Rain Influences On High Wind Backscatter: SeaWindsStudies .
Rain Influences on High Wind Backscatter:SeaWinds Studies during Hurricanes Dennis,Isabel, and Claudette.Additional Studies with ASCATDavid E. WeissmanHofstra UniversityHempstead, New York 11549Mark A. BourassaCenter for Ocean Atmosphere Prediction StudiesFlorida State UniversityTallahassee, FLOCEANS 2008 - September 17
Studies of Interest:USING the QuikSCAT Scatterometer and NEXRADMeasurements of the 3-D Rain Volume:1. To measure the effect of rain impacts on total sea surfaceroughness (Ku-band NRCS), as a function rain rate, windspeed, polarization and azimuth look direction2. Develop a model for the change (increase or decrease)of the surface normalized radar cross section (NRCS forKu-band) as a function of rainrate, wind speed, etc.3. Interpret the results in terms of the interaction betweenwind waves and the splash products of rain impacts(ring waves, crowns and stalks)
DATA:a) QuikSCAT Level 2A NRCS data, H-pol and V-polb) Simultaneous NEXRAD 3-D VolumeReflectivity (S-band) within scatterometer beamc) Surface winds from NOAA Hurricane ResearchDivision Analysis and buoys
NEXRAD Level II, Base Reflectivity,in dBZ,H 500 m, KTLH,10-July-05, t 10:5532.545Circles 50km324031.535Latitude, 5.5 276Longitude, Deg276.5277277.52780
Hurr. Dennis 1055 UTC10 July 2005Product ofNOAA/AOML/HRDCourtesy of Mark PowellMax. 1-min sustainedsurface winds (kt)QS Azim. 2310 degQS Azim. 1210 degBox 1
Method of Computing SeaWinds Volume Backscatter andAttenuation using NEXRAD data in discrete parallelogramsSubdivision of SCAT Footprintinto 5 km Square, 2 km HighCells (0-to-8km)NEXRAD Reflectivity is determinedin each cell, weighted with EGGpattern gain Correction to SIGMA0 ineach cell - 3 dB Contours ShownRelative Positions of Reflectivity CellsAlong Incident Beam, V-pol Case
Electromagnetic Model of the NRCS (σax) Measured by theSeaWinds Scatterometer and Rain Impact NRCS, σrn0Use of “x” subscript below will represent either “h” or “v” polarizationσax Total measured NRCS at Receiver; Sum of Contributions from Sea Surface andRain Volume L2A dataσwdx sea surface radar cross section due to wind driven roughness alone (wind-NRCS)σrnx sea surface radar cross section due to rain impact roughness alone (rain-NRCS)αx(r) attenuation, in nepers/m for each polarization, function of local volume rainrate orprecipitation water contentσox (r) surface equivalent of volumetric rain RCS, constant * Zx(the radar reflectivity factor for Ku-band, Zx, varies with position, “r”)lenx path length of radar beam for each polarization len/Cos(θx)(rain column height, over scatterometer footprint len, θh 46o & θv 54o)lenxlenxσax 0 σox e 4 α x ( s ) dsrdr (σwdx σrnx ) e 4 0lenx α x ( s ) ds
σrn0 model function for the normalized radar cross sectiondue to rain impact; depends on wind magnitude and rainrateAfter solving for the total surface NRCS (σwdx σrnx) from arain affected area, the wind-driven term alone is estimatedfrom a nearby rain-free area: σwdx. Then their ratio σrn0 iscomputed, producing:σ rn 0 (σ wdx σ rnxσ wdx)
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #1, 10-July-05, KTLH-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5H-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #1, 10-July-05, KTLH-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5H-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #1, 10-July-05, KTLH-5V-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5V-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #1, 10-July-05, KTLH-5V-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5V-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #2, 10-July-05, KTLH-5Measured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5H-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #2, 10-July-05, KTLH-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS50H-pol-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #2, 10-July-05, KTLH-5V-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5V-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Measured and atmospheric rain-corrected NRCS, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #2, 10-July-05, KTLH-5V-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20-110010Mean Rainrate, Lowest Elevation, mm/hr110Ratio of NRCS with and without Rain, dBRatio of Total Surface NRCS with Rain/Relative to Wind Only NRCS5V-pol0-5-110010Mean Rainrate, Lowest Elevation, mm/hr110
Quick Summary:The rain-induced backscatter for H-pol and V-pol bothincrease with rainrate, but by smaller amounts than atlower winds ( 8 m/s). Effects start at 4 mm/hr.BUT σov σoH and σov can be negativeat high rainrateInterpretation: the velocity of raindrops striking the surface atwinds above 10 m/s are now affected by both gravity and thewind vector.The roughness effects need to be interpreted interms of the rain splash products (stalks & ring waves), themodified surface turbulence and wind generated waves(including breakers). Could the wind stress be decreasingwith rainrate?
