The seventh tropical depression of the 2009 Atlantic Hurricane Season was born  yesterday at about 5 a.m. EDT, 160 miles south of the Cape Verde Islands.

Fred strengthened into a tropical storm by 11 p.m. EDT that evening.  Fred’s forecast track will keep the storm at sea, mostly in the east Atlantic – far from the United States making it more difficult to monitor and provide accurate forecast data since it’s out of reach from land based Doppler radar, and even the Hurricane Hunters.

But, thanks to NASA’s TRMM satellite and its on-board Precipitation Radar – forecasters have some amazing technology to help provide more reliable and accurate forecast.

The  TRMM Satellite is providing meteorologist with a lot of unique opportunities to study tropical rainfall rates, and tropical cyclones.   TRMM (Tropical Rainfall Measuring Mission) has an on-board radar that can scan storms with incredible accuracy from space.

The radar operates at 13.8 Ghz and has a minimum measurable rain rate of 0.5 mm/h with a range resolution of 250 m, a horizontal resolution of about 4 km, and a swath width of 220 km. A 128-element active phased array system is adopted to achieve contiguous scanning within the swath.

This incredible technology allows forecasters to have an upper-hand when tropical cyclones are too far from land for a land-based Doppler radar’s beam to reach the storm.  And while using satellites such as GOES is nothing new for tropical forecasters – the TRMM’s Precipitation Radar is the first of its kind.

GOES satellite data yields an incredible amount of information about tropical cyclones, and they are still the primary remote sensing tool used by tropical forecasters.  This is because the GOES satellites are geo-synced to the Earth’s location and provide reliable data – with an update at about every 15 minutes.

The TRMM is not a “fixed location” satellite, its orbit is circular and is at an altitude of 218 nautical miles (350 km) and an inclination of 35 degrees to the Equator.  The spacecraft takes about 91 minutes to complete one orbit around the Earth. This orbit allows for as much coverage of the tropics and extraction of rainfall data over the 24-hour day (16 orbits) as possible.

Technology Challenges and Advances

Among the three primary instruments on TRMM, the most innovative is the Precipitation Radar. Other instruments similar to the TRMM Microwave Imager (TMI) and the Visible and Infrared Scanner (VIS) VIRS have operated in space before, but to date there has not been any radar in space for the purpose of measuring rainfall.

Although weather radars on the ground have been used ever since World War II to estimate rainfall, there were many technical challenges that had to be overcome before an instrument of this kind could be used from space.

Power

A fundamental requirement is ensuring that the spaceborne radar has enough power to detect the weak return echo from the rain drops when seen from TRMM’s orbital height of 250 miles (402 kilometers) above the Earth.

3-D Resolution

Another challenge is to gather high resolution three-dimensional maps of the rain during the brief time that the satellite overflies local storms. To meet these demands, Japan’s Communications Research Laboratory elected a radar frequency about three times higher than that of a typical ground-based radar. Despite the restrictions on the size of the antennas in space, the use of a higher frequency provides good resolution and high quality images of storms. An active-phased array antenna, and sophisticated signal- processing techniques are used to scan the antenna beam electronically and rapidly, while ensuring that the transmitted and received radar pulses are synchronized. Solid state power amplifiers (128) are used both to conserve power and to provide a design that is robust.

Focusing the Radar Beam

Another problem to be overcome for the spaceborne radar was the need to produce a narrow radiating beam so that the target area would be small enough to bring out the features of interest on the ground— that is, good ground resolution. And still another problem was to be able to make the beam sweep out a path on the ground that would be wide enough to give good coverage as the satellite moves along its orbit. All of these design considerations were met through developments at Japan’s Communications Research Laboratory. Both the narrow beam size and the ability to steer the beam are achieved through the design of a “phased array” antenna, which uses electronic steering of the beam.

The Tropical Rainfall Measuring Mission is NASA’s first mission dedicated to observing and understanding the tropical rainfall and how this rainfall affects the global climate. It is a joint mission with JAXA of Japan. The primary instruments for measuring precipitation are the Precipitation Radar, the TRMM Microwave Imager, and the Visible and Infrared Scanner. Additionally, TRMM carries the Lightning Imaging Sensor and the Clouds and the Earth’s Radiant Energy System Instrument. These instruments all can function individually or in combination with one another.

Tags: forecasting, satellite, technology