RADIATIVE-DYNAMICAL CONSEQUENCES OF DRY TONGUES IN THE TROPICAL TROPOSPHERE

Dry layers are frequently observed in atmospheric soundings from the c limatologically humid western Pacific warm pool region. Some 2400 soun dings from the TOGA COARE field program were objectively examined for humidity drops (layers in which relative humidity decreases rapidly wi th height), indicative of the bases of dry layers. These occur through out the lower and middle troposphere, with frequency peaks near 550 (t he 0 degrees C level), 800, and 950 mb. A composite constructed from t hese sounding data indicates the following. 1) Almost all dry layers a re too dry (and not warm enough) to be interpreted as conservative ver tical displacements. Rather, they apparently consist of filaments or t ongues of low moist static energy air advected into the column, often from the subtropics. 2) Dry tongues are anomalously virtually warm nea r their bases with a slight cool layer below; that is, they sit atop s harp stable layers or inversions. The authors hypothesize that radiati on is responsible for the thermal structure of dry tongues. The radiat ive effects of humidity structures in the troposphere are reviewed and illustrated. A composite-derived radiative heating perturbation, acti ng for 3.5 days in an idealized model of a dry tongue similar to 300 k m in width (values consistent with case studies), reproduces fairly we ll the high vertical wavenumber components of the composite thermal st ructure. Dynamics acts to spread the effect of the radiative heating p erturbation over a wider area and to concentrate the temperature pertu rbations near the dry tongue base, as observed. The deep layer-mean wa rmth of the composite dry tongue arises from a slight correlation betw een dry tongue occurrence in this dataset and a similar to 1 degrees C global-scale intraseasonal variation of tropical tropospheric tempera ture. A dry tongue affects convective clouds both directly, through it s thermal structure, and indirectly, through dry air entrainment. Low- level dry tongues can prevent deep convection outright, while the stab le layers associated with dry tongues at higher altitudes may cause co nvection to detrain mass. Humidity drops, stable layers, and a proxy f or layer clouds all have similar altitude distributions.