Transcript Microphysical variability of tropical and mid
Microphysical variability of tropical and mid-latitude rainfall as revealed by polarimetric radar
S. Rutledge
1
, D. Wolff
2
, B. Dolan
1
, P. Kennedy
1
, W. Petersen
2
and
1
V. Chandrasekar
1 Colorado State University 2 NASA/Wallops Is. 2013 AGU Fall Meeting San Francisco, CA 9-13 December 2013 Session H42A
• • • • Reflectivity-based rain estimation central to TRMM and GPM.
We will investigate the polarimetric radar derived “structure” of rainfall at several locations around the globe. These structures reveal regimes where the melting of graupel and hail contribute strongly to rainfall vs. where coalescence dominates (tropical warm rain). These structures reflect basic differences in drop size distributions. These structures have implications for the A coefficient in Z=AR b relationships, which is how rain rate is estimated based on TRMM PR observations. We will conclude by discussing how A derived from polarimetric radar compares to that from the PR algorithm. This comparison will be done using both the TRMM V6 and V7 datasets.
Z
Rainfall microphysics seen through combination of polarimetric variables
K DP is a measure of the difference in wave propagation between H and V polarizations; sensitive to non-spherical particles 50 40 30 0 Rain 0.5 deg elevation angle 2 Based on computations of Z and Kdp from DSD assumptions Oklahoma 4 6 K dp proportional to mass content and mass-weighted oblateness ratio
K
dp
K
dp
= µ
C
l 1 ò
D
3 (1 -
LWC
· (1 -
r
m
)
r
)
N
(
D
)
dD K
dp
µ
LWC
·
oblateness
Mass weighted…..
b
Z Z
» µ ò
D
6
N
(
D
)
dD D
6 Reflectivity
Z dr
=
10 log
10
(
Z HH
/
Z VV
)
a Differential Reflectivity
r
=
b
/
a
K dp deg/km Difference in H,V phase in degrees
Application of polarimetric data……
Gorgucci et al. (2006, JTECH) showed that a parameter space formed by K dp / Z vs. Z dr was useful for characterizing precipitation physics. Figure on the right shows results of scattering simulations for various Gamma DSD’s with mean diameters (D m ) ranging from 1.5 to 3.5 mm.
Variations in D m are evident as well-defined curving paths in K dp /Z vs. Z dr space.
This technique can also be used to distinguish warm rain-coalescence situations (high freezing level and active drop coalescence processes, smaller drop sizes) from rain derived from the melting of graupel and hail (larger drop sizes), as distinguished by K dp / Z; Z dr pairs.
For a given rainfall regime, behavior of K dp /Z vs. Z dr represents precipitation physics.
D m , mm Smaller drops, large liquid water contents Modest Z; high K dp Large drops from melting ice Large Z
An illustrative example; contrasting the FNL flood case with a nearby bow echo storm… NLDN lightning for 5 hour period Heavy rain, little lightning FNL storm BEC storm 10 inches of rain in a 5 hour period Lightning with bow echo storm
Z=139R 1.47
High values of K Z DR DP /Z indicate large water contents with low Z; small , small drops Ft. Collins flood example; tropical like heavy rain event. Z=139R 1.47
Nearby strong convective storm; Z=300R 1.4
Z=300R 1.4
Larger Z DR values indicating melting ice particles Normalized density of points expressed as a percentage
Yellow Red
70% 50%
Blue
30%
All points > 30 dBZ used
Flood (tropical like) event distinguished from bow echo by reduced Z and Z dr . K dp /Z shifted to higher values for FNL (flood) case. Implies large LWC consisting of relatively small drops.
Polarimetric variables consistent with Z-R forms for these events
Small drops, high LWC, small A (FLOOD) Large drops, large A (BOW ECHO)
Shift to the tropics…..
TRMM LBA, Jan-Feb 1999
NCAR S-pol radar deployed for TRMM-LBA Documented east-west regime with 7-10 day variability (Petersen and Rutledge, 2002)
24 Feb 1999 WEST case.
Lower CAPE, monsoon-like regime. WEST Subtle differences between East and West EAST 26 Jan 1999 EAST case.
Stronger convection, higher CAPE.
WEST EAST LARGER Z DR VALUES IN EAST CASE COMPARED TO WEST. INDICATIVE OF LARGER DROPS (MELTING ICE) CONSISTENT WITH HIGHER CAPE/STRONGER CONVECTION IN EAST PHASE.
IN WEST PHASE, LARGER K dp /Z INDICATING SUBSTANTIAL LWC CONTAINED BY SMALL DROPS.
Following Bringi et al. 2004,
J. Atmos. Ocean Technol
., the A coefficient in Z=AR b is calculated via the so-called pole-tune method where a Gamma distribution is assumed
Z
= [
a
¢ ( m ) /
N w
]
R
1.5
Here A is a function of Z, D 0 and μ. Bringi et al. argue that A derived in this manner continuously tracks the variability in DSD.
