32nd Conference on Broadcast Meteorology/31st Conference on Radar Meteorology/Fifth Conference on Coastal Atmospheric and Oceanic Prediction and Processes

Wednesday, 6 August 2003: 11:45 AM
Orographic modification of baroclinic flows and precipitation: Airborne Doppler perspectives from MAP and IMPROVE-II
Bradley F. Smull, NOAA/National Severe Storms Laboratory and the University of Washington, Seattle, WA
Recent technical advances in signal processing and quasi-automated editing of airborne Doppler radar data have made possible more expansive airborne-Doppler views of reconstructed airflow and precipitation than were heretofore possible. This paper focuses on the unique ability of airborne platforms to specify details of orographically modified flow and precipitation both directly over and surrounding major mountain barriers.

In accordance with idealized theory regarding the occurrence of varying degrees of “upstream blocking”, in which an appreciable fraction of the incident flow is forced to divert around (vs. surmounting) a major mountain barrier, data from the NOAA P-3 platform are processed for two diverse events: (1) a case of profoundly blocked flow exhibiting minimal precipitation enhancement directly over the orography but with appreciable low-level convergence and echo extension upstream of the Alps during IOP8 of MAP (the Mesoscale Alpine Programme, conducted in autumn of 1990), and (2) a case of relatively unimpeded (i.e. “unblocked”) flow exhibiting significant orographic precipitation enhancement over the windward (western) slopes of the Oregon Cascades during IOP11 of IMPROVE-II (the second phase of the “Improvement of Microphysical PaRameterization through Observational Verification Experiment”), conducted during late 2002.

In both cases, the arrival of strong, moist pre-frontal flows supported the development of deep, horizontally extensive precipitation shields that in turn provided the means to illuminate large atmospheric volumes over and around a zone of sharply rising terrain. Composite analysis approaches encompassing multiple flight legs are employed in both cases to gain a spatially expansive “barrier-scale” view of mesoscale flow and precipitation patterns adjacent to (or in the case of IMPROVE, on both sides of) the mountain crestline. The execution of overlapping dual-Doppler scans as obtained via a regimented “square wave” flight pattern employed during IMPROVE-II allows evaluation of traditional upper-boundary condition assumptions via a direct quad-Doppler solution technique first employed during TOGA-COARE. This test is particularly relevant in the presence of an intense mountain-wave signature, such as that shown to extend well beyond echo top in high-resolution MM5 simulations of the IMPROVE IOP11 case. More spatially limited but temporally continuous data from other platforms (e.g., ground-based S-POL and vertically-pointing S-band measurements confined to the mountain slopes) being discussed by other IMPROVE-II PI’s making submissions to this conference serve to provide desirably comprehensive description of these events.

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