Lightning Variability on the Lake Maracaibo Basin

Lightning Variability on the Lake Maracaibo Basin - Venezuela
Á. G. Muñoz
Centro de Modelado Cientı́fico (CMC). Universidad del Zulia. Maracaibo, 4004. Venezuela.
Department of Earth and Environmental Sciences. Columbia University. NY 10964-1000. USA.
J. Dı́az-Lobatón
Centro de Modelado Cientı́fico (CMC). Universidad del Zulia. Maracaibo, 4004. Venezuela.
Abstract
The Catatumbo Lightnings is the local name of a set of electric storms that take place mainly in the southwestern
quadrant of the Lake Maracaibo Basin (LMB), in Venezuela, between the Bravo and Catatumbo riverbeds. Recently,
this place has been reported to be the location with the highest flash (lightning) density in the world, as evidenced by
LIS/TRMM data. Several local factors help trigger the formation of thunderstorms in the zone, namely the topography
configuration, moisture availability, wind circulation and orographic convection. From a regional point of view, the
monthly lightning anomaly in the Catatumbo zone evidence a modal cycle which follows the Inter Tropical Convergence
Zone (ITCZ) migrations, with a minimum in January and a maximum in August-September, the latter in correspondence
with minimal yearly values of meridional winds and the Caribbean Low Level Jet (CLLJ) seasonal weakening. An
energetic characterization of the diurnal and yearly cycles of the lightning activity is also presented using a high reslution,
non-hydrostatic numerical model (WRF), revealing that the work done by the winds against the pressure field in the
boundaries of a control parcel and the mass-energy advection provided by the same winds play a key role in the production
of lighting in the LMB.
Keywords: Catatumbo, Lake Maracaibo, LIS.
1. Introduction
though they are mainly concentrated to its southwest portion near the Catatumbo river mouth. The appearance
Lighting activity in Northwestern Venezuela is so common that it has a proper name: the Catatumbo Lightnings
(CL). This phenomenon has been reported as the set of
of this electro-atmospheric event has been recurrently reported for approximately 500 hundred years (Zavrotsky,
1991).
electric storms with the highest flash (lightning) density
on Earth (Stock et al., 2011; Albrecht et al., 2009). In the
Several authors have suggested in the past different ex-
last decade, the Lighthouse of Maracaibo, as it is locally
planations about the CL origin (e.g. Zavrotsky, 1991, and
known, has been object of renewed interest in the scien-
references therein). In particular, Falcón et al. (2000) have
tific community due to its inherent characteristics and also
suggested a microphysical model based on the symmetry
for the lack of information on its occurrence and behav-
properties of methane gas and its critical concentration
ior. These electric storms possess an inter-annual variabil-
on the upper side of the storm cell, but in our opinion
ity ranging from 180 to 260 nights per year (Stock et al.,
such a model must be carefully revisited due to the fact
2011) throughout the Lake Maracaibo Basin (LMB) al-
that the proposed mechanism is not viable if updraft and
Preprint submitted to Elsevier
February 10, 2014
downdraft circulations within the clouds are considered.
On the other hand, Stock et al. (2011) and Albrecht et al.
(2009) have described the ligthning phenomenology of the
LMB in terms of satellite observations using the Lightning
Imaging Sensor-LIS (Boccipio et al., 2002), while Mapes
et al. (2003) have discussed the rainfall daily patterns
on Northwestern South America using both a numerical
model (MM5) and satellite observations (GOES) for a 10
day period in 2000.
