Description

The ER-2 X-band Doppler Radar (EXRAD) is a dual-beam, 9.6 GHz X-band Doppler radar used for airborne weather observations. It is designed to operate at high altitudes, onboard the NASA ER-2 research aircraft (ER-2). EXRAD operates using two offset-feed parabolic antennas including a fixed nadir-pointing antenna and a scanning antenna that can scan conically or cross-track (Figure 1). EXRAD was originally built to operate onboard Global Hawk, however, it was reassigned to operate primarily onboard the ER-2 to replace the aircraft’s aging ER-2 Doppler Radar (EDOP). EXRAD and EDOP are very similar instruments and therefore have comparable measurement capabilities. The main difference is the conical/cross-track scanning antenna that is included with EXRAD compared to the fixed forward-pointing antenna with EDOP.

Weather radars use electromagnetic radiation to detect meteorological targets in the atmosphere. EXRAD transmits radiation at a frequency of 9.6 GHz (X-band), which gives it high-sensitivity to precipitation while also maintaining the ability to penetrate the precipitation without extensive attenuation. In addition, X-band is ideal for airborne observations because the relatively small wavelength allows for a smaller antenna diameter that is more suitable for operation on an aircraft. EXRAD’s Doppler capabilities allow it to measure the velocity of air motions within precipitation. The conical/cross-track scanning antenna enables the calculation of horizontal winds flowing along and across the aircraft flight path, provides a wider field-of-view for observations, and allows the characteristics of precipitation particles such as orientation and phase to be determined. The nadir-pointing beam measures vertical motions beneath the plane using the vertical velocities of precipitation particles. Together, the dual-beam measurements allow the 3-dimensional structure of precipitation and winds below the plane to be constructed (Figure 2). These EXRAD data are used to improve and validate satellite precipitation estimates and to examine various weather phenomena such as convective precipitation systems and tropical cyclones.

Measurements

Measures the backscatter of precipitation particles in the atmosphere to determine the radar reflectivity, Doppler velocity, and linear depolarization ratio. Reveals the 3-dimensional structure of precipitation and air motions below the aircraft. Provides information on the phase, orientation, size, and other characteristics of precipitation particles.

Applications

• Satellite validation
• Precipitation and cloud studies
• Hurricane research
• Weather observations