Tropical Inversions near the 0°C Level

Soundings taken from the tropical western Pacific warm pool region during TOGA COARE reveal the common occurrence of temperature and moisture perturbations near the 0°C level. The perturbations frequently are characterized by shallow layers of increased stability (or occasionally temperature inversions) and reversals or inflections in the vertical profile of specific humidity. Similar temperature and moisture inversions have been observed elsewhere in the Tropics and midlatitudes but have not received much attention. Isothermal layers are known to exist just below the melting level in stratiform rain regions; however, not all stable layers observed over the warm pool are confined to precipitation systems. The perturbation in the specific humidity profile accounts for the often-observed double-peak structure in the apparent moisture sink Q2 in tie Tropics. Stratification of the data based on relative humidity criteria indicates that the stable layers near the 0°C level generally fall into two main classifications: anomalously cool–moist conditions at and slightly below the 0°C level and anomalously warm–dry conditions at and just above. The former occur primarily within or in close proximity to precipitating systems, suggesting they are a result of the direct effects of melting. Soundings in the latter group typically occur outside convective areas. Mechanisms for formation of the warm–dry stable layers are unclear at this time, but advective, radiative, gravity wave, and melting effects may all play some role. In some cases they may simply be remnant melting layers from past convection. There is evidence to indicate that the stable layers near the 0°C level affect tropical cloud populations. Convection impinging upon or penetrating the stable layers may detrain significantly near the 0°C level, thereby contributing to perturbations in the moisture profile there. Midlevel cloud layers that are commonly observed in the Tropics may be evidence of this detrainment. Another primary finding is the frequent occurrence of a trade wind stable layer over the warm pool. Though not widely recognized, this finding is consistent with the prevalence of trade cumulus clouds in the western Pacific region. The trade inversions often coexist with, but are distinct from, the inversions near the 0°C level.