The aim of this paper is to discuss the lightning activity
present on the LMB, studying the daily and annual behavior of different variables. Due to the lack of enough quality
data on the basin, the Weather and Research Forecast system (WRF, see Skamarock et al., 2005) will be used, after
being evaluated in terms of its performance to represent
the mean observed vertical behavior of different thermody-
Figure 1: Lake Maracaibo Basin and surrounding orogra-
namic variables. A specific (diagnostic) question we want
phy. See section 2
to address here is: what is the role of the spatial and temporal atmospheric energy flux distribution in the observed
1). The first one is the Andes Range which is divided, in
lightning phenomenology? 1 . The rationale for this is to
Venezuela, into two well defined arms: the Perijá Range
study the available energetic states of the system in order
(extending along the Venezuela-Colombia border) and the
to further undertsand the observed phenomenology. Bear-
Venezuelan Andes (extending to the Northwest until dis-
ing these ideas in mind, the distribution of the paper is as
appearing in front of the Caribbean Sea). With mean
follows. Section 2 describes the main topographic features
heights between 1500 m and 3700 m, both arms encircle al-
of the region of interest, and section 3 briefly presents some
most completely the second phenomenon: the Maracaibo
general phenomenological aspects of the lightning activity
Lake, the biggest in South America. In the Southwestern
on the LMB. Then a general discussion about the total
quadrant of the MLB is placed the Catatumbo River, the
energy flux is presented in section 4 and section 5 contains
most important one for the Maracaibo Lake hydrodynam-
the WRF model configuration and evaluation. Finally,
ics and whose name is shared by both the surrounding
the general results are discussed in section 6 and the main
region and the lightning activity since the times of the
conclusions are included in section 7.
Spanish Colony.
The location of the MLB is affected by several global-
2. The Maracaibo Lake Basin
to-regional climatic phenomena, e.g. El Nio-Southern Oscillation (ENOS), the Inter Tropical Convergence Zone
The region of interest (8N-11N and 73W-70W) is dom-
(ITCZ) migrations, intrusions of high tropospheric troughs
inated by two important geographic phenomena (Figure
from Northern Hemisphere, synoptic perturbations, the
the near future it is possible to modify the present approach
trade winds and the Caribbean Low Level Jet (CLLJ) sea-
so the dry/moist static energy can be used instead the total energy.
sonal variability (Pulwarty et al., 1998; Velásquez, 2000;
1 In
2
Figure 2: LIS/OTD lightning time series for LMB, for
1995-2005
Amador, 2008). Locally, the wind circulation and convection activity on the MLB is defined by the orographic
configuration, the influence of the trade winds, the lake
Figure 4: Location of the lightning hotspots in the LMB,
breezes and occlusion between cold air masses coming from
using LIS data. Units are in f lashes/km2 /yr
the Andes and Perijá Cordillera, and warmer air masses
coming from the Lake and the Caribbean.
for the period 1998-2008, is 181 f lashes/km2 /year (Stock
et al., 2011), which suggest that LMB possesses the higher
3. Phenomenological Aspects and Observations
lightning activity place on the planet, in agreement with
The time series detected by LIS/OTD 2.5 Degree Low
the report of Albrecht et al. (2009). Stock et al. (2011) also
Resolution Time Series (LTRS, see Boccipio et al., 2002) of
mention, based on Centro de Modelado Cientı́fico (CMC)
annual f lashes/day in the region of interest between 1995
of Universidad del Zulia field campaign observations be-
and 2005 is shown in Figure 2. Using granule (high reso-
tween 2002 and 2010, that the most frequent electrical
lution) LIS data Stock et al. (2011) report that the annual
activity in the Catatumbo region is intra-cloud (IC). New
flash minima take place in January and maxima roughly
evidence using a Boltek StormTracker lightning detector
in September (see Figure 3, left panel). This behavior
located at CMC shows that approximately the 52% of
seems to be related with the intensity of the meridional
2011’s March-April discharges in the MLB are IC+, and
wind component (Figure 3, right panel): there is a max-
that maximum lightning activity tends to occur daily be-
imum for January-February (about -1 m/s) and a min-
tween 22 hr and 4 hr (local solar hours), in agreement with
imum between May and September (-0.1 to +0.2 m/s).
Mapes et al. (2003) analysis of the same phenomenon.
The LIS yearly climatological spatial distribution of lightning evidence two hotspots in the LMB, one between the
4. Total and Potential Energy Flux
Venezuela and Colombia border and the most intense one
located nearby the Catatumbo River mouth (see Figure
Here we discuss the total energy flux in a control volume
4), which is coincident with the location suggested by lo-
located in the LMB. The analysis will be used later in the
cal observers (Zavrotsky, 1991; Falcón et al., 2000) along
discussion of the diurnal and annual cycles, and the goal
the years.
is to study the different energy states available for the
The reported flash density for the Catatumbo hotspot
system. For the sake of simplicity we consider an inviscid
fluid.
