Tracking the Tropics With TRMM

EDIT: On 9 April, 2015, the TRMM satellite was turned off due to the exhaustion of its fuel supply after nearly 18 years of service. It burned up upon re-entry on 16 June, 2015.

Recently I wrote a post about NASA’s Terra and Aqua satellites, with a focus on the MODIS instrument they both carry. In this post I’ll focus on my favorite satellite: TRMM. The Tropical Rainfall Measurement Mission, or TRMM (pronounced like “trim”), is a joint mission between the US and Japan that revolves around a very unique satellite, also referred to a TRMM, launched in November of 1997.

The TRMM satellite and its instruments

The aim of the mission is to closely study tropical rainfall patterns, something that had never been done before. In order to constantly observe the tropics, and not waste time tracking over the mid-latitudes or the polar regions, TRMM has an unusual orbit path that is at a 35 degree angle to Earth’s equatorial plane as opposed to polar orbiting satellites which are at an angle close to 90 degrees, thus everything TRMM observes are in the Tropics or Subtropics. On board are five instruments: the Clouds and the Earth's Radiant Energy System (CERES), the Lightning Imaging Sensor (LIS), the Visible and Infrared Scanner (VIRS), the TRMM Microwave Imager (TMI), and the Precipitation Radar (PR).





CERES

LIS

The first two, CERES and LIS, are not directly part of the mission, they are instruments for other Earth observing purposes that were able to fit on the spacecraft. The other three instruments, VIRS, TMI, and PR, are designed to work together to paint a complete picture of rainfall.

VIRS

VIRS:
The VIRS package is a relatively standard system for observing weather in much the same way other meteorological satellites do. However, since it observes the same swath at the same time as the other instruments, VIRS allows for direct comparison of traditional IR and visible imagery with data obtained by the TMI and PR. For example, it can be used to see a particular cloud cluster, while the other instruments gather data on the rain (or ice crystals) that are being produced by those clouds.

TMI

TMI:
This microwave imager allows TRMM to look through the clouds and observe the structure at many different levels. The TMI gathers the same types of data that many other microwave observing meteorological satellites, but at a very high resolution. However, in order to make such detailed observations, the TMI’s swath is much narrower than that of the systems on other orbiters. Of the several frequencies of microwaves it can detect, perhaps the most notable are 85GHz and 37GHz, which provide a view of the details near the top of tall rain clouds and near the base of the clouds, respectively.

PR

PR:
The Precipitation Radar is TRMM’s primary instrument and the first space borne radar. With PR’s radar data the 3-D structure of clouds can be visualized and the intensity of precipitation anywhere within the clouds can be determined. These observations, along with data from the TMI, enable rainfall and the mechanisms that produce it to be understood in a more complete manner than ever before. One important characteristic of the PR is that unlike the other instruments on TRMM, or most satellites for that matter, is that it is an active system, as opposed to a passive one. This difference can be better understood with the aid of an analogy using more familiar tools. Suppose you are at the park and wish to take a photo of a tree. All that must be done is to press a button, the camera’s shutter will open for a split second, light will enter the camera, and the image of the tree will be recorded on the film. This example would be analogous to the TMI recording the microwaves that it collects. Now suppose that at the park is a gathering of all your family and extended family and you wish to take a group photo. First you would have to go around the park and gather all your family members together and arrange them for the picture, then you would set the timer on the camera and race over to the group so you would also be in the photograph, finally, after the camera takes the picture and the film is developed you will get to see how the picture turned out. This situation is sort of like the PR which has to first emit a beam that is scatted by precipitation , then it must receive any of the beam that is scattered back toward TRMM and use it determine the location and intensity of precipitation.






TRMM PR data of Hurricane Isaac

One significant use of TRMM observations is in the study of tropical cyclones. Using PR data the full 3-D structure of the storms’ features, such as the eyewall and rainbands, can be analyzed based on the precipitation within them. Databases now exist that provide this data for every time the PR’s swath passed over a tropical cyclone.