The Tropical Rainfall Measuring Mission (TRMM) is a joint U.S.-Japan satellite mission to monitor tropical and subtropical precipitation and to estimate its associated latent heating. TRMM was successfully launched on November 27, at 4:27 PM (EST) from the Tanegashima Space Center in Japan.

The TRMM Precipitation Radar (PR), the first of its kind in space, is an electronically scanning radar, operating at 13.8 GHz that measures the 3-D rainfall distribution over both land and ocean, and defines the layer depth of the precipitation.

PR 2A23 produces a rain/no-rain flag. Its main objectives are (1) to detect bright band (BB), (2) to classify rain type, and (3) to detect warm rain.

2A23 uses two different methods for classifying rain type: (1) vertical profile method (V-method) and (2) horizontal pattern method (H-method).  Both methods classify rain into three categories: stratiform, convective, and other. To make the results user-friendly, 2A23 outputs a unified rain type. Further information about 2A23 can be found in Awaka et al. (1998).

The V-method starts with the detection of BB. This detection is made by a spatial filter, based on the second derivative of Z with respect to range, and by imposing several conditions on BB (e.g., Z above BB peak should decrease appreciably, the height of BB should appear almost at the same height). One of the major conditions imposed on BB is that the height of BB must be located in a BB window, whose range is from freezH - 1.5 km to freezH + 1.5 km, where freezH is the height of freezing level estimated from a climatological surface temperature at sea level, Tsurface, by the following formula:

  freezH = Tsurface / Tlapse,

and where Tlapse is the lapse rate of temperature (2A23 assumes that Tlapse = 6.0 (deg/km)).

After the BB detection, the V-method goes on to classify rain type. The outline of rain type classification by the V-method is as follows:

  1. When BB exists, rain is basically classified as stratiform.

  2. When BB is not detected, and maximum value of Z at a given angle bin exceeds a convective threshold, rain type for this angle bin is classified as convective.

  3. Other type is defined as not-stratiform and not-convective.

It should be noted that other type of rain by the V-method is defined as not-convective and not-stratiform, i.e., (1) there exists appreciable radar echo, but it is not strong enough to be convective and (2) BB is not detected.

The H-method also classifies rain into 3 categories: stratiform, convective, and other. However, their definitions are different from those of the V-method.

The H-method is based on the University of Washington convective/stratiform separation method (Steiner et al., 1995), which examines the horizontal pattern of Z at a given height, where Z has a 2 km horizontal resolution. In 2A23, the following modifications are made:

  1. Instead of examining a horizontal pattern of Z at a given height, a horizontal pattern of Zmax is examined; here, Zmax is the maximum of Z along the range for each antenna scan angle below freezH (minus 1 km margin).

  2. Parameters are changed so that they may be suitable for the TRMM data with 4.3 km horizontal resolution. The parameters were chosen before the launch of TRMM, using test Ground Validation (GV) data in such a way that the 4.3 km resolution data produce almost the same result as that with a 2 km resolution data.
   
  3. Other type of rain is introduced to handle noise.

In the H-method, detection of convective rain is made first. If one of the following conditions is satisfied for a pixel, which corresponds to the angle bin data being considered, it is judged that the pixel is a convective center: (1) Zmax exceeds a convective threshold or (2) Zmax stands out against the background area.

Rain type for a convective center is convective, and rain type for the pixels nearest to the convective center is also convective.

In the H-method, if rain type is not convective and if the...