Moisture and stability controls on raindrop size distribution including breakup signature within convective clouds

Title: Moisture and stability controls on raindrop size distribution including breakup signature within convective clouds

The intensity of organised convective systems has been linked to environmental conditions; however, variability across cases suggests non-linear relationships, raising the question of whether diversity or other factors contribute to this variability. Especially, an understanding of cloud microphysical processes in these systems is necessary to bridge gaps between them from both observational and modelling perspectives.
At first, the relationships between raindrop size distributions within convective clouds and their environments were investigated using available observations near Kumagaya, the eastern part of Japan. Collisional coalescence of cloud droplets and raindrops, and conversion from cloud droplets to raindrops, were likely to coincide within convective clouds. Stronger vertical wind shear and higher instability are more likely to be sensitive to more vigorous rainfall intensity, as a proxy for increasing median volume diameter in the drop-size distribution. Weaker vertical wind shear and higher moisture content in the lower layer are likely to be more sensitive to larger rainfall amounts, serving as a proxy for increased liquid water content.
In addition, numerical experiments were conducted using the Weather Research and Forecasting model under idealised conditions based on the observed relationships. This study focused on spatial and temporal features of cloud microphysics with a 250 m horizontal and ~125 m vertical grid spacing across an 80 km x 80 km domain extending 20 km vertically. Initial conditions, obtained from the mesoscale model of the Japan Meteorological Agency for 06 UTC July 12, 2022, were used as input for the sounding and included variations in humidity, temperature lapse rates, and vertical wind shear. A spectral-bin microphysics scheme was primarily used to represent cloud microphysical properties in this study. Results showed that a more moist case in the lower levels led to increased rainfall intensity due to greater drop concentration of relatively smaller raindrops and higher liquid water content within convective clouds compared to the control simulation. Larger temperature lapse rates lead to larger raindrop sizes in convective clouds, which in turn contribute to stronger rainfall intensity. Stronger shear conditions generally lead to stronger rainfall intensity, whilst weaker shear conditions, with smaller temperature lapse rates or in humid environments, lead to larger rainfall amounts. These results may reflect midlatitude-type convection and tropical-type convection, including microphysical interpretations, and were consistent with the observational relationships.
These findings suggest that the established relationships between raindrop size distributions within convective clouds and environments could be extended as a baseline for operational quantitative precipitation estimation and to improve the microphysical scheme.