Vertical Motions in Orographic Cloud Systems over the Payette River Basin. Part I: Recovery of Vertical Motions and Their Uncertainty from Airborne Doppler Radial Velocity Measurements

Vertical motions over the complex terrain of Idaho’s Payette River basin were observed by the Wyoming Cloud Radar (WCR) during 23 flights of the Wyoming King Air during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) field campaign. The WCR measured radial velocity V_r, which includes the re-flectivity-weighted terminal velocity of hydrometeors V_t, vertical air velocity w, horizontal wind contributions as a result of aircraft attitude deviations, and aircraft motion. Aircraft motion was removed through standard processing. To retrieve vertical radial velocity W, V_r was corrected using rawinsonde data and aircraft attitude measurements; w was then calculated by subtracting the mean W (W) at a given height along a flight leg long enough for W to equal the mean reflectivity-weighted terminal velocity V_t at that height. The accuracy of the w and V_t retrievals were dependent on satisfying assumptions along a given flight leg that the winds at a given altitude above/below the aircraft did not vary, the vertical air motions at a given altitude sum to 0 m s²¹, and V_t at a given altitude did not vary. The uncertainty in the w retrieval associated with each assumption is evaluated. Case studies and a projectwide summary show that this methodology can provide estimates of w that closely match gust probe measurements of w at the aircraft level. Flight legs with little variation in equivalent re-flectivity factor at a given height and large horizontal echo extent were associated with the least retrieval uncertainty. The greatest uncertainty occurred in regions with isolated convective turrets or at altitudes where split cloud layers were present.