THE ROLE OF WATER-VAPOR AND CONVECTION DURING THE CENTRAL EQUATORIAL PACIFIC EXPERIMENT FROM OBSERVATIONS AND MODEL SIMULATIONS

Field measurements from the Central Equatorial Pacific Experiment (CEP EX) conducted from March 7 to April 5, 1993, are used to study the lin k between water vapor, convection, and sea surface temperature (SST) i n a region of particular importance to global climate. The data are co mpared with results from a general circulation model (GCM). Three high -resolution simulations were carried out with slightly different initi al conditions using European Centre for Medium Range Weather Forecasts (ECMWF) analyses from March 1, 1993, and forced with the observed SST as lower boundary condition. Radiosondes released between the equator and 5 degrees S show a dry region east of the dateline collocated wit h a clear sky region observed from the Japanese Geostationary Meteorol ogical Satellite (GMS) in the first 2 weeks of CEPEX, which both vanis h in the second 2 weeks. The model is able to reproduce this change in convective activity, Moreover, a comparison of the anomalies of the r elative humidity profiles grouped according to SST and infrared bright ness temperature (IRBT) indicates that the GCM correctly simulates the observed behavior, The atmosphere is relatively moist when the SST is warm and low IRBTs occur at the same time, that is, when convection t akes place, and dryer than on average for the opposite case. In genera l, however, the model is too dry in the midtroposphere and too wet in the upper troposphere. Very good agreement is found between the simula ted and observed ice water content, in particular with respect to its increase with in-cloud temperature.