Bringi, V. N.

Gatlin, Patrick N.

Petersen, Walter A.

Thurai, Merhala

Wingo, Matthew

NASA Technical Reports Server

A new perspective on the 
raindrop size distribution and its 
implications for retrievals of 
light rain
Patrick N. Gatlin, NASA Marshall Space Flight Center
Merhala Thurai, Colorado State University
Walter A. Petersen, NASA Marshall Space Flight Center
V. N. Bringi, Colorado State University
Matthew Wingo, University of Alabama in Huntsville
1
American Geophysical Union Fall Meeting 2017 
New Orleans, December 13th
H33J-03
https://ntrs.nasa.gov/search.jsp?R=20170012452 2019-08-30T16:47:30+00:00Z
Why are we doing this?
• GPM retrievals in light rain
• Minimum detectable by DPR:  13 dBz (Ku) & 12 dBz (Ka-HS)
 0.5 and 0.2 mm/hr (Hou et al. 2014; Toyoshima et al. 2015) 
 43.1% and 11.3% of precipitation events (Lin and Hou 2012)
• Minimum detectable by GMI:  0.17 mm/hr (ocean) 0.38 mm/hr (land)  
(Munchak and Skofronick-Jackson 2013)
• Impact of small drops on DSD parameters 
• Dm significantly lower at light rain rates
• Nw significantly larger
• Disdrometer size range limitations cause problems for the gamma 
model to describe the DSD at both tails of the raindrop size spectrum
2
(based on a few cases presented 
in Thurai et al. 2017, JAMC)
Experimental Setup
Easton, Colorado (near CSU-CHILL) Huntsville, Alabama
MPS 2DVD
Parsivel2
Climate:  Semi-arid (Köppen–Trewartha)
April-October 2015
13 rainfall days (documented)
Climate:  Humid subtropical (Köppen–Trewartha)
Number of DSD minutes:  7,692
March 2016-June 2017 (ongoing)
41 rainfall days
3
Combining the MPS and 2DVD
• Better resolve the tails of the 
DSD, especially the small drops
• Good agreement between MPS 
and 2DVD in overlap region
• Combined DSD:  
• MPS: 0.1 ≤ D ≤ 1.2mm
• 2DVD: D>1.2mm
150 mm/hr
2 mm/hr
2DVD: D > 1.2mmMPS: 0.1 ≤ D ≤ 1.2mm
2016-07-31 Huntsville
4
Parameter MPS 2DVD
Horizontal Resolution 50 µm 170 µm
Measuring Area 20 x 3.1 cm 10 x 10 cm
Size range 50 µm-3.1 mm > 0.6 mm
Summary of Rainfall events from Huntsville
5
Huntsville, Alabama DSD parameters
Dm for combined MPS+2DVD is 
smaller than 2DVD alone
Nw for combined MPS+2DVD is 
larger than 2DVD alone
MPS+2DVD
2DVD
6
Dm Nw
Small drops add more water to the DSD
Not much difference in 
rainfall rate except at low 
intensity (COMB.>2DVD)
MPS+2DVD has 
larger LWC
7
What happens to Dm at light rainfall rates?
8
At light rainfall rates, the 
MPS+2DVD Dm is much 
smaller than that from 
the 2DVD
17 April 2016:  Easton, Colorado
How does it affect Nw- Dm variability?
Rainfall Rate [mm/hr]
9 Less variability when small drops are included
Loop with Nw-Dm from 2DVD and MPS+2DVD; 
synchronized with CHILL RHIs of dBZ and LDR, 
1824 – 1924 UT. Every 6 minutes.
Note the jump over the set of lines during 
the passage of the convective storm at 
around 18:55 UTC. 
Also a weaker one at 18:40 where some points 
lie on the lines.
(Animations courtesy Pat Kennedy, CSU CHILL) 
From: Pat
MPS+2DVD site at 13 km range
Bringi et al. 
lines
10
How does this affect Convective-Stratiform Partitioning
Stratiform Precipitation
Easton, Colorado
2DVD-blue
MPS+2DVD-orange
CHILL Radar RHI Scans over MPS+2DVD
11
Ze LDR
Zdr ρhv
Large Nw scatter between 2DVD 
with and without the MPS 
Not much BBand Heavily rimed SN/low 
density GR?
DPR would likely misclassify this as convective (e.g., Houze 2017 PMM)
Convective Precipitation
Easton, Colorado
2DVD-blue
MPS+2DVD-orange
CHILL Radar RHI Scans over MPS+2DVD
12
Ze LDR
Zdrρhv
Much closer DSD agreement 
for convective period 
Why the generalized gamma?
13
Gamma DSD Generalized 
Gamma DSD
Generalized Gamma provides a better fit across the DSD spectrum, 
especially at the tails (Thurai and Bringi 2017, in review)
(see also Lee et al. 2004; Raupach and Berne 2017, JAMC)
𝑁 𝐷 = 𝑁𝑤ℎ𝐺𝐺 𝑖,𝑗,𝜇,𝑐  𝐷 𝐷𝑚
𝑁 𝐷 = 𝑁𝑤ℎ𝑆𝐺 𝑖,𝑗,𝜇  𝐷 𝐷𝑚
SG in red
Bi-modal 
DSDs
30 November 2016
Huntsville, Alabama
3-minute DSDs
3 regions of DSD:
1. Drizzle mode   
Deq < 0.7 mm
2. Plateau region
3. Precipitation 
mode Deq > 1mm
Equilibrium 
DSD?
What are the implications for retrieving light 
rain from GPM?
• Small drops affect the DSD parameters, especially for light rainfall
• Also affect the DSD-based partitioning of Convective-Stratiform
• Generalized gamma accounts for bi-modality of the DSD (i.e., drizzle and 
precipitation portions, including the plateau/shoulder region) and provides better 
fit at the tails
15
 G-G model may provide a better retrieval/validation of light 
rainfall (R, Dm and Nw), but need to further examine variability of 
shape parameters 𝑁 𝐷 = 𝑁𝑤ℎ𝐺𝐺 𝑖,𝑗,𝝁,𝒄  𝐷 𝐷𝑚
Acknowledgements
• A portion of this work was funded by Dr. Ramesh Kakar through 
NASA’s Precipitation Science Mission
• M. Thurai and V.N. Bringi also acknowledge support of the National 
Science Foundation Award #1431127 - AGS (Atmospheric and 
Geospace Sciences), Atmospheric & Geospace Sciences Division
16


A New Perspective on the Raindrop Size Distribution and Its Implications for Retrievals of Light Rain

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