THE ROLE OF SPACEBORNE MILLIMETER-WAVE RADAR IN THE GLOBAL MONITORINGOF ICE-CLOUD

The purpose of this paper is to assess the potential of a spaceborne 9 4-GHz radar for providing useful measurements of the vertical distribu tion and water content of ice clouds on a global scale. Calculations o f longwave (LW) fluxes for a number of model ice clouds are performed. These are used to determine the minimum cloud optical depth that will cause changes in the outgoing longwave radiation or flux divergence w ithin a cloud layer greater than 10 W m(-2), and in surface downward L W flux greater than 5 W m(-2) compared to the clear-sky value. These o ptical depth values are used as the definition of a ''radiatively sign ificant'' cloud. Different ''thresholds of radiative significance'' ar e calculated for each of the three radiation parameters and also for t ropical and midlatitude cirrus clouds. Extensive observational dataset s of ice crystal size spectra from midlatitude and tropical cirrus are then used to assess the capability of a radar to meet these measureme nt requirements. A radar with a threshold of -30 dBZ should detect 99% (92%) of ''radiatively significant'' clouds in the midlatitudes( Trop ics). This detection efficiency may be reduced significantly for tropi cal clouds at very low temperatures (-80 degrees C). The LW flux calcu lations are also used to establish the required accuracy within which the optical depth should be known in order to estimate LW fluxes or fl ux divergence to within specified limits of accuracy. Accuracy require ments are also expressed in terms of ice water content (IWC) because o f the need to validate cloud parameterization schemes in general circu lation models (GCMs). Estimates of IWC derived using radar alone and a lso using additional information to define the mean crystal size are c onsidered. With crystal size information available, the IWC for sample s with a horizontal scale of 1-2 km may be obtained with a bias of les s than 8%. For IWC larger than 0.01 g m(-3), the random error is in th e range +50% to -35%, whereas for a value of 0.001 g m(-3) the random enter increases to between +80% and -45%, This level of accuracy also represents the best that may be achieved for estimates of the cloud op tical depth and meets the requirements derived from LW flux calculatio ns. In the absence of independent particle size information, the rando m error is within the range +85% to -55% for IWC greater than 0.01 g m (-3). For the same IWC range, the estimated bias is less than +/- 15%. This accuracy is sufficient to provide useful constraints on GCM clou d parameterization schemes.