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The SIR-C/X-SAR Rain Experiment

(last update: July 10, 2003)

The image above shows a vertical section through the atmosphere of the echoes measured by the SIR-C radar aboard the space shuttle Endeavour during a pass over Tropical Cyclone Odille off Madagascar in the Indian Ocean on April 11, 1994, around 12:42 GMT (at C-band with VV polarization). This was the first detection of rain from space using radar, a remarkable milestone in environmental remote sensing.

In order to make measurements such as those in the image above, we had to monitor the weather around the clock and around the globe during the two SIR-C flights, in April 1994 and October 1994. The capability to overlay real-time geostationary-satellite weather data (obtained thanks to the University of Hawaii and the University of Reading) with the projected Shuttle tracks was crucial in allowing us to predict and request 48-hours ahead of time a specific "data take":

METEOSAT IR image with Endeavour track overlaid
(11 April 1994, 13:00 GMT - 2.5 Megabytes)

As you can see, the pass over Odille could easily have turned into a near-miss. We had better luck during the second Shuttle flight. On two further occasions over the Western Pacific (orbits number 54 and 103),

  • our data take requests coincided with the passage of Endeavour over and across Typhoon Seth,
  • our request to have the Shuttle roll so the radar antenna beams were perpendicular to the Earth was granted,
  • and the data takes were executed with the correct radar operating parameters!

The resulting images are spectacular. In general, rain falls into two general categories, namely

  • "convective cells" extending vertically over a significant fraction of the troposphere, and
  • "stratiform rain" with negligible vertical air motion, in which the precipitation is largely confined to the lower troposphere.
The former are associated with more intense precipitation, and, consequently, greater backscattered signals over several kilometers, while the latter are usually characterized by weak signals with the strongest backscatter occurring near the melting level.

The 3 panels above show vertical sections (through the atmosphere) of the radar reflectivities measured over a convective cell during the beginning of data take 103.0, at C-, X- and L-bands (VV polarization). The X-band signal shows some residual contamination by the echo from the surface. The blue horizontal tick marks are 3 km apart.

These 3 panels show vertical sections of the radar reflectivities measured over another convective cell towards the end of data take 103.0, at C-band (VV and VH polarizations), and L-band (VV polarization). The cross-polarized returns are due to "frozen hydrometeors", located mostly near the freezing altitude but showing some spread around it due to the convection in the cell.

The 4 panels above show vertical sections of the radar reflectivities measured over a stratiform system during data take 54.0, at C-band (VV and VH polarizations), and X- and L-bands (VV polarization). The "bright band" at the freezing level is quite visible, particularly on the cross-polarized return.


To retrieve a figure-less copy of the preprint (accepted in Remote Sensing of the Environment, July 1996) in which SIR-C's observations of rain storms are described and analyzed:

Go to the top page of the JPL TRMM site

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