Vertical variability of cloud hydrometeors in the stratiform region of mesoscale convective systems and bow echoes

During the Bow Echo and Mesoscale Convective Vortex Experiment, the NOAA P-3 research aircraft executed 17 spiral descents to the rear of convective lines to document the vertical variability of hydrometeors above, within, and below the stratiform melting layer. Ten spirals were behind lines that exhibited bowing at some stage in their evolution. Although quick descents on some spirals forced sampling of different particle zones, clear trends with respect to temperature were seen. For 16 spirals, the ambient relative humidity with respect to ice was in the range of 100% ± 4% at temperatures between -10°C and the melting layer, but exhibited steady decreases below the melting layer to an average relative humidity with respect to water of 77% ± 15% at 9°C. In contrast, one spiral conducted on 29 June 2003 directly behind a developing bow echo had a relative humidity with respect to ice averaging 85% at heights above the 0°C level and relative humidity with respect to ice further decreased below the 0°C level to a minimum relative humidity with respect to water of 48% at 9°C. Vertical profiles of particle shapes, size distributions (SDs), total mass contents (TMC), number concentrations, and parameters of gamma distributions fit to SDs were computed using optical array probe data in conjunction with measurements of radar reflectivity from the P-3 X-band tail radar. For spirals with humidity at or near saturation above the melting layer, melting particles occurred through about 300 m of cloud depth between 0° and 2° or 3°C. Above the melting layer, number concentrations, dominated by smaller crystals, decreased at 19% ± 10% °C^-1, faster than the 10% ± 7% °C^-1 decrease of TMC dominated by larger particles. Increases in the numbers of crystals with a maximum dimension <2 mm (N_<2) and in the slope parameter with temperature also occurred. To the extent that in-cloud heterogeneity did not complicate observed trends, these trends suggest aggregation dominated the evolution of SDs. Observations on 29 June differ from other days and are explained by the unique position and timing of the spiral in subsaturated air behind a developing bow. On 29 June the presence of an isothermal layer at 2.5°C suggested that sublimative cooling delayed the onset of melting. Ice at 7°C showed that melting particles were present through 500 m of cloud depth. A slight decrease in N_<2, but no decrease in the slope parameter, with temperature suggested that sublimation modified the impact of aggregation. Sublimative cooling would only have been significant at the location of the 29 June spiral. For other spirals, evaporative cooling below the melting layer in subsaturated regions was the most important diabatic processes in the stratiform regions at the time of the observations.