Active and passive remote sensing of precipitation over ocean surfaces

The Advanced Microwave Precipitation Radiometer is an acrosstrack scanning, four-channel, total power radiometer system specifically designed to mount and operate within the NASA ER-2 aircraft. Nadir resolutions on the Earth's surface are 2.8 km at 10.7 and 19.35 GHz, 1.5 km at 37.1 GHz, and 640 meters at 85.5 GHz. A stepper motor spins an offset reflector through 50 increments covering ±45° across-track, and directs the upwelling scene emission towards lens/ feedhorn assemblies. The can rate is sufficient to provide contiguous mapping at 85.5 GHz, with spatial oversampling occurring for the three lower frequencies. For an ER-2 flight speed of ≈13 km min^-1 and an altitude of 20 km, the total across-track swath width on Earth is ≈40 km. The AMPRR feedhorn is configured such that the receive polarization state smoothly varies from horizontal (pilot's right) to vertical (pilot's left) across each scan; consequently, the nadir state is a linear 45°. On 12 August 1991 during the Convection and Precipitation/ Electrification (CaPE) experiment near Kennedy Space Center, Florida, measurements were gathered with the AMPR and the National Center for Atmospheric Research (NCAR) CP-2 dual-polarization radar system.