A Combination of Microwave Observations from Satellites and an EOF Analysis to Retrieve Vertical Humidity Profiles over the Ocean

Abstract

A semistatistical retrieval technique is presented to derive humidity profiles over the oceans from passive microwave measurements. The procedure is based upon the vertical empirical orthogonal functions (EOFs) of the specific humidity extracted from a large sample of radiosonde measurements over the North Atlantic Ocean during the period from April to October 1979 (FGGE-year). The North Atlantic is divided into seven regions and the EOF-analysis is carried out for each region separately. The first three eigenvectors of the EOF-expansion explain up to 90% of the total variability within each region and it is shown that they are statistically significant and stable. The eigenvectors of the first order mainly describe variations of the total precipitable water (W), while the second and third-order EOFs are related to the ratio WG/W, with WG as the precipitable water of the planetary boundary layer, and the sea surface temperature (SST), respectively. This fact is used to develop a technique for the estimation of atmospheric moisture profiles assuming that W, WG, and SST can be retrieved from satellite observations. A comparison of the SMMR (Nimbus-7) derived humidity profiles using only W as input data yields a retrieval accuracy of 0.9 g kg?1 in the surface layer and 1.4 g kg?1 at 800 hPa, which corresponds roughly with the top of the PBL. Above this layer the retrieval error falls off rapidly. Case studies demonstrate the capability of the algorithm to resolve typical structures of the humidity field, e.g., synoptic scale disturbances in midlatitudes or the intertropical convergence zone (ITCZ) in the tropics.