Angela K. Rowe

Elevation-Dependent Trends in Precipitation Observed during NAME

Radar data from the 2004 North American Monsoon Experiment (NAME) enhanced observing period were used to investigate diurnal trends and vertical structure of precipitating features relative to local terrain. Two-dimensional composites of reflectivity and rain rate, created from the two Servicio Meteorologico Nacional (SMN; Mexican Weather Service) C-band Doppler radars and NCAR’s S-band polarimetric Doppler radar (S-Pol), were divided into four elevation groups: over water, 0–1000 m (MSL), 1000–2000 m, and greater than 2000 m. Analysis of precipitation frequency and average rainfall intensity using these composites reveals a strong diurnal trend in precipitation similar to that observed by the NAME Event Rain Gauge Network. Precipitation occurs most frequently during the afternoon over the Sierra Madre Occidental (SMO), with the peak frequency moving over the lower elevations by evening. Also, the precipitation events over the lower elevations are less frequent but of greater intensity (rain rate) than those over the SMO. Precipitation echoes were partitioned into convective and stratiform components to allow for examination of vertical characteristics of convection using data from S-Pol. Analyses of reflectivity profiles and echo-top heights confirm that convection over the lower terrain is more intense and vertically developed than convection over the SMO. Warm-cloud depths, estimated from the Colorado State University–NAME upper-air and surface gridded analyses are, on average, 2 times as deep over the lower terrain as compared with over the SMO. Using a simplified stochastic model for drop growth, it is shown that these differences in warm-cloud depths could possibly explain the observed elevation-dependent trends in precipitation intensity.

Microphysical characteristics of MJO convection over the Indian Ocean during DYNAMO

The microphysical characteristics of precipitating convection occurring in various stages of the Madden-Julian Oscillation (MJO) over the Indian Ocean are determined from data obtained from the National Center for Atmospheric Research dual-polarimetric Doppler S-band radar, S-PolKa, deployed as part of the Dynamics of the MJO (DYNAMO) field experiment. Active MJO events with increased rainfall occurred in October, November, and December 2011. During each of these active MJO phases, in addition to enhanced rainfall, convection became deeper and ice-phase microphysics played a greater role. S-PolKa consistently showed nonoriented small ice particles dominating the radar echoes at altitudes of 9–10 km, dry aggregates concentrated between 7 and 9 km, and wet aggregates and graupel near the melting level (~5 km). Graupel occurred mainly in actively convective towers, while the wet aggregates occurred almost exclusively in the stratiform regions of mesoscale convective systems (MCSs). During each of the three multiweek MJO active phases, the maximum rainfall occurred in short bursts lasting a few days. Each multiday rainy period began with deepening convective elements and a concurrent increase in occurrence of dry aggregates, which maximized just prior to organization into MCSs. The peak rainfall occurrence coincided with the maximum coverage of the radar domain by MCSs, reflecting large stratiform regions that exhibited the most frequent occurrence of wet aggregates. During the December active MJO phase, however, the MCSs were shallower and had a slightly lower tendency for wet aggregates in the stratiform regions and, therefore, generally weaker brightbands.

The Olympic Mountains Experiment (OLYMPEX)

OLYMPEX is a comprehensive field campaign to study how precipitation in Pacific storms is modified by passage over coastal mountains.

Diurnal Circulations and Rainfall in Taiwan during SoWMEX/TiMREX (2008)

AbstractThe diurnal cycle of the local circulation, rainfall, and heat and moisture budgets is investigated in Taiwans heavy rain (mei-yu) season using data from the 2008 Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX). Comparisons are made between an undisturbed (UNDIST; 22–29 May) and disturbed period (DIST; 31 May–4 June). Many aspects of the diurnal evolution in surface flows and rainfall were similar during both periods. At night and during early morning hours, the low-level southwesterly flow was deflected around Taiwans main topographic barrier, the Central Mountain Range (CMR), with rainfall focused near areas of enhanced offshore confluence created by downslope and land-breeze flows. During the day, the flow switched to onshore and upslope, rainfall shifted inland, and deep convection developed along the coastal plains and windward slopes. Atmospheric budget analysis indicates a day-to-evening transition of convective structure from shallow to deep to...

Cloud organization and growth during the transition from suppressed to active MJO conditions

During the Dynamics of the Madden-Julian Oscillation/Atmospheric Radiation Measurement Madden-Julian Oscillation (MJO) Investigation Experiment field experiment in the Indian Ocean, the National Center for Atmospheric Research dual-polarimetric S- and Ka-band radar (S-PolKa) radar observed three active Madden-Julian Oscillation (MJO) events. These events were separated by suppressed periods characterized by shallower, more isolated convection and relatively little rainfall. The sensitivity of S-PolKa allowed investigation of the initiation and organization of both nonprecipitating and precipitating clouds. Early in the suppressed periods, shallow nonprecipitating clouds occurred in shear-parallel lines along apparent boundary layer rolls during early morning. Once some of the clouds began to precipitate, small cold pools formed below the showers. By afternoon, the lines all but disappeared with nonprecipitating clouds instead forming along the edges of cold pools. All such convection was limited in depth early in suppressed periods. As the suppressed environment gained moisture, the nonprecipitating clouds were able to grow to larger size, with the deepest precipitating clouds occurring in clusters at intersections of cold pool boundaries by afternoon. Upscale growth into mesoscale convective systems was observed as the suppressed periods transitioned into active MJO phases, contributing to overnight precipitation during the later part of the suppressed period. This study demonstrates the need for models to accurately represent the organization and evolution of nonprecipitating clouds in association with boundary layer dynamics under suppressed conditions of the MJO, prior to the occurrence of precipitating clouds and their cold pools.

Evolution of Precipitation Structure During the November DYNAMO MJO Event: Cloud‐Resolving Model Intercomparison and Cross Validation Using Radar Observations

Evolution of precipitation structures are simulated and compared with radar observations for the November Madden-Julian Oscillation (MJO) event during the DYNAmics of the MJO (DYNAMO) field campaign. Three ground-based, ship-borne, and spaceborne precipitation radars and three cloud-resolving models (CRMs) driven by observed large-scale forcing are used to study precipitation structures at different locations over the central equatorial Indian Ocean. Convective strength is represented by 0-dBZ echo-top heights, and convective organization by contiguous 17-dBZ areas. The multi-radar and multi-model framework allows for more stringent model validations. The emphasis is on testing models’ ability to simulate subtle differences observed at different radar sites when the MJO event passed through. The results show that CRMs forced by site-specific large-scale forcing can reproduce not only common features in cloud populations but also subtle variations observed by different radars. The comparisons also revealed common deficiencies in CRM simulations where they underestimate radar echo-top heights for the strongest convection within large, organized precipitation features. Cross validations with multiple radars and models also enable quantitative comparisons in CRM sensitivity studies using different large-scale forcing, microphysical schemes and parameters, resolutions, and domain sizes. In terms of radar echo-top height temporal variations, many model sensitivity tests have better correlations than radar/model comparisons, indicating robustness in model performance on this aspect. It is further shown that well-validated model simulations could be used to constrain uncertainties in observed echo-top heights when the low-resolution surveillance scanning strategy is used.