Observations and Simulations of the Mesoscale Environment in TOMACS Urban Heavy Rain Events

Abstract

 During the Tokyo Metropolitan Area Convection Study for Extreme Weather Resilient Cities (TOMACS) intensive observation period (IOP) in 2011-2013 summers, atmospheric environment of several heavy rainfalls was observed by means of radiosonde soundings in the Tokyo metropolitan area. We investigated formation and development processes of an extremely developed thunderstorm (Case 1 on 26 August 2011) and a moderately developed thunderstorm (Case 2 on 18 July 2013) observed in the TOMACS IOP, utilizing the radiosonde sounding data. Compared to Case 2, the mesoscale environment of the severe storm in Case 1 featured a lower level of free convection and a deeper layer of easterly flow. We carried out numerical simulations to investigate the formation processes of the convective systems in the two cases, using the Non-Hydrostatic Model (NHM) of the Japan Meteorological Agency (JMA) incorporating the Square Prism Urban Canopy (SPUC) 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 higher temperatures caused by urbanization can lead to increased rainfall in Tokyo by intensifying convergence and ascending motion.