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

Spectral Broadening Due to Shear

A Simple Simulation of BroadeningDue to Turbulence

Non-Rayleigh FeaturesObserving Precipitation with a Cloud Radar—Why Mie?

(Lhermitte, 1988; Kollias et al. 2002)

Air Velocity Retrieval in Stratiform Rain

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

Spectrum Skewness Captures Information Missed by Spectrum Width

s = f(st,ss,sDSD)

Spectrum Skewness Captures Information Missed by Spectrum Width

s = f(st,ss,sDSD)

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).

Drizzling Stratocumulus Retrievals

Luke and Kollias, 2013, JTECH

Insect Clutter Spectra

Luke et. al., 2008, JTECH

Insect Clutter DetectionLuke et. al., 2008,JTECH

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

http://www.gim.bnl.gov/armclouds/radar_spectra_shortcourse_erad2014/