“A” coefficients derived from the TRMM-LBA dataset using the NCAR S-pol radar. DSD ’S ASSUME BROAD RANGE OF VALUES WITHIN EACH REGIME.
K DP /LIN Z Z DR
N-Pol in
MC3E
sporting its new center-fed parabolic antenna and other upgr ades
Midlatitude Continental Convective Clouds Experiment May-June 2011
Observations from MC3E…..
25 April 2011: Multiple Convectiv e Cores
DZ HID VR Kdp Zdr Rhohv
24 May 2011 Severe storm
70+ dBZ up to 10 km - Large (+5 º/km) K dp at the surface - Signature of large hail (in RH and ZDR) - Strong tilted updraft and divergence aloft - Data of high quality at significant ranges HID DZ Kdp VR Zdr and Kdp column Zdr Rhohv
Warm rain
Normalized Self-Consistency for CHILL 70 % 50 % 30 % 29 July 1997 Ft. Collins Flood Tropical-type Sounding
Colorado events for comparison; Warm rain vs. melting hail examples
+ 3 July 2010 Denver International Airport Continental High Plains
Ice based
+ 0 0 Z dr (dB) N-Pol in Oklahoma, MC3E 0 0 1 3 2 5 Normalized Self-Consistency for N-Pol 25 APRIL 2011 4
N-POL
70 % 50 % 30 %
Results consistent with active coalescence growth and ice-based precipitation.
25 April 2011 0800-1000 UTC MC3E Oklahoma Location Continental High Plains
Resulting from higher moisture contents, higher freezing level, collisional breakup, etc.
+ 1 2 Z dr (dB) 3 4 5 2 1 0 0 3 5 4 Normalized Self-Consistency for N-Pol 24 May 2011 70 % 50 % 30 %
N-POL
1 + 2 Z dr (dB) 3 24 May 2011 0045-0300 UTC MC3E Oklahoma Location Continental High Plains 4 5
Rainfall: NASA GPM GV Measurement Infrastructure for IFloods
NASA IFLOODS DEPLOYMENT INSTRUMENTATION Radars: Rain mapping, 4-D precip structure, DSD, rates
• NPOL S-band transportable, scanning dual-pol radar • D3R radar: Dual-frequency (KA KU), dual-polarimetric, Doppler radar.
• 3 Metek Micro Rain Radars (K band), vertically pointing (one on order)
Point-Network Disdrometer/Gauges: Precip character/reference
•
5
2D Video Disdrometers • 16-20 Parsivel-2 Disdrometer with MetOne 12 ” TB Rain Gauge • 25 dual-gauge Met One TB rain gauges with soil T/Q
Dual-Gauge Net Precipitation Video Imager (PVI) JW
May 2, 2013 – Cold, light rain [70 km]
TRMM-LBA West regime Note: Axis change
May 26, 2013 – Convection
Very similar to MC3E case
Squall line example from IFLOODS
Large drops from melting graupel and small hail
Trailing stratiform region from same squall line
Smaller drops from melting of aggregates
IMPLICATIONS FOR TRMM rain mapping
Normalized Self-Consistency for CHILL 70 % 50 % 30 % 29 July 1997 Ft. Collins Flood Tropical-type Sounding
A values
+
Towards small
Z dr (dB) + 3 July 2010 Denver International Airport Continental High Plains
The precipitation physics revealed by these polarimetric data have a direct bearing on Z-R based rain estimation and Z based attenuation correction Shift to upper left implies smaller “A” coefficient in Z=AR b and more rain for a given Z.
Shift to lower right implies larger “A” coefficient in Z=AR b and less rain for a given Z.
Have seen clear examples of these distinctive shifts…….which are due to microphysical variations in the production of rainfall
Towards large A values
TRMM V6 underpredicted intense rain
Kind of the crux of the matter…..
Comparison of A coefficient in Z=AR b between TRMM 2A25 and those derived from S-pol polarimetric radar Rain physics variability was not well captured in Version 6, reflected by the restricted range of A
100.00
10.00
1.00
0.10
LBA SPOL-TRMM V7 Coefficient Conv A in Z = A*R B SPOL-EAST SPOL-WEST TRMM-EAST TRMM-WEST 0.01
0 100 400 200 Coefficient A Value 300 SPOL Resolution = 4km Reasons for the improvement 500 100.000
10.000
Introduced NUBF correction, increase in high rain rates 1.000
Addition of 0.5 dB to PIA; increase in heavy rain over land.
Changes to α in the k=αZ e β specific attenuation calculation 0.100
0.010
0.001
Rainfall physics much better captured in TRMM V7!
For both regimes TRMM derived A has a double peak, coalescence and melting ice. Consistent with the range of polarimetric radar derived A values.
LBA: Convective RR TRMM V7 SPOL-EAST SPOL-WEST TRMM-EAST TRMM-WEST 20 40 60 80 Rain Rate (mm hr -1 ) 4 km sampling 100 120 140