# Boundary Layer forcing mechanisms of the low-level jet events during SALLJEX {#boundary-layer-forcing-mechanisms-of-the-low-level-jet-events-during-salljex .unnumbered}

John Mejia

School of Meteorology, University of Oklahoma

Boundary Layer Meteorology class project

## Abstract {#abstract .unnumbered}

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.

# Introduction {#introduction .Titles}

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.

# Climatology And Regional Description {#climatology-and-regional-description .Titles}

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.

![](media/image1.jpeg){width="5.13125in" height="4.309722222222222in"}

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.

![](media/image2.jpeg){width="6.309027777777778in"
height="2.6597222222222223in"}

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

# Data {#data .Titles}

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.

# Diurnal Cycle {#diurnal-cycle .Titles}

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.

![](media/image3.jpeg){width="6.719444444444444in"
height="1.7555555555555555in"}

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.

# Boundary Layer Processes {#boundary-layer-processes .Titles}

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.

![](media/image4.jpeg){width="6.684027777777778in" height="3.38125in"}

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.

![](media/image5.jpeg){width="5.5375in" height="7.048611111111111in"}

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 agains 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 mentined earlier regarding the thermal wind law.

# Terrain effect {#terrain-effect .Titles}

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.

![](media/image6.jpeg){width="5.0875in" height="5.446527777777778in"}

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)

![](media/image7.jpeg){width="6.74375in" height="2.060416666666667in"}

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).

[]{.mark}

[]{.mark}

# Discusion {#discusion .Titles}

Significat 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 maximun 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 fase 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 maximun wind zone was found. Dynamically, this baroclinic zone
is enhenced 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 heigh and wind maxima heigh do not coincide. To that
end, and taking into account that both phenomenas NALLJ and SALLJ are
relative similars, 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.

# References {#references .unnumbered}

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Blackadar, A. K., 1957: Boundary layer wind maxima and their
significance for the growth of nocturnal inversions, Bull. Amer. Meteor.
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on the poleward water vapour transport in the central region of South
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Bolivia,

5-7 February 2002.

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Hartman, M.K. 1999: An Investigation on the Formation, Wind Flow, and
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5-7 February 2002.

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Velasco, I., and J. Fritsch, 1987: Mesoscale convective complexes in the
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# Walters, C. K., 2001: Airflow Configurations of Warm Season Southerly Low-Level Wind Maxima in the Great Plains. Part II: The Synoptic and Subsynoptic-Scale Environment, *Wea. Forecasting*, 16, 531--551. {#walters-c.-k.-2001-airflow-configurations-of-warm-season-southerly-low-level-wind-maxima-in-the-great-plains.-part-ii-the-synoptic-and-subsynoptic-scale-environment-wea.-forecasting-16-531551. .unnumbered}

#  {#section .unnumbered}

# Walters, C. K. and J. A. Winkler, 2001: Airflow configurations of warm season southerly low-level wind maxima in the Great Plains. Part I: Spatial and temporal characteristics and relationship to convection. *Wea. Forecasting*, 16, 513--530. {#walters-c.-k.-and-j.-a.-winkler-2001-airflow-configurations-of-warm-season-southerly-low-level-wind-maxima-in-the-great-plains.-part-i-spatial-and-temporal-characteristics-and-relationship-to-convection.-wea.-forecasting-16-513530. .unnumbered}

Zhong, S., J. D. Fast, and X. Bian, 1996: A Case Study of the Great
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# Electronic References {#electronic-references .unnumbered}

# Nogues-Paegle, J.  and J. Paegle, 2000: American Low Level  jetS {#nogues-paegle-j.-and-j.-paegle-2000-american-low-level-jets .unnumbered}

# A Scientific Prospectus and Implementation Plan, [***http://www.met.utah.edu/jnpaegle/research/ALLS.html***](http://www.met.utah.edu/jnpaegle/research/ALLS.html) {#a-scientific-prospectus-and-implementation-plan-httpwww.met.utah.edujnpaegleresearchalls.html .unnumbered}