The climatic features of the distinctive early-spring dry spell in the southeastern margin of the Tibetan Plateau and the relationship between this dry spell and topographic forcing are investigated. The results show that the early-spring dry spell is characterized by the lowest relative humidity and highest evaporation in the year and by an extremely low amount of rainfall in March and April. The dry spell is a regional phenomenon, as centers of high relative humidity and large rainfall amounts are located to both the east and west of the dry region. In the major dry region, more than 80% of the days in March and April are “dry” days, during which the daily precipitation amounts to no more than 0.1 mm. The mean duration of a dry period between two consecutive rainfall events is found to be 7 days. Further analyses indicate that the relationship between the prevailing westerly flow and the topographic conditions plays an important role in the formation of the dry spell. Strong orographic uplift occurs as the westerly wind flows over the north-south-oriented mountain ranges to the west of the dry region; this in turn causes heavy rainfall over the windward side. After flowing over the mountains, the air subsides over the dry region on the leeward side, suppressing precipitation and increasing the surface air temperature. On the basis of sensitivity experiments carried out using a regional climate model, the role of the topographic conditions in modulating the local climate is confirmed. Reduction in the height of the western mountains as well as lowering of the elevation of the eastern highlands can lead to significant increases in the early-spring rainfall over the dry region.
In this study, mixed layer (ML) height information is extracted from the range-corrected signal-to-noise ratio (SNR) data of wind profilers operated by the Japan Meteorological Agency (JMA) and the Korea Meteorological Administration (KMA). The altitude where the maximum backscattered power is received by a wind profiler is regarded as ML height. Four sites (Kumagaya and Mito in Japan; Munsan and Gangneung in Korea) were tested on clear sky days, which were chosen using a sunshine ratio. The ML height information obtained was utilized to analyze the spatiotemporal variations of ML heights over Japan and to qualitatively evaluate the model mixing depth predicted by a KMA regional numerical weather prediction model. The frequency of the wind profiler is 1.36 GHz (1.29 GHz for sites in Korea), and the temporal resolution of the SNR data is 10 min. In the vertical, the operating resolutions are 300 m for the sites in Japan and 70 m for Korea. The quantitative analysis of the mixing depths estimated by JMA shows day-to-day, seasonal, and spatial variations in the measured mixing depths. Comparison with nearby radiosonde observations, as well as surface station and satellite observations, is conducted. The estimated ML height tended to be somewhat high in the early morning presumably due to the minimum detectable range and a residual layer (RL) effect. Mean ML heights at the Kumagaya site in July and October 2001 show a seasonal difference of about 500 m. The sea breeze effect can be seen in ML heights at the Mito site located near the sea coast. On the other hand, the evaluation of model-predicted ML heights against KMA SNR data reflected the nonlocal K-profile boundary layer scheme's well-known feature of having faster development with greater depth for an ML. It is proposed in this paper that the ML heights estimated from the Munsan and Gangneung sites can be applied to the observational analysis of mixing depths and be employed as an auxiliary tool for evaluating model ML heights on an operational basis.
In this study, various probability distribution functions for the entire intensity range of short-term precipitation, such as the hourly and daily precipitation intensities during the snowless season in Japan, were examined. The traditional distribution functions, i.e., the exponential, Weibull, Gamma, generalized Gamma, log-normal, and Johnson's SB distributions, were insufficient to express the precipitation intensities within the rainy periods. Three types of distribution functions were newly proposed in association with the extension of the Weibull distribution. The new distributions were constructed to vary asymptotically from the exponential distribution on the weak intensity side to the Weibull distribution on the strong intensity side. The number of parameters of the distribution functions was four except for the parameter of the rainy period ratio. One or two parameters were fixed as unity parameters in all observation points in the parameter estimation in order to overcome a multi-solution problem caused by the strong non-linearity of the distribution functions. The unity parameters were determined to be the values at which the all-points average of mean square errors for logarithms of exceedance probabilities had the minimum value. Other parameters were estimated by the maximum likelihood estimation method. The new distribution functions were more suitable to express the short-term precipitation, including both weak and strong intensities, than the traditional distribution functions were. For the hourly precipitation, the boundary intensity between the exponential and Weibull distribution properties was considered to be associated with that between the main domination ranges of stratiform and convective precipitations. A two-parameter (except for the rainy period and the unity parameters) function with Type-I and three-parameter functions with Type-I and -III were proposed as the best functions for hourly precipitation intensities. A two-parameter function with Type-III was also proposed for daily precipitation.
