Determining Geostrophic Winds Using a Satellite‐borne Infra‐red Radiometer

Abstract

White Sands Missile Range, New MexicoETERMINING the wind field using current satellite instrumentation is very D difficult even under optimum conditions. Two techniques at present receiving much attention are the inference of winds by clouds or moist tongue motion (via visible and infra-red data) and the use of the correspondence between sea-surface wind and the height of sea waves, the latter being inferred from the reflectivity of the sea surface in the visible or the emissivity of the sea surface in the microwave (Ohring 1972). These techniques are primarily limited by the availability of suitable cloud tracers in the first case and to retrieving only seasurface winds in the other. Other obvious problems include the apparent motion of clouds caused by dissipation and building and the difficulty of accurately determining cloud height to locate the winds vertically.It appears feasible that a useful approximation to the wind field for much of the atmosphere might be calculated from data obtained by those satellitebased infra-red radiometers that are specifically designed to sound the atmospheric thermal structure. The wind that might be obtained using such instruments is the geostrophic wind, which provides a reasonable approximation to the actual wind for a wide range of probable meteorological conditions for latitudes greater than 10"-20" north or south and altitudes greater than 1-2 km. THE GEOSTROPHIC APPROXIMATIONIn the geostrophic approximation it is assumed that the significant horizontal forces in the atmosphere are the pressure gradient and Coriolis forces, and that these forces are in equilibrium. Thus, the geostrophic wind resulting from these approximations is given by (see Holton 1972) + where k is the unit vertical vector, p is the density, ,,p is the horizontal pressure gradient, and f=2Q sin 4, the Coriolis parameter (where Q and q5 are the angular velocity of the earth and the latitude, respectively). Another useful form of the geostrophic equation is easily obtained from (1) and the hydrostatic equation : g-Vg =-k X VpZ, f -rwhere Z is the geopotential height, g is the acceleration due to gravity, and D is the gradient along constant pressure surfaces. 191