(the so called Catatumbo Lightning), as measured by LIS
3
Figure 3: Flash mean frequency (left) for each Cartesian quadrant over the MLB (after Stock et al. (2011)) and meridional
winds climatology in m/s (right) using NOSA30k (Muñoz & Recalde, 2010).
Following Peixoto and Oort (2005), it is possible to show
The diabatic heating can be written as
that the total potential energy flux per mass unit is
d
(Φ + I) = gw − pα∇ · v + Q
dt
(1)
Q = Qh + Qf
(3)
Qh = −α∇ · Frad − L(e − c) − α∇Jcond
(4)
where
where v = (u, v, w) is the 3-velocity vector, with in general
non-zero zonal (u), meridional (v) and vertical (w) com-
that involves radiation, latent (e is evaporation, c stands
ponents, g is the gravity acceleration, Q is the diabatic
for condensation and L is the specific latent heating) and
heating, p is preassure and α is the inverse of the density
conduction heating, respectively. Qf is related to dissipa-
ρ.
tion by friction.
Similarly, the kinetic energy flux can be written as
d
K = −gw + pα∇ · v − α∇ · (pv + τ · v) + Qf
dt
We consider now a control volume V located, for the
sake of simplicity, over the Catatumbo region, with the lat-
(2)
eral boundaries far enough of the Perijá and the Venezue-
where τ is the stress tensor.
lan Andes mountain ranges and the bottom face of the
At equations (1) and (2) the terms gw and −pα∇ · v
volumne located over the Planetary Boundary Layer.
represent, respectively, the work done by the parcel against
Also, the upper face is chosen to be at the Equilibrium
the gravitational field and the compression work against
Level or a few hundreths meters above it.
the pressure field. The fact that they appear with opposite
the friction and surface-atmosphere stress termscan be ne-
signs in the equations means that these terms are related
glected in equations (1) and (2). Moreover, the caloric
to conversion between kinetic and potential energy. The
conduction term in equation (4) can also be neglected and
term −α∇ · (pv + τ · v) corresponds to the boundary work
we also drop the friction dissipation term Qf , because the
done by pressure and friction.
fluid is inviscid.
4
Therefore,
both the magnitude of the winds and the mass-energy ad-
Then, after a slight manipulation, we can write
Z vection. For simplicity, we consider constant mean vertical
1
∇ · pv + Fp · v + Qh dV
gw −
velocities, w = w = constant. Then, the following cases
ρ
V
V
(5)
Z
Z
arise:
d
KdV =
[−gw + Fp · v] dV
(6)
R
R
V dt
V
– I – V dV |pα∇ · v|
>
dV |Qh |
=⇒
V
R
2
where Fp ≡ − ∇p
is
the
force
per
mass
unit
related
to
d
ρ
dV dt2 (Φ + I) < 0: the potential energy
V
changes in pressure.
in the volume decreases non-linearly. Depending
It follows that the potential energy increases in the conon the original amount of the potential energy,
trol volume if the latent, sensible and radiative heats inin this case some convection cells may be present
crease, and/or if the vertical velocity increases. Nonetheand produce precipitation and lightning. Howless, it decreases if the pressure and wind fields increase
ever we claim that they will not occur for large
in the boundaries, because they involve an increase in the
extensions of space or time.
R
R
total external pressure over the volumen (a compression ef– II – V dV |pα∇ · v|
=
dV |Qh |
=⇒
V
R
fect), and therefore a work against the pressure field take
2
d
this is an equilibrium
dV dt
2 (Φ + I) = 0:
V
place to ballance this effect. The kinetic energy, on the
case between advection and heating. The poother hand, increases with the same wind and pressure
tential energy flux remains constant, but the
fields at the boundary, but decreases with the appearece
potential energy may linearly increase, be conof work done against the gravitational field.
stant or linearly decrease depending on the value
Now we claim that equation (5) suggests an important
of w, and it will be equal to g∆z(t), where ∆z(t)
conclusion in terms of occurrence of convective cells in the
is the mean vertical displacement.
control volume. The incident wind field on the boundaries
R
R
dV |Qh |
=⇒
<
– III – V dV |pα∇ · v|
V
has two different effects on the energetic of the system.
