Various Paragraphs

George J. Huffman

13 August 2008

Acronyms

GPCP / Global Precipitation Climatology Project
SG / Satellite-Gauge monthly combined precip product
1DD / One Degree Daily precip product
TMPA / TRMM Multi-satellite Precipitation Analysis
TRMM / Tropical Rainfall Measuring Mission
MW / microwave (sensor channel)
IR / infrared (sensor channel)
HQ / High-Quality (MW) precip estimate
VAR / Variable Rainrate (IR) precip estimate
TMPA-RT / Real-Time version of the TRMM Multi-satellite Precipitation Analysis
3B40RT / PPS identifier for HQ product in real time
3B41RT / PPS identifier for VAR product in real time
PPS / Precipitation Processing System
PMM / Precipitation Measuring Missions
GPM / Global Precipitation Measurement project
3B42RT / PPS identifier for TMPA-RT combined MW-IR product
TOVAS / TRMM Observations Visualization and Analysis System
ASCII / Text, or character data
GDISC / Goddard Data and Information Services Center
CoCoRaHS / Community Cooperative Rain, Hail, and Snow network

Satellites Estimate Precip, Rather than Measuring It

Although we sometimes say that particular satellites measure precipitation, more precisely satellites measure the radiant energy in various parts of the electromagnetic spectrum that allow scientists to estimate precipitation. As the energy upwells from the Earth’s surface through the atmosphere, it is modified by the gases, aerosols, clouds, and precipitation that make up the atmosphere, and the satellite channels are chosen to be sensitive to various combinations of those things. Some channels are “window” channels, relatively insensitive to gasses and aerosols and mostly responding to the highest-elevation object that the satellite sees, either a dense cloud or the surface. Examples include visible and infrared channels. Other channels sense the precipitation particles. In some cases, the channels respond to the emission of additional radiant energy by the liquid precipitation, while others detect reductions in the upwelling radiant energy due to icy precipitation particles scattering away the upwelling energy. Examples include microwave channels. The microwave sensors are more accurate, but fly only on satellites that have “low” altitude orbits and therefore provide only occasional snapshots of any particular spot on Earth. These and other issues prevent completely accurate estimates of precipitation from satellites and emphasize the need to consider satellite results only as estimates.

What Does a Typical Global Precipitation Picture Look Like?

<this could point to the lower “week accumulation image” on trmm.gsfc.nasa.gov> At the largest scales, namely global images averaged over weeks or longer, the precipitation takes on a fairly stable pattern. There is a narrow band of nearly continuous moderate-to-heavy precipitation that encircles the Earth fairly close to the Equator, the Inter Tropical Convergence Zone. At slightly higher latitudes there tend to be large areas of very light precipitation, located in the “subtropical high pressure” centers. In mid-latitudes the precipitation is again systematically higher, due to the repetitive occurrence of low pressure centers and frontal systems in the “storm tracks”. Animations of average monthly data around the annual cycle reveal systematic shifts of these basic features with the seasons, principally showing that the precipitation maxima tend to move north-south as the Sun moves north-south during the year. The largest departure from this usual picture occurs in the Equatorial Pacific Ocean in El Niño and La Niño events, which occur every 3-7 years and take most of a year to run their course.

<this could point to the upper “3-hr image” on trmm.gsfc.nasa.gov> Moving to successively shorter time scales reveals that precipitation occurs in discrete patterns, as you would expect from your personal experience, which makes it hard to infer the broadscale patterns that become apparent with enough time averaging. In the tropics, the systems tend to appear as relatively shapeless blobs, except for the occasional tropical cyclone. At higher latitudes the precipitation is frequently organized into lines and arcs in the vicinity of fronts and low pressure systems. Even at higher latitudes, in the absence of strong dynamical organization strong convection will also appear in blobs. At time scales shorter than a day, one sees the typical cycles of precipitation that occur over the course of the day. These diurnal cycles occur more or less strongly depending on location and season, but land areas generally have stronger diurnal cycles than the oceans. Tropical and subtropical coastal areas tend to show land/sea breeze effects that are coupled. When you view the image loop of 3-hourly precipitation on trmm.gsfc.nasa.gov, the “flashing” of individual features partly represents real short-term variability, but mostly it arises from disagreements among the satellite estimates.

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