Various Paragraphs
George J. Huffman
13 August 2008
Acronyms
GPCP
SG
1DD
TMPA
TRMM
MW
IR
HQ
VAR
TMPA-RT
3B40RT
3B41RT
PPS
PMM
GPM
3B42RT
TOVAS
ASCII
GDISC
CoCoRaHS
Global Precipitation Climatology Project
Satellite-Gauge monthly combined precip product
One Degree Daily precip product
TRMM Multi-satellite Precipitation Analysis
Tropical Rainfall Measuring Mission
microwave (sensor channel)
infrared (sensor channel)
High-Quality (MW) precip estimate
Variable Rainrate (IR) precip estimate
Real-Time version of the TRMM Multi-satellite Precipitation Analysis
PPS identifier for HQ product in real time
PPS identifier for VAR product in real time
Precipitation Processing System
Precipitation Measuring Missions
Global Precipitation Measurement project
PPS identifier for TMPA-RT combined MW-IR product
TRMM Observations Visualization and Analysis System
Text, or character data
Goddard Data and Information Services Center
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?
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<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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