Advancements in NEXRAD in Space (NIS)

Recent advances in space technology have made possible, within the next decade, the use of a sophisticated Doppler precipitation radar at Ka-band frequency for flight on a GEO satellite platform for observations of rain microphysics and kinematics within tropical cyclones (TCs), i.e., tropical storms and hurricanes, on a high-frequency time-lapse basis. The notional design would produce 1-hour sampling with 12 km spatial resolution at nadir, extending to ~14 km resolution at the radar's 4º scan limit -- yielding a 5300-km diameter Earth disk (equivalent to coverage of approximately 48º lat by 48º lon). Because of its Doppler capacity and repeat scan-imaging capability, the name NEXRAD-In-Space (NIS) has been chosen to denote its ground-radar heritage. The NIS radar is designed to measure line-of-sight (LOS) 35 GHz reflectivity and Doppler velocity profiles over the entire 5300-km Earth disk scene, with an along-beam resolution of 300 m enabled by pulse compression, and a sensitivity (minimum detectable signal) of 5 dBZ. Along with moderate and heavy precipitation, the relatively high sensitivity would enable detection of light rain, freezing rain, and precipitating ice hydrometeors -- including snow flakes, aggregates, and graupel. Doppler acuity is ~0.3 m s-1, and for off-nadir views when the actual horizontal velocity vector is not orthogonal to the viewing plane of the radar beam, it would be possible to recover horizontal winds along with the vertically-oriented velocities. The NIS radar samples by use of a 35-m diameter (illuminated over a 28-m diameter spot) spherical antenna made from a strong, lightweight mesh material in which two transmit-receive array pairs glide along a rotating arm (boom) atop the satellite facing the antenna, thus creating a dual-beam, spiral-feed combined profile image of both reflectivity and Doppler velocity. With this notional feed design, the combined images are repeated every 60 minutes, with an allowance for increased sampling frequency by adding a pair of secondary rotating arms to the main boom arm. In the case of a storm that moves beyond the notional scan limit of ±24ºlatitude, the radar-satellite system could be articulated up to ~2.5º which would extend the monitoring range to beyond ±40º latitude. Notably, the NIS design does not require rotation of the radar or of the spacecraft, and eliminates the complexity of electronic scanning.

The hourly scan frequency and Doppler velocity capacity of the NIS satellite provide more than just an evolutionary step in the ability to observe mesoscale processes around and within TC environments. Such observations could be taken of a storm from genesis through its breakup over land, i.e., from out-to-sea through to landfall and beyond. By combining high sensitivity reflectivity and Doppler velocity information, it would be possible to follow the 3-dimensional structure of a storm, acquiring dense observations which through direct data assimilation could guide the prediction of the storm with a high resolution, nonhydrostatic cloud resolving model. Such a model would be able to capture the various stages of storm intensification and precipitation in conjunction with eyewall replacement, which arise through mesoscale interactions involving: (a) surface heat and moisture fluxes, (b) fine-scale vorticity growth and transports, (c) fine-scale vertical fluxes of momentum and vorticity, (d) development and mergers of vortical hot towers (VHTs) and eyewall mesovortices, (e) formation of secondary eyewalls, and (f) mesoscale convergence of moisture within the TC boundary layer. Moreover, over the larger domain, the observations could capture outer storm features related to steering flow that might help establish better predictions of track and along-track velocity.