Chapter 20
TRMM: The Tropical Rainfall
Measuring Mission
Anthony D. Del Genio
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025
USA
20.1 Introduction
Precipitation is one of the most important parts of both the global water and energy cycles,
yet it is one of the most poorly observed climate parameters. Globally, the net latent heating
associated with precipitation accounts for about 80% of the total diabatic heating required to
balance radiative cooling to space. The spatial pattern and temporal variability of this heating
serves as the primary driver for the tropical general circulation. The local balance between precipitation and evaporation over ocean helps determine the thermohaline component of the ocean
circulation and may influence both tropical El Nino-Southern Oscillation (ENSO) events and
high latitude deep water formation. Over land, the precipitation-evaporation balance controls
the tendency for drought to occur and directly influences the surface temperature by altering
the relative contributions of sensible and latent heat fluxes to the surface energy budget. Within clouds, precipitation formation is thought to be the major sink of cloud water and thus
affects the reflectance of bright tropical cumulus anvils. Drizzle formation in low-level marine
stratocumulus may help detach those clouds from surface moisture fluxes as the drizzle evaporates in the sub cloud layer, thereby contributing to the stratocumulus-trade cumulus transition.
Land- ocean differences in precipitation efficiency, associated with different concentrations of
nucleating aerosols, may cause cloud feedback in a changing climate to differ for marine and
continental clouds.
Despite its importance to the climate system and its practical consequences in our everyday
lives, current precipitation data sets are woefully inadequate. Global precipitation rates are
uncertain to at least 20%, and monthly precipitation estimates over oceans can disagree by
50% or more depending on the estimation technique (cf. Arkin and Meisner, 1987). The main
problem over oceans is the lack of direct sampling of precipitation. Existing ocean climatologies
historically were simple extrapolations from coastal measurements or were heavily weighted
by a few island stations which may give biased estimates due to the different atmospheric
stability structure and diurnal cycle over land and ocean. More recently, ocean rainfall has
NATO ASI Series. Vol. 145
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
@Springer-VeriagBerlinHeidelberg 1996
TRMM: The Tropical Rainfall
Measuring Mission
Anthony D. Del Genio
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025
USA
20.1 Introduction
Precipitation is one of the most important parts of both the global water and energy cycles,
yet it is one of the most poorly observed climate parameters. Globally, the net latent heating
associated with precipitation accounts for about 80% of the total diabatic heating required to
balance radiative cooling to space. The spatial pattern and temporal variability of this heating
serves as the primary driver for the tropical general circulation. The local balance between precipitation and evaporation over ocean helps determine the thermohaline component of the ocean
circulation and may influence both tropical El Nino-Southern Oscillation (ENSO) events and
high latitude deep water formation. Over land, the precipitation-evaporation balance controls
the tendency for drought to occur and directly influences the surface temperature by altering
the relative contributions of sensible and latent heat fluxes to the surface energy budget. Within clouds, precipitation formation is thought to be the major sink of cloud water and thus
affects the reflectance of bright tropical cumulus anvils. Drizzle formation in low-level marine
stratocumulus may help detach those clouds from surface moisture fluxes as the drizzle evaporates in the sub cloud layer, thereby contributing to the stratocumulus-trade cumulus transition.
Land- ocean differences in precipitation efficiency, associated with different concentrations of
nucleating aerosols, may cause cloud feedback in a changing climate to differ for marine and
continental clouds.
Despite its importance to the climate system and its practical consequences in our everyday
lives, current precipitation data sets are woefully inadequate. Global precipitation rates are
uncertain to at least 20%, and monthly precipitation estimates over oceans can disagree by
50% or more depending on the estimation technique (cf. Arkin and Meisner, 1987). The main
problem over oceans is the lack of direct sampling of precipitation. Existing ocean climatologies
historically were simple extrapolations from coastal measurements or were heavily weighted
by a few island stations which may give biased estimates due to the different atmospheric
stability structure and diurnal cycle over land and ocean. More recently, ocean rainfall has
NATO ASI Series. Vol. 145
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
@Springer-VeriagBerlinHeidelberg 1996