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Timing of Convection Initiation During the
WETANC-LBA
Luis Gustavo de Paiva Pereira Maria Assunção Faus
da Silva Dias Augusto José Pereira Filho Paulo
Takeshi Matsuo
Abstract The impact of deforestation has been
of interest to scientists around the world. Great
interest arises from interactions between
vegetation and atmosphere, and how that may
affect the growth of Cumulus clouds and the
distribution of the precipitation in the Amazon
Basin. A study published by Cutrim et. al. (1995)
has shown that in Rondonia, during the dry season
July October), there is a bias of Cumulus
growth over deforested areas, specially along
highway BR-364. This study intends to identify
the average time of convection initiation in
Rondonia during the wet season. In this analysis,
images of the S-pol radar obtained during the
WETAMC-LBA, as well as a vegetation map were
used. The images of the radar show that the
first shallow Cumulus were formed over forest
areas. This signature was observed on 100 of the
convective days analyzed in this study,
indicating a completely different pattern from
that observed during the dry season. In addition,
some evidences point out to a tendency of clouds
to acquire higher reflectivity values in
areas where the forest neighbors pasture
areas. Although interface areas accounted
for about 40 of the cases where the first
convective cell of he day began to form, the
observation of the development of Cumulus clouds
show that the deeper clouds formed over the
forest interface areas.
Timing of Convection Initiation
Location of Convection Initiation
The S-pol radar images, available,
from January 16th to February
27th, were pre-selected, eliminating the
days where stratiform clouds were masking the
correct time of convection
initiation. The convection was considered
initiated when the pixels reached the 15 DBz
threshold. The average time for the
convection initiation, during the the 30
days analyzed, was 1403 UTC, with standard
deviation of 1 hour and 4 minutes. As it can
be seen in the histogram, in the majority of
the days the convection initiated
between 1331 and 1435 UTC (or 931 and
1035 local time).
Observing the first convective cell of
the day, it could be noticed that these
cells were formed over forest regions in 100
of the cases analyzed. On the left, there
is a vegetation map on Rondonia,
and on the right the number of cases
where the first convective cell of the day
formed in each of the sectors illustrated.
The letter S, represents the center of the
S-pol radar.
Interface Regions
Conclusions
The convection initiated over forest on
100 of the cases analyzed. Cutrim et. al
(1995), analyzing satellite images of
Rondonia State found evidences that there
was a significant increase of shallow
Cumulus over deforested regions during the
dry season (July to October), specially along
highway BR-364. On he other hand, the
results obtained in this study show that
there is a significant different situation during
the wet season. The location and time of
convection initiation represent an important
element and for numerical models for
atmospheric simulations. If this
conclusion is verified, this result should be
considered in the physical parametrizations of
the surface processes in the Planetary
Boundary Layer and Cumulus
formation. Interface regions, where forest
neighbors pasture, showed that the clouds were
growing deeper, due the higher reflectivity
values observed in these locations.
Analyzing the location where the first
convective cells were formed, two
interesting patterns could be observed first,
in all cases the first cell of the day formed
over forest regions as it can be seen on the
graphic above second, in 40 of the cases, the
first cell of the day formed over regions
of forest neighboring pasture
areas. Furthermore, it was also observed that
the cells that formed over forest
interface areas acquired higher
reflectivity values. On the figures beside,
the squares in the bottom vegetation maps
indicate interface regions.
This research is sponsored by FAPESP (Fundação
de Amparo a Pesquisa do Estado de São
Paulo)