Humidity Profiles over the Ocean

Abstract

The distribution of water vapor in the atmosphere affects climate change through radiative balance and surface evaporation. The variabilities of atmospheric humidity profile over oceans from daily to interannual time scales were examined using nine years of daily and semidaily radiosonde soundings at island stations extending from the Arctic to the South Pacific. The relative humidity profiles were found to have considerable temporal and geographic variabilities, contrary to the prevalent assumption. Principal component analysis on the profiles of specific humidity were used to examine the applicability of a relation between the surface-level humidity and the integrated water vapor; this relation has been used to estimate large-scale evaporation from satellite data. The first principal component was found to correlate almost perfectly with the integrated water vapor. The fractional variance represented by this mode increases with increasing period. It reaches approximately 90% at two weeks and decreases sharply, below one week, down to approximately 60% at the daily period. At low frequencies, the integrated water vapor appeared to be an adequate estimator of the humidity profile and the surface-level humidity. At periods shorter than a week, more than one independent estimator is needed. High-frequency surface humidity can be estimated if additional information on the vertical structure of the humidity profile is available or if the integrated water vapor in the boundary layer, instead of the entire atmospheric column, can be measured accurately by spaceborne sensors.