Download Sources of Precipitation Over Equatorial Central Africa

SOURCES OF PRECIPITATION
OVER
EQUATORIAL
CENTRAL
AFRICA
1
1
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2
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Owen Collins , Ellen Dyer , Dylan Jones , David Noone , Jesse Nusbaumer
1University
of Toronto, 2University of Colorado Boulder
3. Meteorology
1. Introduction
Equatorial Central Africa (hereafter referred to as
ECA) is a major global convective region that plays a
large role in the global circulation. Additionally, ECA is
climatically important due to its extensive rainforest;
second largest in the world after the Amazon. Many of
the farmers in the area depend on the high natural
rainfall and do not have any other form of irrigation.
ECA is a chronically understudied region. This is
compounded by the lack of gauge data in the region.
Climate models show a wide range of variance in
precipitation over the region, differing by as much as a
factor of 3.
We present here a study of the factors driving
precipitation over ECA, with a particular focus on
March, April, & May (hereafter referred to as MAM).
The ECA region spans from 5°S to 5°N, & 12.5E° to 30°E. This is
the same region used in many papers.
2. Tools & Techniques
Python was used with NumPy & Matplotlib to process data and
create plots
Data Sources:
ECMWF (European Centre for Medium-range Weather Forecasts):
ERA Interim reanalysis data, which is based on real-world
observations that have been combined with a climate model (that
includes atmosphere, land surface, ocean, sea ice, and the carbon
cycle processes). Earliest ERA Interim data is 1979. It is a continuation
of the older ERA-40 dataset. Contains a large number of atmospheric
variables. All of plots, except for those from the watertagging, use
this data.
MAM precipitation is centered around the equator
(location of the ITCZ at this time). Dominant winds are
southeasterly below the equator, and northeasterly
above the equator (trade winds).
In MAM (left) convergence and uplifting (ITCZ) clearly occurs in ECA
(between 5°S & 5°N). Compare to JJA (top-right) and DJF (bottom-right),
where the ITCZ is north and south of ECA, respectively. These winds were
averaged from 12.5°E to 30°E.
Seasonal cycle of evaporation/precipitation.
Precipitation exceeds evaporation in almost all
months, suggesting that non-local moisture sources
are crucial. Note the MAM rainfall maximum.
4. Moisture Flux & Divergence
Isocam CESM 1.2:
A new version of the CESM model that has the capability to
tag water vapour from the surface in user-defined regions and
track where the vapour from those regions gets transported
and precipitated. This data set is used for all of the
watertagging plots. Earliest data is 1980. Both CESM models
were run by Ellen Dyer.
FAMIP CESM 1.0.4:
Uses active atmosphere and land models with data ocean.
Shows similarities with isocam data, however it has different
cloud cover and convection schemes. Earliest data is 1940.
CESM (Community Earth System Model):
A climate model that uses pre-defined sea surface temperatures
(data ocean).
F /  
P0

0
F 
qV / 
g
dp
  F dl    F dl
A
(1)
( 2)
Equation 1 is used to quantify
moisture flux through a column of
air. Specific humidity (q) is
multiplied by meridional (ϕ) or
zonal (λ) wind velocity (V). It is
integrated across a vertical column
assuming hydrostatic equilibrium.
Equation 2 integrates the net fluxes
along the boundaries of a region,
then divides by the area to
calculate divergence (Divergence
Theorem).
Moisture flux is dominated by southeasterly systems, especially
from the western Indian Ocean. ECA is generally convergent. Note
the correlation between convergence and precipitation.
Throughout MAM, the net flux tends to be convergent at lower levels, and divergent at higher
levels, as the Walker & Hadley Cells would suggest.
Meridional net flux is convergent at a higher level, which approximately corresponds to the
African Easterly Jet (AEJ). This can be explained as convergence between the northern &
southern AEJs. The northern & southern AEJs can be seen as two cores in the net zonal plot.
5. Watertagging: CESM 1.2
Moisture source regions.
Moisture recycling ratio of the areas surrounding ECA. This is the ratio of
precipitation from the DRC region to total precipitation. Southern ECA was
seen to have more moisture convergence ∴ less local moisture sources.
Relative contributions of each region’s evaporated
moisture to MAM ECA rainfall.
Moisture flux is convergent (negative) all year. Minima in
divergence align with maxima in precipitation. The MAM
minima is largely brought about by meridional convergence.
Net vertical flux of moisture. The ITCZ is clearly visible as a zone of upward
movement.
6. Conclusions
• Quantitative results have been examined for March-April-May
precipitation, winds, moisture flux, and watertagged precipitation.
MAM is one of two yearly precipitation maxima that coincides
with the passing of the ITCZ through the region.
• Moisture flux is mainly easterly, and is convergent over ECA during
MAM. The largest source of this is mid-level meridional
convergence, likely caused by the AEJ-north and AEJ-south.
• Watertagging results suggest that East Africa and the western
Indian Ocean are the most important non-local sources of
moisture. These sources experience significant interannual
variability during MAM, which is related to the Indian Ocean
Dipole.
Monthly precipitation cycle, 1980-2001, by source. DRC, Ind4, and EAFS are the
most important sources in MAM.
Overall, differences are positive, except for ARHL & Other. Regions to the east of
ECA have the largest positive difference.
Yearly changes in MAM rainfall. A 3-4 year cycle is visible in many of the sources to the east of ECA, such
as Eastern African (EAFS) & the Indian Ocean (Ind) sources. This may be caused by the Indian Ocean
Dipole, a fluctuation in the east-west sea surface temperatures. An index of the Indian Ocean Dipole is
provided (IOD, dotted line).
• To improve understanding of the precipitation and climate in the
region, more observational data is needed. The number of
reporting weather stations in the region must be increased.
Radiosondes and AMDAR could be a suitable short-term solution.