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




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.

Figure 1.               Panel showing the long term meridional wind for North America (upper plots), during the boreal winter (a) and summer (b), and for South America (lower plots) during the austral summer (c) and winter (d).  Source: NCEP/NCAR Reanalysis.


Figure 2.               (a) Topographic distribution in the SALLJ region. (b) PPBB network deployed during SALLJEX.


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.


4          Diurnal Cycle

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.

Figure 3.               Mean wind speed for the Pilot Balloon  stations (a) upstream the SALLJ predominant region, (b) close to the Andean elbow and (c) downstream the SALLJ predominant region. Only taking into account LLJ events that occur during the Austral Summer. The filled circles represent the AM sounding at 1200 UTC and the empty one are at 2100 UTC.


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.



Figure 4.               NOAA P-3 flight during February (a) 06 and (b) 07/2003. Barbs and stream lines were plotted for the 850 mb level. The black squares indicate the location of the sounding plotted in Figure 5. Transect c-c’ indicates the location of cross section plotted in the Figure 6b.



Figure 5.               NOAA P-3 sounding (1 second time resolution) for February (a-b) 6 and (c-d) 7/2003. From the left column to the right there were plotted the temperature and dew point, the wind speed, the potential temperature and the specific humidity.  The soundings (a) and (b) correspond to the same place at different times, the same for sounding (c) and (d), see Figure 4. The continuous horizontal lines indicate the PBL top level at the time of the sounding and the dash lines indicate the PBL top level of the previous pass.



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)



Figure 7.               Topography cross section (a) along the jet and (b) across the jet in the area of the Andean elbow. (c) depicts the geographical location of (a) and (b).



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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Electronic References


Nogues-Paegle, J.  and J. Paegle, 2000: American Low Level  jetS

A Scientific Prospectus and Implementation Plan,