NEXRAD Level II, Base Reflectivity,in dBZ,H 500 m, KMHX,18-Sep-03, t 15:583745Plot SIG0 Cell Locations36.5H-polV-pol40363535.530Latitude, Deg352534.5Box , Deg286287288
Azim. #1 30deg Box #2Azim #2 185degBox #1
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #1, 18-Sep-08, KMHX-5Measured NRCS, " "H-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #1, 18-Sep-03, KMHX-5Measured NRCS, " "H-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #2, 18-Sep-03, KMHX-5Measured NRCS, " "H-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #2, 18-Sept-03, KMHX-5Measured NRCS, " "H-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #1, 18-Sep-03, KMHX-5Measured NRCS, " "V-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10V-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #2, 18-Sept-03, KMHX-5Measured NRCS, " "V-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10V-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Quick Observation for Hurr Isabel:Winds in Box # 1 40-50 kts and Box #2 50-80 ktsIn Box #1 changes in NRCS start at about 10 mm/hrand can be a few dB for H-pol and V-polIn Box #2 negligible changes in NRCS for allrainrates.
Wind: 17 m/s@ 58 degat #42035BuoyNEXRAD Level II, Base Reflectivity,in dBZ,H 500 m, KHGX,15-July-05, t 04:0131.545Azim #2 305 degLatitude, Deg314030.535303029.525292028.51510285Box #1 27.5Azim # 1 205deg263264265Longitude, Deg266267
NEXRAD & BUOY LocationsKHGX42035KCRP4201942020Buoy #Winds Speeds4205:42019:42020:8.6 10 m/s8.5 11 m/s4.5 5.5 m/s
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #1, 15-July-03, KHGX-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #1, 15-July-03, KHGX-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #1, 15-July-03, KHGX-5V-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10V-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #1, 15-July-03, KHGX-5V-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10V-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #2, 15-July-03, KHGX-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #2, 15-July-03, KHGX-5H-polMeasured NRCS, " "Corrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10H-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #1, Box #2, 15-July-03, KHGX-5Measured NRCS, " "V-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10V-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Measured and atmospheric rain-corrected NRCS, dBRatio of NRCS with and without Rain, dBMeasured NRCS and Atmosphere Corrected NRCS, Azim #2, Box #2, 15-July-03, KHGX-5Measured NRCS, " "V-polCorrected NRCS, "square"-10-15-20 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101Ratio of Total Surface NRCS with Rain/Relative to Wind Only NRCS10V-pol50-5 -210-101010Mean Rainrate, Lowest Elevation, mm/hr101
Summary for Hurricane Claudette:The properties of the NRCS vs Rainrate aresimilar in many respects to those seen inHurricanes Dennis and Claudette; V-pol hasnotable differences with respect to H-pol.Which polarization is more representativeof the surface stress properties ?
GENERAL SUMMARYAt low wind speeds ( 7 m/s), the changes in NRCScaused by rain roughness was very similar for bothH-pol and V-polAs the wind speed increases, and approaches theterminal velocity of gravity driven falling rain, thedifferences become progressively pronounced.This indicates there is an interaction between the rainimpact products and the wind driven wave spectrumthat produces new roughness characteristics thathave a distinct and differential polarization response.
ASCAT Studies of the NRCS vsRainrate were made. These werelocated in the Gulf of Mexiconear the Texas and LouisianaCoastTwo events have been studied:1.April 26 near the Houston Nexrad(KHGX)2. March 7, near the Lake Charles,La. Nexrad (KLCH)
Acknowledge with appreciation the contributionof Scott Dunbar who provided us with the dataand software that we needed.
NEXRAD, Base Reflectivity,in dBZ,H 500 m, KHGX,26-Apr-08, t 15:3631.545Latitude, Deg5 km Resolution314030.535Winds 8 , Deg2662670
oo2Locations of Rear & Fwd Beams Within Rain Distribution, 50 incid 55 , Apr 10Latitude, deg28.427.6644264264.526586265.5Longitude, deg266266.5267
ooNRCS vs Rainrate, FWD and REAR Beams, (50 Incidence angle 55 )-20Forward Beam XRear Beam "RED-SQUARE"-22NRCS "SIG0", in dB-24-26-28-30-32-34 -110010Mean Rainrate, (45 km square), mm/hr101
ASCAT SIG0 Data vs Incidence Angle, (April 26), Locations Outside Rain Area-12Rear Beam Forward Beam -14Mid- Beam-16NRCS, "SIG0", in dB-18-20-22-24-26-28-303035404550Incidenge angle, deg556065
NEXRAD, Base Reflectivity,in dBZ,H 500 m, KLCH, 07-Mar-08, t 03:4860325 km Resolution5031.531Latitude, Deg4030.5303029.5Winds 15 m/s20291028.528264.5265265.5266266.5 267 267.5 268 268.5Longitude, Deg2690
ooLocations of Rear & Fwd Beams Within Rain Distribution, 50 incid 55 , Mar 1128.4628.328.21Latitude, deg628.628.1265.5266266.5Longitude, deg267
ooNRCS vs Rainrate, FWD and REAR Beams, (50 Incidence Angle 55 )-14-15NRCS, "SIG0", dB-16-17-18-19-20Rear Beam -21Forward Beam -22 010110Mean Rainrate, (45 km square), mm/hr102
ooNRCS vs Rainrate, Mid-Beam, (39 Incidence Angle 44.5 ), Mar. 07, 2008-12-12.5-13NRCS, "SIG0", dB-13.5-14-14.5-15-15.5-16-16.5 010110Mean Rainrate, (45 km square), mm/hr210
Rain Influences on High Wind Backscatter: SeaWindsStudies during Hurricanes Dennis, Isabel, and Claudette. Additional Studies with ASCAT David E. Weissman Hofstra University Hempstead, New York 11549 OCEANS 2008 - September 17 . sds wdx rn
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