Radiative-Dynamical Consequences of Dry Tongues in the Tropical Troposphere

Abstract

Dry layers are frequently observed in atmospheric soundings from the climatologically humid western Pacific warm pool region. Some 2400 soundings from the TOGA COARE field program were objectively examined for humidity drops (layers in which relative humidity decreases rapidly with height), indicative of the bases of dry layers. These occur throughout the lower and middle troposphere, with frequency peaks near 550 (the 0°C level), 800, and 950 mb. A composite constructed from these sounding data indicates the following. Almost all dry layers are too dry (and not warm enough) to be interpreted as conservative vertical displacements. Rather, they apparently consist of filaments or tongues of low moist static energy air advected into the column, often from the subtropics. Dry tongues are anomalously virtually warm near their bases with a slight cool layer below; that is, they sit atop sharp stable layers or inversions. The authors hypothesize that radiation is responsible for the thermal structure of dry tongues. The radiative effects of humidity structures in the troposphere are reviewed and illustrated. A composite-derived radiative heating perturbation, acting for 3.5 days in an idealized model of a dry tongue ∼300 km in width (values consistent with case studies), reproduces fairly well the high vertical wavenumber components of the composite thermal structure. Dynamics acts to spread the effect of the radiative heating perturbation over a wider area and to concentrate the temperature perturbations near the dry tongue base, as observed. The deep layer-mean warmth of the composite dry tongue arises from a slight correlation between dry tongue occurrence in this dataset and a ∼1°C global-scale intraseasonal variation of tropical tropospheric temperature. A dry tongue affects convective clouds both directly, through its thermal structure, and indirectly, through dry air entrainment. Low-level dry tongues can prevent deep convection outright while the stable layers associated with dry tongues at higher altitudes may cause convection to detrain mass. Humidity drops, stable layers, and a proxy for layer clouds all have similar altitude distributions.