precipitation studies with radars and use of wp/rass by s.h. damle
TRANSCRIPT
![Page 1: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/1.jpg)
• Precipitation studies with RADARS
and
use of WP/RASS
By
S.H. Damle
![Page 2: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/2.jpg)
• In precipitating conditions a vertically pointing Doppler Radar Effectively measures the apparent fall velocity of the falling hydrometers/snow particles
• The scattering in most cases is presumed to be Rayleigh • The apparent/observed fall velocity deviates from true
fall velocity because of• Ambient clear air velocity & turbulence• Pressure dependence of fall velocity • Deviations from the assumed Rayleigh scattering
mechanism
![Page 3: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/3.jpg)
• The ambient clear air velocity & turbulence are the most important and must be known with reasonable accuracy/ confidence level to obtain the true fall velocities & other related parameters from the observed ‘apparent velocity
• The classical work of Atlas and Srivastav (1973) shows that the clear air updrafts/downdrafts should preferably be measured with accuracies of 0.25 m/sec or better to obtain reasonable estimates of the rain related parameters
![Page 4: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/4.jpg)
• The conventional S/C/X band Radars can not quite measure the turbulent clear air motion – the scale sizes of turbulence to which they could respond being much smaller ~ λ /2
• In the absence of clear air velocity & turbulence , Roger used an empirical relation like to utilise radar precipitation data
<Vf> = 2.65 Z ** 0.107 m/secsince <Vobs > = Vf - w where w is positive for upward air motion.
• The Roger’s empirical relationship is approximate within ±1 m/sec
![Page 5: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/5.jpg)
• The advent of U/VHF Radars has now made possible direct measurement of ambient clear air updraft/downdraft and the turbulence.
• Simultaneous measurement of clear air turbulent air velocities and the hydrometeor (apparent) fall velocities make possible the precipitation studies with these Radars.
![Page 6: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/6.jpg)
The use of clear air wind profiler Radars
• Typical clear air Radar operating frequencies 50, 400, 1380 MHz.
• Clear air scattering mechanism – Bragg scatter - λ-1/3 dependence
• Precipitation - Rayleigh scatter - λ-4 dependence• If one equates the expressions for the volume reflectivity
for the two cases one obtains the relationship between the equivalent reflectivity factor Ze & the Cn2 Thus
• dBZe = 10 log Ze = 10 {log 10 Cn2 + log λ11/3 +15.31} where λ is in meters and Ze is in mm6/m3
• Equation is valid for scattering from water droplets• Typical clear air Cn2 values are in the range of 10-15 to
10-18 m-2/3
![Page 7: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/7.jpg)
Illustrative Table of Cn2 & Ze
C2n (m
-2/3) MST (53 MHZ) WP(404 MHZ) 1357 MHZ LAWP(Gadanki)
10-13 50.603 (dB) 18.205 (dB) -1.11 (dB) 10-14 40.603 8.205 -11.11 10-15 30.603 -2.205 -21.11 10-16 20.603 -12.205 -31.11 10-17 10.603 -22.205 -41.11
![Page 8: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/8.jpg)
From this table one can note• 50 MHz system would almost always measures the clear
air velocity unless the precipitation rises beyond rain rate of few mm/hr
• 1380 MHz profiler is sensitive to smallest of drizzle (rain rate 0.01mm/hr) and swamps the clear air signal.
• This prompted research workers to use co-located or near by radars- one at 50 MHz and the other 915/1380 MHz for precipitation study.
• The 400 MHz system case is intermediate between these two extremes and thus the 400 MHz profiler may be amenable to simultaneous observations of Bragg & Rayleigh scatter.
![Page 9: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/9.jpg)
• Relationships to dropsize distribution
Z = ∫ D**6 * N (D) dD• The first task is to relate fall velocity with drop
diameter
Vf = a*D**b - unrealistic• GK data shows that fall speed approaches an
asymptotic value of 9.2 m/sec for D > 6 mm• Atlas , Srivastava modification
V=965-1030 exp (-6D) cm /sec, where D is in cm
![Page 10: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/10.jpg)
• Modified GK expression is valid near the ground • For higher heights Foote & Dutoit suggested a
density correction factor to use them at higher heights
Vh =vg (ρg/ρh)**0.4• Thus if true fall velocity is calculated from radar
observation, sample D values, within the limitation of spectral resolution, can be obtained for a given precipitation event.
![Page 11: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/11.jpg)
Key factors to prepare algorithm development
Location in the Doppler plane of the Bragg & Rayleigh scatter signal
• Ralph’s prescriptions of velocity thresholds for Rayleigh scatter identification for rain
• The Rayleigh scatter signal for height levels just below and above the zero degree isotherm level
![Page 12: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/12.jpg)
• The bright band signature- Large velocities just below freezing level
heights and steep fall in the speed towards freezing level heights – indicative of presence of snow at/above freezing level
- Absence of Bright Band – indicative of presence of super cooled water droplets above freezing levels
• The (Doppler) velocity value in the case of snow/ice particulates is less than that of water droplets because of lower density of ice/snow compared to that of water
![Page 13: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/13.jpg)
Signal flow in a profiler
Objective Determination of noise power;Hildebrand/Sekhon algorithm
• Obtain average noise power/bin ……… Pn& standard deviation ……σnFor a given range bin
• Subtract average noise power from spectral value• Identify clear air spectral peak• Identify precipitation spectra peak• Fit Gaussian curve for clear air spectra and estimate
clear air velocity and spread• Deconvolve the ppt spectrum by clear air spectrum and
obtain ppt spectral width
![Page 14: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/14.jpg)
Observed Spectrum
Deconvolution of Window function
Find clear-air spectrum : St
Fitting clear-air spectrum
Find precipitation spectrum : SD
Set initial precipitation spectrum
Convolution : St * SD
Comparison with observed spectrum
Convergence decisionIteration
Retrieval of raindrop size distribution
![Page 15: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/15.jpg)
• For lower rain rates it may be acceptable to fit Gaussian curve to ppt spectra and estimate apparent fall velocity and apparent spectral / observed width
σtp**2 = σpo**2- σcl**2 • True fall velocity = observed velocity –
clear air velocity• Calculate dBZ from estimated S/N
![Page 16: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/16.jpg)
• Illustration with common size distributionN(D) = N0 exp (-λD) V = 965 -1030 { (λ/(λ+6))} 7 cm /sec
• With knowledge of λ and sample value of D as obtained earlier one may estimate N0 for every measurement height by relating to Z as indicated earlier
• Variation of N0 and λ with height may provide by extrapolation estimation of expected distribution of ground
• The knowledge of variation of the distribution with time could possibly correlated with cloud models to study the evolution.
![Page 17: Precipitation studies with RADARS and use of WP/RASS By S.H. Damle](https://reader036.vdocuments.net/reader036/viewer/2022082711/56649ef35503460f94c05713/html5/thumbnails/17.jpg)
Concluding Remarks
Difficulties arise when• Separate Bragg and Rayleigh scatter peaks can not be
identified in the spectrumThe topic is of current research interest : April 2005 : JOAT
• Use of iterative deconvolution. • Works with additional external constraints in the
algorithm since iterative methods can lead to divergent solutions
• Model spectra fitting is carried out using gamma distribution – 400 MHz
• More work needed for the case when two distinct peaks can not be identified.