R
2
d
> 0: the potential energy
dV dt
2 (Φ + I)
V
On one hand, it decreases the potential energy flux via the
flux increases with time due to mass advection
term in parethesis in equation (5), as explained in the prefrom the boundaries and the potential energy
vious paragraph. On the other hand, it may also increases
increases non-linearly. We claim that there is
the potential energy flux via the term −L(e − c) present in
a greater probability of finding convective cells
Qh , due to the mass-energy advection into de system. and
in the control volume, with greater spacial and
considering that in general in the Tropic and also in very
temporal extension and vertical development
humid zones the total condensation is greater than the tothan in the previous cases.
tal evaporation; in addition |α∇ · Frad | < |L(e − c)|, so
Z
d
(Φ + I) dV =
dt
Qh > 0. (Given that the kinetic energy in general can
It is obvious that the original amount of potential en-
be neglected with respect to the potential energy (< 5%
ergy in the volume is of key importance for this analy-
of the total energy) we will not further consider it in our
sis. The system will go from one state to another through
analysis).
the change of the flux of potential energy. It is imme-
We now proceed to study different cases of interest,
diate to note that the following especial subcases of in-
comparing the work done against the pressure field at
the boundaries (pα∇ · v) and the heating term (Qh =
terest may arise when the advection is null or negligible,
R
dV |pα∇ · v| ∼
= 0 and therefore the system is nor doV
−α∇ · Frad + L(c − e)). In principle this will depend on
ing any work against the pressure field neither ingesting
5
humidity from the boundaries:
to a post-processing of the NOSA30k dataset produced by
the Andean Observatory and CMC (Muñoz et al., 2010;
a) Qh < 0 =⇒ L(c − e) < 0: evaporation is greater than
Muñoz & Recalde, 2010).
condensation and therefore as time passes the total hu-
Both simulations use the same geographical domain: be-
midity content in the control volume decreases. The
tween 8 and 11 degrees latitude and -73 to -70 degrees lon-
clouds tend to desappear and a clear atmosphere oc-
gitude, and where forced with the NCEP NCAR Reanal-
curs with time.
ysis Project -NNRP- (Kistler et al., 2001). In addition,
b) Qh = 0 =⇒ |α∇ · Frad | = |L(c − e)|: this is the
for the sake of comparison, the same set of physical pa-
convective-radiative equilibrium case. We have w = 0
rameterizations were used, which includes Kessler scheme
and the system doesn’t ingest humidity. The balance
for microphysics and RRTM and Dudhia schemes for long-
between convection and radiation constraints the pre-
wave and shortwave radiation, respectively. The Monin-
cipitation and lightning activity.
Obukhov (Janjic) scheme for surface-layer, and thermal
c) Qh > 0 =⇒ L(c − e) > 0: condensation is greater than
difussion with 5 soil levels for land-surface physics were
evaporation, so there is a high probability of cloud pro-
also selected. The Mellor-Yamada-Janjic TKE scheme was
duction. Precipitation and lightning activity are con-
employed for the boundary-layer option, and the SST was
strained to the available humidity and potential energy
updated daily.
in the control volume.
The model results are discussed in the next section, but
In our analysis of the LMB diurnal and annual cy-
before we think it is important to evaluate the WRF rep-
cles, we will use the Convective Available Potential En-
resentation of certain key variables on the LMB.
ergy (CAPE) as a proxy of the potential energy studied
here. The rationale is that we expect them to be directly
5.1. Model Performance
proportional and the numerical model and the (observed)
To evaluate the model, radiosonde observations from La
Skew-T plots provide in a very easy way the CAPE, while
Cañada station (10 31’ 0” N, 71 39’ 0” W, 13 meters above
the potential energy flux involves further computations.
sea level) and surface data from Base Aérea Rafael Urdaneta -BARU- station (10 33’ 30” N, 71 43’ 40” W, 76
5. Numerical Simulations
meters above sea level) are contrasted with the simulated
In order to study the diurnal and annual cycles, we use
variables. This comparison is done on a more qualitative
a numerical model. This is due to the fact that observed
basis as the time periods for the observations and the sim-
vertical profiles are very scarce (availability exists for 18
ulations are not the same. By ’qualitative’ we mean that
months between 1983 and 1984) in the LMB, and only
the average monthly behavior is computed for the avail-
for 12Z. Two different datasets are employed, both using
able variables and then these means are compared. We are
the Weather Research and Forecasting -WRF- model (Ska-
aware of the limitations of this approach.