Temporal progress of forest reduction by deforestation in the Asian tropical region was simulated using a global climate model that included a new land-surface ecosystem model. The horizontal resolution of the model was 1.875°. Two experimental areas were defined: the Indochina peninsula (ICP) area, and the Maritime Continent (MTC) area. With a control simulation under the actual vegetation condition, a grassland experiment (C4 experiment) and a bare-soil experiment (BS experiment) were carried out. Forest vegetation in the experimental areas was gradually changed (at the latest actual rate) to non-forest type vegetation. The natural growth of C4 grass was reproduced in the C4 experiment, and the plant growth was deterred in the BS experiment. Each experiment was run for 100 model years. Temporal changes of the energy and the carbon cycle balances under the vegetation transition were examined. Compared with the control, the net radiation (RNE) decreased because of the increase in the land surface albedo. The sensible heat flux decreased because of the decreases in RNE and other particular reasons. The results for latent heat flux (E) in this study differed from those in our previous study. In the C4 experiment, E increased, especially in the ICP area; in the BS experiment, E decreased in both experimental areas. In the MTC area, the surface temperature in the C4 experiment increased because of the large influence of decreased precipitation. Decreases of the soil wetness in the C4 experiment were more significant in the MTC area, due to increased transpiration and the decreased precipitation. The change from forest vegetation to C4 grass vegetation induced the reduction of carbon absorption by the land surface. Continuous deforestation in the Asian tropical region certainly induces the elevations of the global atmospheric carbon dioxide concentrations, even if the deforested area is not replaced by bare soil surface condition.
Four-dimensional variational (4D-Var) data assimilation (DA) experiments using Global Positioning System (GPS)-derived precipitable water vapor (PWV) were conducted for the tropical cyclone (TC) Nargis in 2008. In order to analyze the initial field at 1200 UTC 30 April 2008, 12, 24, 36, and 48 h sequential DA experiments with 3 h assimilation windows were performed. The initial fields made by these DA experiments were applied to subsequent forecast experiments using a nonhydrostatic model (NHM) with a horizontal resolution of 10 km. NHM predictions using initial fields produced by DA experiments that used only ordinary observational data (without GPS PWV) exhibited a large variation of predicted maximum TC intensity (958 to 983 hPa) for each experiment. In these experiments, a longer assimilation period did not necessarily result in better prediction. The DA of GPS PWV yielded a smaller variation of predicted maximum TC intensity (964 to 974 hPa), and a longer assimilation period tended to bring deeper depression of TC central pressure. Overall, TC intensities determined by DA experiments with GPS data were closer to the best track produced by the Regional Specialized Meteorological Centre (RSMC) New Delhi than the DA experiments without GPS data. The 48 h DA without GPS PWV resulted in the weakest prediction of TC development with the deepest TC central pressure of 983 hPa, while 48 h DA with GPS PWV successfully predicted rapid TC development with the deepest pressure of 967 hPa. One cause of the incomplete development of Nargis in the 48 h DA experiment without GPS PWV was insufficient observations in the Bay of Bengal, especially in the first 12 h. Underestimation of precipitation was conspicuous in the first 12 h of the DA. Implementation of GPS PWV into the DA contributed to increasing the precipitation and changed the fields of pressure and wind in the bay. In the first several hours, modifications of the fields of pressure and wind around the Andaman Islands were conspicuous. These affected areas extended with time and created a more favorable environment for TC development.
This study investigates the elevation dependency of the present-day climate and future climate change in temperature and precipitation over Korea. A dynamically downscaled fine-scale climate simulation (20 km) shows reasonable agreement with two types of observations maintained by the Korea Meteorological Administration. The model exactly captures the strong relationship between the elevation and local climatology as seen in observed temperature and precipitation patterns. The behavior of the elevation dependency shown by the present-day climate simulation is also appeared in the climate change signal. The warming amplification is highly correlated with elevation. The warming is more pronounced at higher elevations than at lower elevations during winter, and maximum warming occurs at minimum temperature, showing an asymmetric response between minimum and maximum temperature. A noticeable differential rate of winter warming in response to the elevation can be explained by the snow-albedo feedback. Precipitation and snow changes also show the relevant topographical modulation under global warming. This study clearly demonstrates the importance of a refined topography for improving the accuracy of the local climatology and suitably reflecting the altitudinal distribution.