Vertical Air Motion Retrievals from Airborne W-band Cloud Radar

In-cloud vertical air motion must be quantified to describe cloud dynamics and their lifecycles. Knowledge of the updrafts and downdrafts in clouds is also critical for parameterizing cloud processes within models at different scales. Short-wavelength (Ka- and W-band) vertically-pointing Doppler radars are often used to observe clouds and investigate the parameters of the atmospheric boundary layer, including the vertical air motion and cloud microphysics. The Doppler velocities measured by cloud radars consist of both vertical air motions and fall speeds of the cloud and precipitation particles. A few methods have been developed to separate the two velocity components, either based on the radar-measured Doppler velocities or the entire Doppler spectra. However, those techniques were typically demonstrated with ground-based vertically pointing Ka-band radar measurements under specific field experiments. Compared to ground-based radar measurements, vertical velocity measurements from airborne radars may involve larger uncertainties due to aircraft motion and high flying speed. The different measurement quality and retrieval mechanism associated with airborne radar will pose challenges to the air motion retrieval techniques developed for ground radar Doppler measurements. This study presents a new, simple, but effective approach for estimating the in-cloud vertical air motion using airborne W-band radar measurements. This approach is applicable to both clouds with and without precipitation particles. First, we establish the relation between cloud and precipitation particle fall speed and radar reflectivity. Then, the particle fall speeds estimated from the radar reflectivity are compared with the radar-measured Doppler velocities to derive the vertical air motions. This technique is demonstrated with the W-band airborne radar measurements during the NOAA Atlantic Tradewind Ocean–Atmosphere Mesoscale Interaction Campaign (ATOMIC) from early January through mid-February 2020, as well as the ship-based W-band radar observation from the Dynamics of the MJO (DYNAMO) field campaign. Comparison with the results reported in the literature indicates that this simple airborne radar retrieval technique performs well and is suitable for real applications.