Observations and simulations of the mesoscale environment in TOMACS urban heavy rain events

Abstract

During the intensive observation period (IOP) of the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS) in the summer months of 2011 - 2013, the atmospheric environment of several heavy rainfalls was observed by radiosonde soundings in the Tokyo metropolitan area. We investigated the formation and development processes of an extremely developed thunderstorm (Case 1 on August 26, 2011) and a moderately developed thunderstorm (Case 2 on July 18, 2013) during the TOMACS IOP by utilizing radiosonde sounding data. Compared with Case 2, Case 1 featured a lower level of free convection and a deeper layer of easterly flow in the mesoscale environment of the severe storm. We performed numerical simulations to investigate the formation processes of the convective systems in the two cases by using the nonhydrostatic model of the Japan Meteorological Agency incorporating the square prism urban canopy scheme. Model results fairly represented the spatial distribution and amounts of the rainfall in both cases. In Case 1, the formation of a distinct convergence zone between easterly and southerly flows was the likely trigger of active convective systems around Tokyo. To further examine the urban impact on precipitation, we performed two comparative simulations: one using realistic current urban surface conditions (CRNT experiment) and the other using less-urbanized surface conditions (LURB experiment). The CRNT experiment yielded more rainfall than the LURB experiment in the central urban area. It appears that the higher temperatures caused by urbanization can lead to increased rainfall in Tokyo by intensifying convergence and ascending motion.