Comparison Of Cloud Boundaries Measured With 8.6mm Radar And 10.6@tm Lidar

Abstract

AS previously published in theCOMPARISON OF CLOUD BOUNDARIES MEASURED WITH8.6 mm RADAR AND IQ.6 _m LIDARTaneil UttalNOAA Wave Propagation Laboratory325 Broadway, Boulder CO 80302Janet M. IntrieriCooperative Institute for Research in the Environmental SciencesUniversity of Colorado, Boulder CO 80309INTRODUCTIONOne of the most basic cloud properties is location; the height of cloudbase and the height of cloud top. The glossary of meteorology defines cloudbase (top) as follows: "For a given cloud or cloud layer, that lowest(highest) level in the atmosphere at which the air contains a perceptiblequantity of cloud particles" _i) $ Our studies show that for a 8.66 mm radar,and a 10.6 _un lidar, the level at which cloud hydrometers become "perceptible"can vary significantly as a function of the different wavelengths, powers,beamwidths and sampling rates of the two remote sensors.THE EXPERIMENT for determining echo boundaries.This allows CLDSTATS to operate ondata sets collected by differentremote sensors, as long as the datais in Common Doppler Exchange Format(4). While we have run CLDSTATSprimarily on vertically pointingdata, the algorithm is sensitive toelevation angle, and in theory canbe run on different kinds of scans,for instance RHI scans.The user specifies a thresholdfield (e.g. reflectivity), athreshold value, and a minimumnumber of consecutive range gates inwhich the threshold value must existfor the in-cloud condition to bemet. To choose successful thresholdvalues, the user must havefamiliarity with the instrument andit's response to backscatteringtargets in the atmosphere. It shouldbe noted that CLDSTATS examines eachbeam of data separately, starting ata lower limit and ending at an upperlimit which is also user specified.Therefore, this algorithm is a I-Dfilter as opposed to similar cloudboundary detection program developedby Penn State University whichimposes a 2-D filter (5).CLDSTATS has been testedextensively on radar data, and wehave settled on a thresholdingcriteria using the normalizedcoherent power field that appears towork well for all but the musttenuous cirrus clouds. Normalizedcoherent power is a measure ofsignal coherence from pulse topulse. The lidar characterizationwas somewhat more difficult, sincebackground values of lidarbackscatter from aerosols weresometimes as high as in-cloudvalues. It was therefore necessaryIn November and December of1991, the First ISCCP RegionalExperiment II (FIRE II) wasconducted in Coffeyville, Kansas forthe purpose of studying cirrusclouds and their effects onplanetary radiation budgets. Thisexperiment was a large multi-organizational effort coordinated byNASA. It brought together a largenumber of surface, airborne, andsatellite-based active and passiveremote sensors.The NOAA Wave PropagationLaboratory (WPL) brought a Doppler,8.66 mm radar (2) and a Doppler,10.6 _/n lidar (3) and operated themside-by-side. Although 6othinstruments have scanningcapabilities, they operatedprimarily in a vertically pointingmode to obtain time-height crosssections of the cloud as it passedover the observation site. The radarpointed in a fixed vertical mode for25 min of every 30 min observingperiod. The lidar pointed verticallyand also rocked back and forth todetermine periods when specularreflection might be occurring.Therefore, the lidar data wasfiltered in the post processing sothat only the vertical beams of datawere included in our analysis.ANALYSISTo determine echo boundariesfrom active, range-gated remotesensors, the NOAA/WPL radar grouphas developed the program CLDSTATS.This program is designed for maximumflexibility so that the user canchoose different threshold criteriaCmbi ned 09ticat-MicrousveEarth and Atmosphere SensingSymposium 22-25 Narch 1993AUou_I_rU,