The role of spaceborne millimeter-wave radar in the global monitoring of ice cloud
- Univ. of Reading, Reading (United Kingdom)
- National Center for Atmospheric Research, Boulder, CO (United States)
The potential of spaceborne 94-GHz radar for measuring global vertical distribution and water content of ice clouds is assessed. Longwave (LW) fluxes for model ice clouds are calculated and used to determine minimum cloud optical depths that will change outgoing longwave radiation or flux divergence within a cloud layer greater than 10 W m{sup -2}, and in surface downward LW flux greater than 5 W m{sup -2}, compared to clear-sky value. Optical depth values are used to define radiatively significant clouds. Thresholds of radiative significance are calculated for radiation parameters and for tropical and midlatitude cirrus clouds. Observational data of ice crystal size spectra from midlatitude and tropical cirrus are used to assess radar capability to meet measurement requirements. A radar threshold of -30 dBZ should detect 99% (92%) of radiatively significant clouds in the midlatitudes. Detection efficiency may be reduced significantly for tropical clouds at very low temperatures (-80 C). LW flux calculations also establish the optical depth accuracy required to estimate LW fluxes or flux divergence. Accuracy requirements are also given in terms of ice water content (IWC) for validating cloud parameterization in general circulation models (GCMs). IWC estimates are derived using radar and additional information to define mean crystal size. IWC for samples with a horizontal scale of 1-2 km has a bias of less than 8%. For IWC larger than 0.01 g m{sup -3}, random error is from +50 to -35%; for 0.001 g m{sup -3}, random error is between +80 and -45%. This is also the best achievable accuracy for cloud optical depth estimates and meets requirements derived from LW flux calculations. Without independent particle size information, random error is from +85 to -55% for IWC greater than 0.01 g m{sup -3} and estimated bias is less than {plus_minus}15%. This accuracy is sufficient to provide useful constraints on GCM cloud parameterization schemes. 34 refs., 15 figs., 6 tabs.
- OSTI ID:
- 273954
- Journal Information:
- Journal of Applied Meteorology, Vol. 34, Issue 11; Other Information: PBD: Nov 1995
- Country of Publication:
- United States
- Language:
- English
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