marock et al., 2005): the first one is a 4 km resolution sim-
For the surface variables (relative humidity, pressure
ulation from 01 Jan 2012 to 31 Jan 2012 with output every
and temperature), simulations do agree with the general
3 hours, aimed to study the daily variability; the second
observed behavior for the BARU station. The skew-T
one is a 30 km resolution simulation over the whole 1998-
plots computed from the soundings at La Cañada station
2008 period, with a total of 132 monthly outputs, aimed
and the WRF simulation at the same point also qualita-
to study the annual cycle. The latter dataset corresponds
tively agree in general (see Figure 5), for both January
6
Figure 5: Climatological Skew-T plot for La Cañada station at 12Z, from observations (left) and WRF model output
(right) and for January (in blue) and September (in red). The solid line corresponds to the parcel temperature, while
the dashed line to the dewpoint temperature. The gray line refers to the US standard atmosphere.
6. Discussion
and September. However, we note that the wind profiles doesn’t match pretty well. In addition, the obtained
Time series for the 1998-2005 period have been ob-
CAPE values agrees for September (observed: 2870J/kg,
tained following a spatial average from the domain to rep-
simulated: 2400J/kg; percentage error: −16%), but not
resent the evolution of CAPE and other variables in the
for January (observed: 1570J/kg, simulated: 400J/kg;
basin. Observing the superposition of the flash density and
percentage error: −74%). We claim that the discrepancies
CAPE evolutions (Figure 6) a relationship is clearly seen
are due to the scarcity of useful observed values for Jan-
(p=0.01 statistically significant Spearman correlation of
uary and the different periods (only a few days possessed
0.787). Both means present a regularly temporal behavior
data) considered for the computation of the statistics. The
similarly through the year although there are differences
WRF output makes sense for January as it corresponds to
in the peak values for different years. These peaks occur
the yearly’s driest month on LMB, and therefore a low
in the third quarter of every year and the lowest values ap-
CAPE is to be expected.
pear at the beginning of the years. It is important to bear
in mind that the values are actually smaller than the ones
present in certain places of the LMB because the points
We conclude that WRF is in general simulating qualitatively well the behavior of the variables of interest in
have been further zonally and meriodionally averaged.
the LMB, and therefore the model outputs will be used in
It can also be seen that the highest peaks are found in
what follows to discuss the daily and annual behavior of
2001 and 2005. Nonetheless, the area under the curve of
CAPE and its relationship to lightning ocurrence on the
flashes is higher for year 2005 which makes it the most
basin.
active in the period. Also, CAPE has its highest peak in
7
Figure 6:
Lightning density (dashed line, units in
2
f lashes/km /day, see left vertical axis) and CAPE (solid
line, units in J/kg, see right vertical axis) time series
Figure 7:
Lightning density (dashed line, units in
2
f lashes/km /day, see left vertical axis) and meridional
2005 which contrasts with year 2001 where mean CAPE
winds (dotted line, units in m/s, see right vertical axis)
has its lowest peak in the whole period. This result sug-
time series
gests that other variables may be needed to explain flash
density activity that year.
From NOSA30k dataset, it has also been possible to
calculate monthly means and the CAPE climatology and
surface (i.e. 10 m) winds for the 1998-2008 period. The
monthly means show that the maximum CAPE computed
value in the 132 months of the 1998-2008 period appears
in September 2005, having reached 2427 J kg −1 . The wind
behavior along the year seems to be dominated by the
meridional component. There is a (p=0.01) statistically
significant Spearman correlation of -0.533 between flash
density and meriodional winds, but no statistically significant correlation for the zonal component was found.
From Figure 7 the meridional winds annual unimodal pattern is clearly observed. It is important to note that 2005
was a historic year concerning hurricane activity in the
Caribbean and it may have implications on the low meridional winds (and also the zonal ones) evidenced in the
basin, as is shown in the series. This issue may require
further research.
6.1. Annual Cycle
Figure 8: 1998-2008 CAPE (J/kg) climatology for January
The first thing to notice is that the first two months
of the year have the lowest levels of CAPE and the highest levels of wind velocity as opposed from the August,
8
Figure 11: Same as figure 10 but for September.
Figure 9: Same as figure 8 but for September.
