Skip to content. | Skip to navigation

Personal tools
You are here: GES DISC Home Precipitation Additional Features Science Focus TDST_SCI TRMM DATA: PRIMARY STATISTICAL PARAMETERS


Page Contents:

Primary Statistical Parameters

Mean and variance of precipitation are two primary parameters. Mean shows an averaged amount of precipitation during a studied time period, whereas variance shows an averaged amount of precipitation variation departing from this mean.Mean is calculated as:

Ri = S ( Rit ) / N ,

here, Rit denotes precipitation value at a space point i on a measuring time t. S meansadding the all N samples of precipitation values for a chosen time domain.

The formula of standard deviation of the variance is:

s(Ri) = { S ( Rit - Ri)2 /N}0. 5

The unit of the standard deviation is of the unit of precipitation itself, such as millimeter per day.

Basic Statistics of TRMM Precipitation Data

The mean annual rainfall, seasonal means, and seasonal variance calculated from TRMM 3B43 are displayed below.TRMM 3B43precipitation estimates are on a calendar month temporal resolution and a1-degree by 1-degree spatial resolution extending from 40oS to 40 oN. The data cover the period of Jan. 1998 - Oct. 2000. The seasonal means were calculated as: first, themeans of the 34-month period for each month were computed; then the three means over a season were averaged to get the seasonal mean. DJF denotes December, January, and February. MAM denotes March, April, and May. JJA is the summer season with June, July, and August, and SON denotes the fall season with September,October, and November. The seasonal variance is also based on the deviation from the monthly mean first, and then the deviation was averaged for the season in consistent with the way of seasonal means calculation.Please note that there is another way tocalculate seasonal means, that considers the all observation of each season as one group (for monthly mean data, there are 3 observations for a season), and calculate the mean for each season.The variancewas then calculated as the departure of each month value from this mean.

TRMM 3B43 includes precipitation rate (mm/hr) and its root-mean-square (RMS) error estimates derived from TRMM and other data sources.For more details of TRMM 3B43 algorithm, please link here TRMM 3B43 Data Description .

The mean annual precipitation (Jan. 1998- Oct. 2000) is plotted below, top panel for the region of 0oE - 180oE, and the bottom for the region of 180oW-0o.

[mean annual rainfall 98-00]

Main features:


  • Three distinct zones of heavy rainfall are located over Africa, South America, and the Southeast Asia.
  • The intertropical convergence zones (ITCZ) in the Atlantic and Pacific oceans are clearly evident.
  • The rainiest region in the tropics is located in the equatorial western Pacific Ocean (warm pool region) and the maritime continents, where annual rainfall exceeds 10 mm/day.
  • Pronounced dry regions are observed in the subtropical high pressure regions in the southern Atlantic, Southern Pacific and Southern Indian oceans, and over the Sahara and Arabian deserts, where less than 1 mm/day is estimated.

The following figures show the seasonal means that show the distribution of heavy rain regions and their movement with seasons:

[rainfall seasonal mean] [rainfall seasonal mean]


Main features:


  • During the DJF, the heavy rain regions take place over South Pacific Convergence Zone (SPCZ), warm pool region (equatorial West Pacific Ocean), the maritime continents, and Brazil.
  • During the MAM, rain belts over SPCZ moves northward, and rain occurs over the eastern Asia, denoting the beginning of Asian monsoon precipitation.
  • During the JJA, the India and Asia monsoon rain reach their seasonal maximum intensity, and the tropical convergence zones are very clear over Pacific and Atlantic. Over lands, precipitation occurs over the Northern Brazil and Northern Africa.
  • During the SON, the Asian monsoon precipitation disappeared, and the precipitation over warm pool region and the maritime continents developed.

The standard deviation of precipitation for each season show the significant variations in precipitation even the entire observational period is short (34 months), due to the El Niño (ending at early 1998) and La Niña (starting from late 1998) (see "Interannual Variation of Tropical Precipitation" below).

[rainfall seasonal standard variance] [rainfall seasonal standard variance]

The following figure shows the zonal mean of the precipitation on latitude-time domain.It clearly depicts the annual movement of heavy precipitation centers: over the Northern Hemisphere, heavy precipitation developed within the belt of 5o-15oN starting from later Spring until later autumn, whereas over the Southern Hemisphere, heavy precipitation developed within the belt of 0o-10oS starting from January until April.The precipitation intensity of the centers over the Southern Hemisphere is weaker than that over the Northern Hemisphere, except during the early 1998 corresponding to El Niño.

[zonal mean]

The overall zonal mean averaged over the 34-month period (below) shows two peaks corresponding to the precipitation center's annual movement between the 10oS-10oN, the one at the Northern Hemisphere has stronger magnitude.This reflects the precipitation intensity of the ITCZ.

[zonal mean]

The zonal means averaged separately for land and ocean show the interesting differences.The figure of zonal means over land shows the evolution of the precipitation centers with time and the strong connection of the precipitation centers between the two hemispheres. By contrast with the land the zonal mean over ocean shows isolated precipitation centers over the Northern Hemisphere, because that the location of ITCZ in general is over the Northern Hemisphere.Also, the precipitation over the Southern Hemisphere was weak corresponding to the La Nin˜ a event.

The climatological zonal mean over ocean and land shows the overall difference: over land the curve is of one peak with maximum at the south side of the equator, probably due to the contribution of South America's precipitation.Over ocean, the curve is of two peaks, similar to the overall zonal mean.

[zonal mean] [zonal mean] [zonal mean]

There was an El Niño and a La Niña event during the period of January 1998 to October 2000.The figure below shows the SST anomaly averaged over the belt of 5oS-5oN on the longitude-time domain.This SST data are from monthly NCEP SST analyzed fields.The monthly optimum interpolation (OI) fields are derived by a linear interpolation of the weekly OI fields to daily fields then averaging the daily values over a month. The analysis uses in situ and satellite SST's plus SST's simulated by sea-ice cover.Before the analysis is computed, the satellite data is adjusted for biases using the method of Reynolds (1988) and Reynolds and Marsico (1993).A description of the OI analysis can be found in Reynolds and Smith (1994). The SST anomaly is with respect to the mean of SST for the period of 1981-2000.The SST over the eastern Pacific was warmer than the normal by above 2oC during the early 1998. Starting from July 1998, the cold SST anomaly developed over the central Pacific Ocean and diminished around April 1999.Then the cold SST developed again around Oct. 1999 and last for a couple of months.

[SSTA longitude-time]

These significant SST variation impacts the atmosphere above it, precipitation variable, as one of the influenced fields, experienced anomalous variation with the same phase as that of the SST variation. The figure of precipitation variation over Equatorial Pacific (below) shows the heavy precipitation over the eastern Pacific during the early 1998.Corresponding to the long-duration of lower SST, the precipitation was relatively weak over the vast region from the dateline extending to the eastern Pacific Ocean.

[rainfall over equatorial
tropical region]

Document Actions
NASA Logo -
NASA Privacy Policy and Important Notices
Last updated: May 27, 2010 04:50 PM ET