Low-Level Inversions over the Tropical Pacific—Thermodynamic Structure of the Boundary Layer and the Above-Inversion Moisture Structure

Abstract

The structure of the boundary layer over a broad region of the equatorial Pacific is studied using dropwinsonde measurements made in January, February, May and June of 1979. Low-level inversions of sufficient strength to inhibit deep convection are found to be present in more than 50% of the soundings. These inversions appear to play a critical role in regulating convective activity over the central and eastern Pacific. The tops of the inversions have an average pressure level of approximately 300 mb and show little latitudinal or longitudinal variation. The majority of the inversion soundings (approximately 70%) have a reversal in the mixing ratio profile (qreversal) above the inversion that appears as a dry layer at the top of the inversion layer capped by a relatively moist layer. This moist layer is on the average 2 g kg?1 more moist than the corresponding soundings that have no qreversal. No systematic regional or temporal variations in the frequency of occurrence of the qreversal or the structure of the boundary layer associated with this feature were observed. In previous studies it was suggested that the qreversal could be formed by nearby convection that is penetrating to higher levels, moistening thou levels, and producing downdrafts that spread out at the top of the inversion as a dry layer. Differences between the thermodynamic structure of soundings with and that without the qreversal support this idea. It is suggested that relatively dry layers may form above inversions of all types and heights in areas where there is nearby convection and associated downdrafts.