September and October have the highest levels of CAPE
and the lowest in wind velocity. CAPE has the biggest
magnitudes over the LMB’s low lands and over Lake Maracaibo.
There is also a region of high convection and
weak winds over the Ciénagas de Juan Manuel de Aguas
Claras y Aguas Negras National Park (CNP), whose center is roughly located at 9.25N and 72.25W (see Figure
1).
Figures 8 to 11 present the January and Septem-
ber CAPE, precipitation and wind circulation from the
NOSA30k dataset.
Trade winds come from the north and enter the basin
following a counterclockwise course due to the orographic
configuration.
During September, winds penetrate the
basin with energies ranging from 4 to 9 J kg −1 and these
decrease to magnitudes of less than 0.5 J kg −1 upon arrival
at the CNP. In February, trade winds enter the basin with
bigger energies (12 to 18 J kg −1 ) and finally lose most of
it but having more than 1 J kg −1 . The wind behaviour
Figure 10: 1998-2008 precipitation (mm) and wind clima-
suggests an advection of humidity from the Caribbean Sea
tology (m/s) for January
into the basin towards the southwestern part of the lake
9
and, in the process, the sweep of lake-generated humidity.
6.2. Diurnal cycle
This is the pattern throughout the year and this is partly
The atmospheric diurnal variability in the LMB is ruled
why the CNP, located in the zone, is mostly a marsh-
by thermal inercia. Key daily events obtained from the
land. Largest mean climatological numbers for CAPE oc-
high resolution numerical simulations are presented in Ta-
cur in September and reach values of 2200 − 2400 J kg −1 .
ble 1.
Minimum peak values occur in January where CAPE tops
The simulations reveal that the LMB possesses at mid-
300 − 400 J kg −1 . These peak values are found to appear
night the higher amount of relative humidity of the entire
over Lake Maracaibo and the CNP marshlands. These re-
diurnal cycle. At this moment, the Lake breeze is oriented
gions are the most active in terms of lightning discharges
towards the center of the lake, transporting the available
in the domain (Albrecht et al., 2009; Stock et al., 2011).
humidity from the surrounding montains. Using again the
From a diagnostic point of view, we can use the cases
energetic cases dicussed in Section 4, it is possible to see
discussed in Section 4 to describe the annual lightning be-
that even when the advection is not negligible, the latent
havior in LMB. We claim that in January and February
heat term prevails (Case III), and condensation takes place
the Case I prevails: the winds are more intense than the
in different regions of the basin, facilitating there the oc-
rest of the year but there is not enough advection of hu-
currence of precipitation and maybe also lightning.
midity, and the radiation for this season is a yearly min-
As the night evolves, the increasing winds coming from
imum; therefore the heating term is not as important as
the mountainrange and directed towards the lake will tend
the work done against the pressure field, and the potential
to balance the winds coming from the Caribbean, in the
energy decreases non-linearly, causing less convection and
North of the LMB. This means that the advection from the
lightning than in other periods of the year.
Caribbean is decreasing gradually. Around 04.30h (local
The following months evidence an increase in the total
time, from now on) the winds from the Caribbean can’t
heating, but also a decrease of the winds at the LMB, with
enter the basin (Figure 12) and are directed now west-
the yearly minimum for the meridional component (which
wards, even when in other places of the Caribbean coast
is the one who possesses a statistically significant correla-
of both Venezuela and Colombia are directed southwards.
tion with lightning) taking place in July but the stronger
The process is similar to the formation of a ’wind gate’ in
northwards winds happening in September. We interpret
the North of the LMB. In terms of the energetic discus-
this as a transition from the Case I at the beginning of the
sion, this period can be associated to Case II. The same
year to the Case II between April and July, and then to the
scenario remains until a few hours after dawn. Then the
Case III for August and September (see the corresponding
temperatures in the basin begin to be more homogeneous.
cases in Section 4). After October the system may return
Around 10.30h the wind velocities present the daily min-
to the Case II before beginning a new year in the Case I.
ima, and the heating is still not dominant (Case II-a).
