Because the airborne Doppler meteorological radar can provide the flexibility to observe the severe weather system, it plays an important role in studying the dynamic, thermal and microphysics structure of the squall line, heavy rainfall, typhoon and other severe weather systems. This paper reviews the development and the application of the airborne meteorological Doppler radar. The airborne Doppler radar technology development, some leading radars, scan strategy, wind retrieval, radar data assimilation and the adaptive observation are discussed.
There are four major airborne Doppler meteorological radars. They are WP-3D airborne Doppler meteorological radar (P-3 radar), ELDORA (Electra Doppler Radar/Analyses Stereoscopic par Impulsions Aeroporte) airborne dual-antennas Doppler weather radar, ER-2 Doppler radar (EDOP) and Wyoming Cloud Radar (WCR).
The P-3 and ELDORA radar operate at X-band with the antenna mounted in the tail of the aircraft. The two radars can provide the reflectivity, radial velocity and Doppler spectral data. They are vertically scanning Doppler radar. The X-band EDOP Doppler radar can provide the dual-polarization information that is useful for studying the microphysics structure of the rainfall system. Furthermore, because the ER-2 aircraft can fly at 20 km high, the EDOP radar can directly measures the vertical reflectivity and wind structure from the top of the deep convective systems. This is generally not possible with other low and medium-altitude airborne radar. The WCR radar is a dual-polarized Doppler radar that operates at 95-GHz (3 mm). It can provide the reflectivity, radial velocity, Doppler spectral and dual-polarization data that are very useful to investigate the dynamic and microphysics structure of the cloud and precipitation processes.
In order to retrieve the three-dimensional wind field with the dual-Doppler radar algorithm, it is desired to have the two Doppler speeds from the different directions. Some radar antenna scan methodologies were developed for getting the pseudo-dual Doppler data. In the 1980′s, the “L-Shaped” and “U-Shaped” flight patterns were designed for the P-3 single-antenna radar. This approach has been successfully used to study a wide variety of precipitation systems such as squall line, storm, mesoscale convective systems and hurricane rainband structure. A more efficient scan methodology is Fore/Aft Scanning Technique (FAST) and Alternative FAST (AFAST). The FAST/AFAST scan method can improve the data temporal resolution compared with the P-3 radar flight pattern scan method. The advantage of the AFAST over the FAST is that the effective horizontal data resolution is 0.7km instead of 1.4km. The P-3 and ELDORA weather radar can operate at FAST methodology. Because the ELDORA has dual-antennas, the data horizontal and temporal resolutions are denser than the P-3 radar′s. Many results showed that the retrieval wind filed with ELDORA radar data was more accurate than P-3 radar's.
The EDOP is dual-beam radar. It makes use of two fixed (non-scanning) radar beams: one points at nadir and the other pointes at approximately 33.5 forward of nadir. The high-altitude viewing perspective of EDOP and its close proximity to deep thunderstorm updrafts and downdrafts allow for relatively high-resolution vertical structure measurements when compared with the typical ground-based radars. The WCR radar is a fixed-beam multi-antennas airborne radar. It can operate at profiling scan mode with fixed antennas looking up and down from the aircraft, vertical-plane dual-Doppler (VPDD) scan mode with two beams lying within a vertical plane, and horizontal-plane dual-Doppler (HPDD) scan mode using two beams lying in a horizontal plane.
The airborne weather Doppler radar wind retrieval methodologies are classified as single-Doppler radar wind retrieval algorithms and dual-Doppler radar wind retrieval algorithms. Some single-Doppler radar wind retrieval algorithms were developed based on the P-3 radar special scan method. Because the dual-Doppler radar wind retrieval is more accurate, many researches focus on it. The three-dimensional wind field can be retrieved from the P-3 and ELDORA radar scan data. The synthetic multiple-Doppler method was used to retrieve the three-dimensional wind by P-3 radar data firstly. Furthermore, some dual-Doppler radar three-dimensional wind retrieval algorithms based on the variational methods were developed which improved the retrieval accuracy remarkably. Because the EDOP and WCR radar are the fixed-beam scanning radar, the two-dimensional wind filed in the fixed beam scan plane can be retrieved with dual-Doppler radar wind retrieval technology.
The preliminary study of airborne Doppler radar data assimilation experiment showed that it could improve the accuracy of the numerical forecast remarkably. The researchers should pay close attention to the airborne Doppler radar data assimilation and the adaptive observation with the airborne Doppler radar in the future. The format of the airborne radar observation data should be standardized. One of the important researches is to increase the accuracy of the retrieval vertical velocity. It can be resolved by means of improving the radar scan strategy and the wind retrieval method.
It addresses that next airborne meteorological radar is phased array weather radar with the rapid-scanning technology and the dual-polarization function. It can supply the temporal resolution and polarization information for understanding the three-dimensional dynamical, thermal and microphysics structure of the cloud and precipitation processes.