Numerical Study on Dynamical Downscaling for Regional Climate Projection —Application to Asian Regions and Development of a Regional NICAM—

  • Masayuki Hara, 2014: Numerical Study on Dynamical Downscaling for Regional Climate Projection —Application to Asian Regions and Development of a Regional NICAM— .

In this paper, we have achieved a series of dynamical downscaling studies over the Asia region and development of a new regional climate model. First, we present the advantages of dynamical downscaling with three applications of a regional climate model (RCM). A RCM generally offers a better representation of atmosphere phenomena near the ground surface because of detailed topographic features, land cover, cumulus convection and local circulation than general circulation model (GCM). Next, we introduce a newly developed RCM to obtain more accurate near-future predictions.

Using this advantageous dynamical downscaling method, we have studied snow and an urban heat island of regional future climate change. A series of numerical experiments was conducted in order to investigate the impact of global warming on snow amounts in Japan during early winter. After confirming the accuracy of hindcast simulations for a high-snow-cover (HSC) year and a low-snow-cover (LSC) year, dynamical downscaling experiments were conducted in order to make future projections using the pseudo-global-warming method. The precipitation, snow depth, and surface air temperature of the hindcast simulations show good agreement with the AMeDAS station data. At the end of December, the decreasing ratios of snow water are more significant in areas with an altitude of less than 1,500 m. The increase in air temperature is one of the major factors influencing the decrease in snow water since the present mean air temperature in most of these areas is near 0 ºC even in winter. On the other hand, the change in the mean areal precipitation due to global warming is less than 15 % in both years. The results indicate that the future snow change is strongly affected by detailed topographic features. Dynamical donwscaling, which can represent detailed topographic features, can produce improved snow projection over those of coarse grid GCMs.

During the past 100 years, the mean surface air temperature (SAT) increased about 3 ºC in Tokyo, while the world mean SAT increased only 0.66 ºC. The major reason of the difference in warming is the effect of the urban heat island (UHI), whose intensity also increased during the period and often the most during winter. This study investigates the change in UHI intensity (UHII) of the Tokyo metropolitan area by the effects of global climate change. The present climate simulation is conducted using a high-resolution numerical climate model, the Weather Research and Forecasting (WRF) model, including an urban canopy sub-model. A future climate run is conducted using the pseudo-global-warming method, assuming the boundary conditions in the 2070s estimated by a GCM under the SRES A2 scenario. The simulation results indicated that UHII would be enhanced more than 20 % during the night due to the global climate change. SAT in the Tokyo metropolitan urban area increases more slowly during the daytime due to the larger heat capacity than in rural areas. Heat release from the buildings in the urban area is larger than that in rural areas at night, when the dispersion of the released heat tends to be restricted to the lower atmosphere because of weak turbulence. These processes are sensitive to cloud fraction and atmospheric stability in the lower atmosphere. The result shows that detailed land cover plays an important role in projecting future urban thermal environments. Dynamical downscaling of a GCM projection using RCM is necessary to obtain a detailed urban thermal environment because dynamical downscaling can capture the detailed land cover.

Next, we investigate precipitation over the Maritime Continent, comparing the precipitation simulated by a 20-km grid Meteorological Research Institute General Circulation Model (MRI-GCM) and the near-surface rain data of TRMM 2A25. The focus is particularly on the diurnal cycle and its phase distribution of precipitation. The features of the simulated precipitation mostly agree with observations made over islands and straits having horizontal scales smaller than 200 km. However, these are quite different around larger islands, such as Sumatra and Borneo, particularly in the phase of the diurnal cycle. The MRI-GCM indicates maximum precipitation in the early afternoon on these islands, while the observed precipitation has its maximum at night. In particular, over the inland areas of the larger islands, the simulated diurnal cycle has an almost reversed phase. The simulated precipitation is remarkably weaker than the observation around the western coast of Sumatra Island, where a large discrepancy is also found in the phase distribution along a line perpendicular to the coast. A higher-resolution simulation using a non-hydrostatic model without convective parameterizations substantially improves the phase distribution over Borneo Island. The non-hydrostatic model simulates well the migration of the precipitation zone and the daily maximum at night in the inland areas. In contrast, the GCM fails to simulate the diurnal cycle over islands whose horizontal scale is larger than 200 km, although the 20-km grid spacing is small enough to reproduce the major aspects of the local circulations. The cause of the difference between the GCM and RCM is the cumulus convective parameterization, which may not adequately represent the coupling of convection and local circulations.

A new regional atmospheric model to target a certain limited area is developed for conducting realistic regional climate simulations based on the stretched horizontal grid version of a regional model based on the global cloud-resolving model, the nonhydrostatic icosahedral atmospheric model (NICAM). The simulation domain of the newly developed regional climate model (hereafter diamond NICAM) consists of one diamond (two triangles in an icosahedron), although the original NICAM global domain consists of ten diamonds (twenty triangles). Preliminary experiments with the diamond NICAM have been performed, and the results show almost the same climate as those of the global NICAM. Usage of the diamond NICAM can reduce the inconsistency in climate simulations between GCMs and RCMs.

Following paper is a part of Section 2 of this thesis. (http://dx.doi.org/10.3178/hrl.2.61)
Following paper is a part of Section 3 of this thesis. (http://www.nagare.or.jp/download/noauth.html?dd=assets/files/download/noauth/nagare/29-5/29-5gencho2.pdf)
Following paper is a part of Section 4 of this thesis. (http://dx.doi.org/10.2151/jmsj.87A.413)