From this analysis, the period with maximum CAPE
The latent heat values and the winds are small, and there-
and lightning activity will be August-September, which
fore there will be not significant increases of potential en-
coincides with the numerical simulations discussed in this
ergy flux, tending to inhibit atmospheric instabilities in
work (see Figures 6 and 9). We also note that this period
the basin at this period.
corresponds to the Caribbean Low Level Jet (CLLJ) sea-
As the heating continues by the effect of the solar ra-
sonal weakening Amador (2008), a relation that may need
diation, the land becomes hotter than the lake, and the
further study in the future.
temperature gradient finally changes direction and the lake
10
Table 1: Some daily key events in Lake Maracaibo Basin, in Venezuelan Time (VET) and Universal Time (Z).
VET
Z (GMT)
Event
Maximum advection from the mountainranges to the LMB. In the North, zonal winds
04.30
09
inhibit advection from the Caribbean to LMB (’wind gate’ stablished).
The surface temperature is homogeneous in the LMB.
10.30
15
Winds at minimum.
Begin of lake breezes
13.30
(Lake → coasts)
18
Maximum of lake breezes. Northern ’wind gate’ is open.
16.30
21
Advection from the Caribbean.
Lake breezes inversion
19.30
(Coasts → lake)
00
Surface temperature minimum in the coasts. Intense
01.30
06
breezes towards the Lake
breeze begin to blow towards the coasts (13.30h). This can
of Mapes et al. (2003) and Albrecht et al. (2009).
be associated to the energetic Case I, taking place when
the work done by the winds is dominating in the LMB.
7. Concluding Remarks
In the meantime, the displacement of mass from the lake
to the coasts has been enabling the advection from the
The lightning activity in the Lake Maracaibo Basin has
Caribbean to be more and more important, being maxi-
been studied in this paper. As it has recently been re-
mum around 16.30h (see Figure 12). This is the opposite
ported by two different authors (Stock et al., 2011; Al-
to the 04.30h case: now the ’wind gate’ at the North of
brecht et al., 2009), Northwestern South America possesses
the basin is completely open.
the highest flash rate in the planet, and specifically it is
located in this basin. The Catatumbo Lightnings, as they
With the decrease of the incident radiation over the
are locally known, present a mean modal behavior in the
basin, the magnitude of the winds directed towards the
flash density variability along the year, with a minimum
coast also decreases and the meridional transport from the
in January and a maximum in September, with mean val-
Caribbean provides humid air to the lake, facilitating the
ues of 181 flashes/km2 /year (Stock et al., 2011). From a
formation of convection. We identify this situation with
general perspective, the convection processes are modu-
the Case III. After 19.30 the winds in the basin are again
lated in the region by the effect of the ITCZ migrations,
directed towards the lake and cold air masses from the
the trade winds, the CLLJ and local topography (Stock
mountains may provide favorable conditions for occluded
et al., 2011). In particular, the meridional wind inten-
fronts, precipitation and lighting activity. This situation
sity in the LMB seems to play an important role in the
usually last until around 04.30h, following the observations
lightning formation, with an inverse correlation: high hur11
Figure 12: WRF’s (4 km) daily surface (10 m) winds in the LMB. Notice the maximum advection from the Caribbean
at 16.30 VET (21Z, left panel) and its minimum at 04.30 VET (9Z, right panel). The winds in the north behave as a
gate for the basin, which inhibits/permits the mass and energy transport into the region of interest.
ricane activity years (e.g. 2005) provide a mean decrease
and therefore a higher probability of lightning occurrence
of southwards wind velocities along the Venezuelan coasts,
are August-September (in the yearly cycle) and especially
enabling suitable conditions to convection activity in the
between 16.30h and 04.30h (local time, in the daily cy-
LMB Southwestern region.
cle). January exhibits the yearly minimal amount of both
CAPE and lightning activity.
After comparing numerical simulations with available
A future study on the matter may consider more closely
observations, we found that the model outputs could be
the role of the dry/wet static energy on the formal com-
used to adequately describe the behaviour of certain vari-
putation of the different energy flux terms, and a possible
ables (e.g. CAPE, winds) in the basin. Considering CAPE
way to forecast the spatial probability of lightning occur-
as a proxy of the total potential energy in the basin, we
rence in the basin, as a tool for building an early warning
showed under certain approximations that its flux can be
system for saving human lifes and properties related to
written as a balance between the work done by the winds
the electrical and oil systems, two of the more affected
against the pressure field in the boundaries of a control
economic sectors in the basin.
parcel and the mass-energy advection provided by the
same winds. We used this idea to describe, from a di-
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