Improving the Conceptual Understanding of Urban Thunderstorm Modification Using the Real Atmosphere Idealized Land-surface (RAIL) Method

Urban thunderstorm interaction is an often-observed example of local, inadvertent weather modification. Storm modification results from the combined influence of land-surface heterogeneity and cloud interaction with urban aerosols. The complex land-surfaces and highly heterogeneous aerosols in real case simulations make it difficult to improve our conceptual understanding of the individual components of urban-thunderstorm-interaction.

This study introduces the Real Atmosphere/Idealized Land-surface (RAIL) method to disentangle the individual contributions of elements of land-surface heterogeneity and aerosols to thunderstorm changes in the urban environment. Within a high resolution (1km), 3D cloud resolving model coupled with a land-surface model, the RAIL method specifies atmospheric conditions based on composites of different pre-storm environments. The land-surface is a flat surface with uniform soil conditions and a homogeneous rural land-type surrounding one or more urban areas. For each of four pre-storm environments (strong/weakly forced, single/multicell) the method tests different land surface sensitivities: rural land type, city size, city shape, development intensity, and multi-city orientation. An aerosol sensitivity study is also conducted, varying concentrations of small mode (cloud-nucleating) and large mode (drizzle enhancing) aerosols, as well as varying the overall urban plume size, shape, and extent upwind and downwind.

The study finds that the urban environment can affect the amount of precipitation from a thunderstorm, reducing precipitation by as much as half in the urban center and increasing precipitation by as much as double downwind. Land surface and aerosol modifications are most prominent in weakly forced environments. Typically, the land-surface heterogeneity causes the storm to weaken upwind of the urban center, but sufficiently high concentration of cloud-nucleating aerosols slow the weakening of the storm. Larger cities, relative to storm motion, suppress the thunderstorm more. As a storm moves downwind, the small mode aerosols are responsible for the continued suppression of the storm as it passes out of the urban center. The large mode aerosols are responsible for downwind invigoration. The proportion of small-to-large aerosols determines whether a storm's precipitation is primarily convective or stratiform. Weekday concentrations of urban aerosol produce smaller regions of heavier convective rain, while weekend concentrations produce larger regions of stratiform rain. The size of the aerosol plume determines how far downstream thunderstorm invigoration will occur. The results of this study will help direct future research and operational weather models to study precipitation in urban environments.