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Single Frequency Observations of Warm
Clouds and PrecipitationEdward Luke with Pavlos Kollias, Frederic Tridon,
Stefan Kneifel, and Alessandro Battaglia
Short Course on Research Applications
of Radar Doppler Spectra
Garmisch-Partenkirchen, Germany
August 31, 2014
On a Mac or Linux:java -classpath netcdfAll-4.3.jar:BNLSpecVisToolkit-current.jar SpectraVisualizationToolkit
On Windows (a semicolon replaces the colon):Java -classpath netcdfAll-4.3.jar;BNLSpecVisToolkit-current.jar SpectraVisualizationToolkit
Visualizer Software Startup
Use the correct path to the two .jar files in the above command if they are not in your current directory.
BNLSpecVisToolkit-1.0.10.jar BNLSpecVisToolkit-current.jar
Non-precipitating CloudParticles have negligible fall velocities, making them tracers of air motion.
s = f(st, ss)
Vair
Non-Rayleigh FeaturesObserving Precipitation with a Cloud Radar—Why Mie?
(Lhermitte, 1988; Kollias et al. 2002)
DSD Retrieval in Stratiform Rain
MeasuredForward Model
Drop size distribution
Radar cross section
RetrievalIncomplete Gamma Fit
Power spectrum
Vertical Air Velocities and Rainfall DSD• Vertical air velocity to an accuracy of 10 cm/s• Exploration of Drop Size Distribution (DSD) parameters in time and height• Excellent agreement with surface disdrometer measurements
(Giangrande et al., 2010)
N(D) = N0e-ΛD
Spectrum Skewness in the Azores
A marked microphysical transition occurs near midday, as indicated in this time-height plot of spectrum skewness.
autoconversion dominated
accretion dominated
Drizzle Onset in the Doppler Spectrum
Drizzle first appears here
The Doppler spectrum of cloud droplets without drizzle is very close to symmetrical due to action of small-scale turbulence. Thus, early drizzle growth should impose a deviation (positive skewness) from the near-zero skewness of the background (cloud PSD Doppler spectrum).
Doppler spectra skewness and microphysics
CloudDrizzle
DrizzleCloud
Cloud dominates 1st, 3rd and 6th moments of N(D) Cloud
dominates 1st, 3rd; Drizzle dominates 6th moment of N(D)
Kollias et al., 2011a, JGR
Starting and transition state
Doppler Spectrum Skewness – A Powerful Radar Observable Sensitive to Drizzle Onset
Drizzle radar reflectivity higher than cloud radar reflectivity
Cloud radar reflectivity higher than drizzle radar reflectivity
Cloud+drizzle radar Doppler spectrum
(Zcloud>Zdrizzle)
Cloud+drizzle radar Doppler spectrum
(Zcloud<Zdrizzle)
Cloud-only radar Doppler spectrum
Positive Doppler spectra skewness (indicator of drizzle onset) is more sensitive than reflectivity to identify drizzle onset
Change in skewnessChange in reflectivity
Luke and Kollias, 2013
Cloud/Drizzle Spectrum PartitioningKey assumption 1: On average, cloud power is equal on either side of spectrum peak.
Key assumption 2: Drizzle particle fall velocities generally exceed the spectrum broadening due to dynamics (violated by heavier drizzle).
In Summary
We have looked at some properties and retrieval approaches for radar Doppler spectra representative of
Drizzling warm marine stratocumulusContinental stratiform rainFair weather cumulusClear air clutter