An Examination of the Diurnal Cycle in Oceanic Tropical Rainfall Using Satellite and In Situ Data

Diurnal variations in tropical cold cloudiness are examined for the period 1986–90 for each 2.5° latitude–longitude area in the global Tropics. The fractional coverage of cold cloudiness, as determined from various IR brightness temperature thresholds, has been used as a proxy for tropical convective precipitation, as direct observations of rainfall are unavailable for much of the earth, especially over the oceans. Variations in fractional coverage of cold cloud for three different temperature thresholds are examined: 235.225, and 215 K. The results of this study indicate that cold cloud is most frequently observed over land between 1800 and 2100 local time and is independent of the temperature threshold used. Over the tropical oceans, however, the time of maximum occurrence of cold cloud varies substantially with the temperature threshold employed. Coldest cloud-top temperatures (< 215 K) are found to occur much earlier in the day than warmer cloud tops and peak between 0300 and 0600 local time, which is consistent with many earlier limited-area studios. This observation is further confirmed from precipitation intensity differences between morning and evening observations from microwave satellite data. An interesting out-of-phase relationship between oceanic and continental convection is also discussed. Ship reports of weather type from the Comprehensive Ocean Atmosphere Data Set are examined as are hourly rainfall amounts from optical rain gauges on moored buoys that were deployed for the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment. Both of these data sources also indicate the preference for predawn oceanic heavy rainfall and convective activity. A cursory examination of the diurnal variations in short-range (6 h) rainfall forecasts from the National Meteorological Center Medium-Range Forecast Model are compared with the satellite and in situ results. The daily variations of these forecasts, which are made four times daily, indicate that the diurnal behavior of the model is in reasonable agreement with that of the satellite and in situ observations.