tends to slow down the speed of the winds in the PBL as you get closer to the
surface and turbulent mixing smooth the velocity profiles due to the vertical
momentum transfer.
Previous
analyses of wind soundings (Berri and Inzunza, 1993; Douglas et al., 1999)
revealed intriguing aspects that are not easily reconciled by theoretical
analyses that have been successfully applied to North American cases and to the
broad calculation doing so far in this paper. The principal discrepancies are a
deep late afternoon, rather than a shallow early morning wind maximum at Santa
Cruz (SCZ), Bolivia (Douglas et al., 1999). It is possible that those
observations do not reflect climatology since they consist of only 90
observations taken over 70 days. Based
upon a numerical modeling, Silva Dias (2001) found consistencies with the
theoretical analysis but did not found how to support the observational
evidence of a late afternoon/early evening maximum rather than a shallow
nocturnal/early morning maximum.
It is worth highlighting
other observable feature of the wind profiles above the wind maximum.
Geostrophic flow decreasing with height is characterized upstream and close to
the Andean elbow. Although downstream the flow aloft seems not to change with
height (characteristic of barotropic atmospheres), it was found that at that
levels the mean diurnal difference is about the same order of magnitude that
was found at low level. Such difference could be attributed to the diurnal
cycle of the geostrophic wind produce by the sloping terrain to the west of
this location.
5
Boundary Layer Processes
Two
NOAA P-3 flights are analyzed in this section. They are the Feb06 and Feb07
both corresponding to a moderate and a strong LLJ events, respectively. Both
flights were designed to capture the 3D structure of such events and following
almost the same track, see path of the flight in Figure 4. The
intentions of this section are, first, to show the vertical profiles of the
principal variables in a location close to the wind maxima and related these
porfiles with the PBL processes that prodece them, and second, to argue for
some possible forcing mechanism that lead such a wind maximum profiles in
relation with its spatial location.
Figure 5 shows the profiles for both flights Feb/06 (Figure 5a and b) and Feb/07 (Figure
5c and d). In either case, two soundings were plotted
at different times, about 3 hours in the first flight and 2.5 hours in the
second one. In general, a wind maximum (about 30 m s-1) is found at
the same level of capping inversion height (about 850 mb). The first day
presents weaker wind maximum (15 m s-1) at heigher level (790 mb),
whereas the second day has a clearer and shallower LLJ structure with winds up
to 30 m s-1. Also, it is observed that the capping inversion height
is rising with time, which is a typical response of the diurnal cycle of the
PBL with a well-mixed layer (ML), which was faster the second day with an
observed change of 50mb/2.5hr against 40mb/hr. This illustrates in a brad way the
efficiency of the additional turbulence produce likely by the wind shear.
Besides the wind, specific humidity and potential temperature nearly contant
within the PBL bring more evidence to say that, at this location, the ML
corresponds to a statically unstable air; the second day shows how this
thermodynamic profiles changes more rapidly from statically neutral (or in some
layer stable) PBL into a statically unstable BL.
The
thermodynamic and dynamic structures of the Feb/07 jet are plotted in Figure 6, which shows a vertical section across this LLJ event
(location of these cross sections are depicted in Figure 4b). There were plotted the vertical distribution of
the wind speed (Figure
6a) and the potential temperature (Figure 6b). It took about 80 minutes to flight from one side
to the other, nonetheless, these variables ware considered stationary along
that period of time. Potential temperatures and winds with in the PBL are
relatively uniform due to the strong vertical mixing, which is more evident in
the zone with higher speed.
Dynamically,
cyclonic vorticity at the west side of the jet wind maximum is associated with
convergence at the surface and then producing deeper PBL, whereas, to the east,
divergence at the surface produced by the anticyclonic vorticity suppresses the
regular growth of the PBL making it shallower.
The
unequal heating from west to east over this sloping terrain produces the
baroclinic environment, which is evidenced in the Figure 6b. Above the capping inversion the potential
temperature is rising from west to east. Moreover, this temperature gradient is
further enhanced due to a deep daytime PBL over the High Plains against a
shallower PBL to the east. At the time of maximum heating, the mesoscale
pressure gradient results in a stronger northerly geostrophic wind in this
region, which bring stronger evidence to support the diurnal change of the
geostrophic wind that was mentioned earlier regarding the thermal wind law.
6
Terrain effect
Several
questions arise regarding the effect of the Andean mountains as a blocking
structure. The observed acceleration of the mean flow east to the Andean elbow,
as mention in section 4, could be attributted not only to the change of
direction of the flow throughout the corner effect, but also to the gap flow
produce for the some smaller topographic structures across the jet in the same
region, see Figure
7. Campetella and Vera (2002) did simulations of the
atmospheric circulation over South America only forced by the presence of the
Andean mountains. Those simulation were capable to reproduce the main features
of the LLJ but but diurnal observed differences are not well simulated. The dynamical effects associated with the
flow along this conbinated structures are not well understood. So that, future
work I this area could be done studying conceptual models for flow patters
using separeted configuration via numerical simulation and using idealize
topographical features.
Figure 6.
wind speed pointing
outward (a) and potential temperature on the vertical cross section in February
7/2003, see cross section c-c’ in the Figure
4b. Da(improve
this plot and drow the PBL top line)
7
Discussion
Significant
low-level jets occur to the east of the Andean mountains in a topographical
pattern very similar to the LLJ that normally develops during summer at the
Great plain in United States in the northern hemisphere.
AM
and PM wind profiles based on PPBB observations were found that agree with the
classical LLJ theory and PBL concepts. Although the mean profiles were made to
illustrate the diurnal differences (AM-PM) and its associated processes, they
necessarily do not to take into account that pure inertial oscillation theory
(Blackadar, 1957), which predicts that LLJ wind maximum should be reached
earlier at higher latitudes that at lower latitudes, where the inertial period
is shorter. To that end, the hours chose to represent the extremes of the
diurnal cycle necessarily do not represent the phase of the actual diurnal and
semidiurnal cycle.
Researh
aircraft observations have been used to study the dynamical structure of the
SALLJ during two events in Feb06 and Feb07 2003. The analyzed flights show a
low level temperature gradient in the region where the maximum wind zone was
found. Dynamically, this baroclinic zone is enhanced around the core of the jet
maximum due to the cyclonic and anticyclonic vorticity produce around the wind
maximum.
In
the analyzed flights the height of the wind maxima coincided with the capping
inversion level. Nevertheless, Stensrud (1996), Walters and Winkler (2001) and
Walters (2001) found episodes in the NALLJ when the capping inversion height
and wind maxima heigh do not coincide. To that end, and taking into account
that both phenomenas NALLJ and SALLJ are relative similar, it is expected that
the wind maximum level is not always expected to occur at the same level of the
capping inversion.
Considerable
extended work over the whole set of observations collected during SALLJEX will
be required for a more completed and systematic analysis of this very complex
phenomena. Also, a quality control has not been performed over the obsevation.
So that, some comments and conclusion could change after some extend control.
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