The 1997 Pan American Climate Studies Tropical Eastern Pacific Process Study. Part I: ITCZ Region

Abstract

The Pan American Climate Studies Tropical Eastern Pacific Process Study (TEPPS) obtained a comprehensive set of observations of the structure of clouds and precipitating storms over the eastern tropical Pacific from 28 July to 6 September 1997. The TEPPS data can address a wide range of problems involving tropical oceanic clouds and precipitation. The main goal of the project was to understand why passive microwave satellite algorithms indicate an E?W gradient in the precipitation pattern in the tropical Pacific with heavier rainfall in the east while infrared satellite algorithms indicate heavier rainfall in the west. Satellite?derived precipitation estimates are based on characteristics of the vertical structure of precipitating clouds: in the case of infrared methods, cloud?top temperature, and in the case of microwave methods, the vertically integrated ice scattering and/or water absorption determined by the vertical profile of hydrometeors. The premise of the expedition was that by obtaining surface?based radar measurements of the vertical structure of precipitation where and when the differences between the infrared and microwave precipitation estimates were large, it could be determined which satellite method yielded a more accurate pattern of precipitation in the Pacific. This paper describes the types of observations obtained during TEPPS and some preliminary results. A single, well?equipped vessel on its maiden voyage, the National Oceanic and Atmospheric Administration ship Ronald H. Brown, was the base for all observations. Scanning C?band Doppler radar and cloud photography documented the three?dimensional structure of clouds and precipitation in the vicinity of the ship. Upper?air soundings were obtained at £ 4 h intervals. Surface meteorological and oceanographic instruments and vertically pointing 915?MHz and S?band profilers characterized conditions at the ship itself. During 28.5 days in the eastern Pacific ITCZ, the shipborne radar observed echoes larger than 50 km in maximum horizontal dimension within 100?km radius of the ship 71% of the time and larger than 100 km 55% of the time. The ship spent 16 days on station at 7.8°N, 125°W and 4 days in the vicinity of Hurricane Guillermo. Samples of surface atmospheric and oceanic data collected during the cruise illustrate the difficulty of interpreting short timescale buoy data time series in the absence of the mesoscale context provided by radar data. The ship sounding data show that the larger?scale, longer?lived convective precipitation activity and organization on timescales of days in the eastern Pacific ITCZ is closely associated with the presence of stronger southerly winds, which in turn suggests that large?scale atmospheric processes such as easterly waves or inertial stability oscillations are a regulating mechanism. Comparison of the ship radar data, satellite IR data, and satellite microwave data shows that part of the reason why the IR and microwave?derived precipitation maps differ is that in the eastern Pacific ITCZ IR cold cloudiness resolves only a subset of the precipitation detected by microwave data. Large precipitating systems (> 100 km scale) of long duration (> 24 h; i.e., the mesoscale organized systems) were reliably associated with cold cloudiness < 235 K. Precipitating systems of shorter duration and/or smaller scale (i.e., the less?organized convection) sometimes reached 235 K and sometimes did not. Satellite microwave data generally agreed with the radar data regarding the location and areal coverage of precipitating regions larger than ~10 km in horizontal scale. However, the microwave algorithm examined in this study had varying degrees of skill in locating the heavier rainfall areas within rainy regions.