Rita wind speeds derived from ASAR image

"Radar images penetrate through clouds and can easily detect the eye replacement cycle of hurricanes which are precursors to further intensification."

Rita was a maximum Category Five on the Saffir-Simpson Hurricane Scale when the ASAR image was acquired. As it continues west through the Gulf of Mexico it has weakened to a still-dangerous Category Four. Rita is expected to make landfall on the Gulf coast during the morning of 24 September.

ERS-2 joins in Rita observations

ERS-2 scatterometer data

The same day Envisat acquired its ASAR image of Rita, its sister spacecraft ERS-2 also made complementary observations of the hurricane's underlying wind fields using its radar scatterometer.

This instrument works by firing a trio of high-frequency radar beams down to the ocean, then analysing the pattern of backscatter reflected up again. Wind-driven ripples on the ocean surface modify the radar backscatter, and as the energy in these ripples increases with wind velocity, so backscatter increases as well. Scatterometer results enable measurements of not only wind speed but also direction across the water surface.

What makes ERS-2's scatterometer especially valuable is that its C-band radar frequency is almost unaffected by heavy rain, so it can return useful wind data even from the heart of the fiercest storms – and is the sole scatterometer of this type currently in orbit.

The ERS-2 Scatterometer results for Hurricane Rita seen here have been processed by the Royal Netherlands Meteorological Institute (KNMI). They are also routinely assimilated by the European Centre for Medium-Range Weather Forecasting (ECMWF) into their advanced numerical models used for meteorological predictions.

"Scatterometer data from the ERS-2 platform provide high-quality wind information in the vicinity of tropical cyclones," states Hans Hersbach of ECMWF. "For a Hurricane like Rita, the combination of such observations with [in-situ] dropsonde data enables the analysis system at ECMWF to produce an improved forecast."

NOAA using Envisat radar altimetry data

Envisat image acquired 22 September

Another Envisat instrument called the Radar Altimeter-2 uses radar pulses to measure sea surface height (SSH) down to an accuracy of a few centimetres.

Near-real time radar altimetry is a powerful tool for monitoring a hurricane's progress and predicting its potential impact. This is because anomalies in SSH can be used to identify warmer ocean features such as warm core rings, eddies and currents.

The US National Oceanic and Atmospheric Administration (NOAA) is utilising Envisat RA-2 results along with those from other space-borne altimeters to chart such regions of 'tropical cyclone heat potential' (TCHP) and improve the accuracy of Hurricane Rita forecasting.

Observing hurricanes

MERIS image acquired 20 September

A hurricane is basically a large, powerful storm centred around a zone of extreme low pressure. Strong low-level surface winds and bands of intense precipitation combine strong updrafts and outflows of moist air at higher altitudes, with energy released as rainy thunderstorms.

Envisat carries both optical and radar instruments, enabling researchers to observe high-atmosphere cloud structure and pressure in the visible and infrared spectrum, while at the same time using radar backscatter to measure the roughness of the sea surface and so derive the wind fields just above it.

Those winds converging on the low-pressure eye of the storm are what ultimately determine the spiralling cloud patterns that are characteristic of a hurricane.

Additional Envisat instruments can be used to take the temperature of the warm ocean waters that power storms during the annual Atlantic hurricane season, along with sea height anomalies related to warm upper ocean features.