Transcript Document

Contrasting Tropical Rainfall Regimes Using TRMM and Ground-Based Polarimetric Radar
Steven A. Rutledge, Robert Cifelli, Timothy J. Lang
Colorado State University, Fort Collins
Steven W. Nesbitt
University of Illinois at Urbana-Champaign
1. Introduction
2. NAME (July-August 2004)
The TRMM satellite has provided unprecedented data for over 10 years.
TRMM precipitation products have advanced our understanding of tropical
precipitation considerably. There are many studies underway that seek to
refine the precipitation products from the TRMM Precipitation Radar (PR).
In this work we build on previously identified influences on tropical
precipitation in Amazonia and in Mexico, the latter associated with the N.
American Monsoon system.
L2
N1
L1
S-Pol Near-Sfc D0 via D0 = 1.529*(ZDR)0.467
S-Pol Near-Sfc D0 via D0 = 1.529*(ZDR)0.467
We used polarimetric radar data from two field projects, TRMM-LBA and
NAME (North American Monsoon Experiment), to characterize the physical
nature of precipitation in these regions. We contrasted these structures with
10-yr statistics from the PR. It will be demonstrated that using regimespecific Z-R estimators could lead to improved rainfall estimates derived
from the PR. In addition, the results demonstrate the possibility of
uncorrected PR attenuation in heavy rainfall events.
Precipitation variability
as a function of
meteorological regime
N2
S-Pol Liquid Water Mass*
Precipitation in Amazonia is strongly influenced by pronounced reversals in
the low-level flow regime (easterly and westerly regimes, or continental-like
vs. maritime-like) (I1, I2). In Mexico, precipitation characteristics vary
markedly with terrain (I3), ranging from frequent showery rain over the high
terrain of the Sierra Madre Occidental to more persistent, heavier rain over
the coastal plains (I4). An even broader contrast exists between land-based
convection and convection over the adjacent Gulf of California.
TRMM-LBA
3. TRMM-LBA (January-February 1999)
S-Pol Liquid & Ice Water Mass*
S-Pol Ice Water Mass*
N4
N3
L4
Height (km)
L3
East
West
NAME
S-Pol reflectivity cumulative frequency distributions
and mean profiles
Precipitation
variability as a
function of terrain
Frequency Distribution of S-Pol Rain Rate
S-Pol a in Z=aR1.5**
Over the water, S-Pol observed a lower frequency of large median volume diameters (D0) than over
land (N1). This was associated with less ice mass aloft (N2). However, liquid water mass was large,
suggesting the relative importance of warm rain processes over the Gulf. Over land, the lowest
elevations had the highest frequencies of large D0 and more ice mass, and these tended to decrease
with increasing elevation. High rainfall rates were more frequent over the low terrain and water (N3)
This was consistent with upscale growth of convection toward lower elevations (Lang et al. 2007).
I1. Easterly and westerly
I3. Terrain map for the NAME
regimes in TRMM-LBA, showing
higher lightning flash rates in the
east regime compared to the
west regime. Mean CAPE was
higher in the east regime
compared to the west regime.
domain. Adapted from Lang et al.
(2007).
The net result was increasing a in Z=aR1.5 from water to high elevations (N4). But this was not
observed by TRMM (N5, N6), which showed an opposite trend, except for the ocean/gulf region.
However, TRMM found the over-water a to be larger near the coast, so this average was misleading.
TRMM conditional rain rate was found to decrease with elevation, which was completely inconsistent
with S-Pol. Given the more frequent heavy S-Pol rain rates at low elevations, how well did the TRMM
attenuation correction handle these extreme events? Reflectivity intercomparisons between TRMM
and S-Pol showed good agreement for a handful of available overpasses (Lang et al. 2008), but
these did not include strong events. TRMM did observe more vertically intense echoes over the lowelevation land (N7), like S-Pol. In addition, TRMM’s ocean/gulf CFAD was consistent with the S-Pol’s
inference of smaller drop sizes (i.e., lower reflectivities despite heavy rainfall) over the water.
N5
TRMM PR Statistics
July-August 1998-2007
> 1500 m 500-1500 m < 500 m Ocean/Gulf
Mean a
a
I2. Diurnal cycle of rainfall
I4. Diurnal cycle of convective rain
for the easterly and westerly
regimes during TRMM-LBA.
rate for the NAME domain. Results
are plotted as a function of elevation
band and over water.
Contact Info: Steven A. Rutledge, CSU Atmospheric Science,
Ft Collins, CO 80523; (970) 491-8283, [email protected]
This research is supported by NASA PMM Grant NNX07AD51G and NSF Grant
ATM-0733396. Larry Carey contributed some of the LBA plots.
173.297
185.151
208.575
176.801
20.216
28.721
49.254
45.943
Mean b
1.5379
1.5421
1.5474
1.5527
b
0.0044
0.0055
0.0085
0.0129
Mean Unc
R (mm/hr)
 Unc R
0.146
0.138
0.116
N6
TRMM PR
(JA 98-07)
a in Z=aRb
Domain:
10x10 deg box
Center at S-Pol
0.0410
N7
0.089
0.111
0.104
0.058
Mean Cond 9.37
R (mm/hr)
9.05
8.56
7.38
 Cond R
2.24
2.98
3.30
5.58
Mean Freq 0.015
0.014
0.013
0.005
 R Freq
0.009
0.010
0.007
0.005
TRMM PR
(JA 98-07)
Reflectivity
CFADs
S-Pol a in Z=aR1.5**
Easterly regime convective precipitation was characterized by larger D0 compared to the
westerly regime (L1). Easterly regime convective precipitation also contained larger liquid and
ice water mass compared to the westerly regime (L2), consistent with the more frequent
occurrence of vertically intense reflectivities in the easterly regime (L3), and heavier rainfall
overall.
The larger drops observed in the easterly regime were manifested in a slightly higher “a”
coefficient for the Z=aR1.5 relation in convective precipitation (L4). This indicated that different ZRs were required to accurately estimate the rainfall in each regime. TRMM did capture this
relative variability in a between regimes (L5), although TRMM’s mode was shifted toward larger
values (L6). However, TRMM conditional rain rates were completely at odds with the S-Pol
results, and the reflectivity CFADs showed little difference between easterly and westerly
regimes (L7). As in NAME, TRMM PR attenuation in extreme rainfall events may have played a
role in biasing the easterly regime results low.
L5
TRMM PR Statistics
DJF 1998-2007
L6
>|0.5 m/s| wind
criteria
Easterly
Westerly
No Regime
number of days
193
536
163
number of
convective pixels
31041
104741
29568
number of sampled 2112463
pixels
5666049
1799237
mean a in Z=aR^b
175.0
170.9
170.1
σ(a)
33.6
18.5
27.4
mean b in Z=aR^b
1.546
1.546
1.546
σ(b)
.007
.005
.007
mean convective
rain rate
2.96
3.59
3.16
mean convective
7.50
conditional rain rate
7.89
7.14
convective
precipitation
frequency
0.007
0.013
0.011
800 hPa winds derived from NCEP/NCAR reanalysis
TRMM used 7.5x7.5 deg box centered on S-Pol
* Methodology described in Cifelli et al. (2002)
** Methodology described in Bringi et al. (2004)
TRMM PR (DJF 98-07)
a and b in Z=aRb
L7
TRMM PR
(DJF 98-07)
Reflectivity
CFADs