Regional-scale evaluation on the changes in environmental stability for afternoon convective precipitation in summer under global warming

Severe local rainfalls induce various kinds of meteorological disasters. These severe rain events in most cases occur un-der the influence of synoptic-scale and/or mesoscale weather disturbances. In addi-tion, local-scale rain events due to rapidly developing cumulonimbus clouds without any significant synoptic forcings some-times occur and have potentially a high societal impact. It is anticipated that such rapidly developing, local-scale rainfalls in synoptically undisturbed environments become more prevalent and more intensified in a future warming climate.

The intensity of convectively induced precipitation is directly controlled by the structure, organization, and intensity of cumulonimbus clouds and thus can be related to and diagnosed by the conditions of the cloud environments. Vertical shear determines the structure and organization of convective systems and hence their intensity, while temperature and moisture conditions characterize static stability. Nomura and Takemi (2011) have investigated the environmental stability for summertime afternoon rainfalls under synoptically undisturbed environments over a plain region that includes the Tokyo metropolitan area. The conventional surface observation data were used to extract hot, sunny days under synoptically undisturbed conditions, and the gridded mesoscale analysis data that cover the Tokyo area were used to examine the difference of the characteristics of environmental stability between no-rain, rain, and strong-rain events in the afternoon by calculating commonly used stability indices and parameters.

Based on the previous studies, we investigate the environmental stability for regional-scale rain events in synoptically undisturbed conditions under the influence of future global warming. According to Chuda and Niino (2005), stability conditions widely change over Japan depending on locations and seasons; therefore, we focus on the summertime cases in the Kanto Plain where many studies have been previously done for environmental conditions. The outputs of high-resolution global climate simulations in global warming climates are used to examine stability parameters for rain events over the Kanto Plain. The changes of the environmental stability for the convective precipitation under global warming are investigated.

The data used in this study are the gridded outputs at 20-km horizontal spacing of the super-high-resolution atmospheric GCM (AGCM) simulations at a resolution of T959L60 (i.e., triangular truncation of 959 with 60 vertical layers) for present, near-future, and century-end future climates, conducted by Meteorological Research Institute (MRI), Japan Meteorological Agency (JMA) under the Innovative Program of Climate Change Projection for the 21st Century (the KAKUSHIN program). The code name of this AGCM is MRI-AGCM3.2S. The GCM simulations are intended to represent the present climate (during 1979-2008), the near-future climate (during 2015-2044), and the last quarter of the twenty-first century (during 2074-2104), each for 30 years with a global warming scenario, that is, the A1B IPCC emission scenario (IPCC 2007). We used the outputs simulated for 25 years that correspond to the periods of 1980-2004 for the present climate, 2020-2044 for the near-future climate, and 2075-2099 for the century-end future climate. We investigated the characteristics of the environmental stability for afternoon precipitation that develops at a regional-scale under synoptically undisturbed conditions in summer.

The comparison of the GCM present-climate simulation under the synoptically undisturbed conditions with the corresponding states revealed by the radiosonde observations at Tateno in the Kanto Plain and the MSM analyses indicated that the GCM present climate well reproduced the characteristics of the temperature and moisture profiles, the stability parameters, and also daily precipitation amounts up to 25 mm. This favorable agreement of the GCM outputs with the real, existing climates enabled us to investigate the changes in the environmental stability under global warming.

In the future climates, temperature lapse rate decreased in the lower troposphere, while water vapor mixing ratio increased throughout the deep troposphere. The changes in the temperature and moisture profiles resulted in the increase in both precipitable water vapor and CAPE, which were evaluated as statistically significant. These projected changes will be enhanced with the future period.

Furthermore, the differences of the stability parameters between the no-rain and rain days under the synoptically undisturbed condition in each simulated climate period were examined. The statistical analyses for these differences indicated that the environmental conditions in terms of the stability parameters that distinguish no-rain and rain events are basically unchanged between the present and the future (including the near-future) climates. This result suggests that the environmental characteristics favorable for afternoon precipitation in the synoptically undisturbed environments will not change under global warming.