The work presented herein deals with the problems of inter-comparison of precipitation values at 8 km × 8 km and 4 km × 4 km scales (herein often referred to as 8 km and 4 km pixel or scale) that represent the approximate instantaneous field of view of thermal infrared measurements from geostationary satellites like Kalpana, Geostationary Meteorological Satellite (GMS), etc. A number of temporal, spatial and spatiotemporal precipitation series at 4 and 8 km scales from TOGA-COARE radar observations are subjected to autocorrelation analysis to study precipitation variability. It was found that autocorrelation of precipitation measurements drop to 0.45 and 0.58 in 10 minutes time for 4 and 8 km pixel, respectively. Similarly spatial autocorrelation drops to 0.64 and 0.61 for 4 km pixel with 4 km displacement and 8 km pixel with 8 km displacement, respectively. The drop in autocorrelation in the spatiotemporal series is shown to be even more rapid. The root mean square fractional error for precipitation shows a value of 0.63 to 0.90 and 0.71 to 0.94 for 10 to 60 minute time lag for 4 and 8 km pixel, respectively. The autocorrelation analysis of precipitation underlines the need for precise geolocation and time measurement at each pixel for meaningful intercomparison. We have also presented results on the precipitation comparison when two observations have different spatial scales. We analyzed the probability distribution P (ri|R), of precipitation measurements ri at 2 km scale within a 8 km pixel for a given pixel averaged rain rate R. It was found that there is only 40% probability that a 2 and 8 km pixel averaged rain will match with a percentage error less than 50% of R. We also presented a Monte Carlo simulated precipitation comparison measured at two different scales 8 and 2 km. This analysis highlights the importance of the comparable scales in precipitation validation.
Interannual variations of both the Baiu precipitation and tropical cyclone (TC) activity in the western North Pacific (WNP) are controlled by large-scale atmospheric circulations associated with the El Niño/Southern oscillation (ENSO) and the tropospheric biennial oscillation (TBO) of the Asian monsoon. This work examines covariability between the Baiu precipitation and the TC activity in the WNP through the ENSO and the TBO. In years when sea surface temperature anomalies (SSTAs) are negative in the eastern tropical Pacific with respect to the ENSO, the number of TCs increases near the Philippines in the Baiu season, June and July. On the other hand, in years of negative SSTAs in the eastern tropical Pacific related to the TBO, the strength of TCs is enhanced to the southeast of Japan. Each of these two TC activities enhances a cyclonic circulation there, which shifts the axis of monsoon southwesterlies and contributes to form the peculiar Baiu precipitation anomalies. These modifications are, however, dependent on the phase of the ENSO and the TBO. In years of positive SSTAs in the eastern tropical Pacific, the anomalous TC activity is small and sometimes has opposing effects on the atmospheric circulations of the ENSO and TBO. Thus, the covariability is strong between the Baiu precipitation and TC activity in the WNP in the specific phase of the ENSO and the TBO, although both covary in large-scale atmospheric circulations.
Typhoon Vera struck Japan on 26 September 1959, causing the largest meteorological disaster in Japan since World War II, with tragic consequences. Over the past 50 years, numerical weather prediction systems and observation networks have been developed that facilitate more realistic simulation of tropical cyclones (TCs). In this study, the operational mesoscale forecast and analysis systems of the Japan Meteorological Agency (JMA) were applied to the case of Vera with some modifications, and the forecast probability of the natural disasters caused by Vera was investigated. In data assimilation, dropsondes deployed near the TC center and TC bogus data were individually used in addition to conventional observations. In order to reduce the discrepancy between the model states and real observations near the TC core region, due mainly to the insufficiency of the model resolution, we adjusted the observational error of the dropsondes as a function of observation-minus-forecast innovations. The results indicate that both the dropsondes and the bogus data contributed to improving the analysis and a subsequent model forecast with 5-km grid spacing. Initialized using the analysis with dropsondes, the weather distribution (e.g., surface pressure, clouds, and fronts) of the forecast result was similar to the observed weather map. Using this result, a storm surge forecast and a downscaling numerical simulation with horizontal resolution of 1km were also conducted. The intensity and track forecast did not improve compared to the forecast with 5-km grid spacing, but precipitation was better captured due to the higher-resolution orography. The storm surge forecast was comparable to observations, suggesting that the strong wind caused by Vera was well reproduced in the forecast model.
On the Korean peninsula, observation is being conducted so densely that the mean distance between stations of the extensive ground-based data network is only 12.7 km. Nevertheless, because of significant mountainous terrain and the fact that most observation sites are situated in low areas rather than mountain tops or ridges, the detailed topographical effect on temperature distribution is not reflected properly. A model using fine-scale grid spacing can represent such a topographical effect well, but due to systematic biases in the model, simulated temperature distribution will be different from the actual observation. This study therefore attempts to produce a detailed mean temperature distribution for South Korea through a method combining dynamical downscaling and statistical correction. For the dynamical downscaling, the Weather Research and Forecast (WRF) model developed by the U.S. National Center for Atmospheric Research (NCAR) is used. We applied a multi-nesting technique to obtain high-resolution climate information (3 km) with a focus on the Korean peninsula. The integration period was 10 years from January 1999 to December 2008. For the correction of systematic biases shown in downscaled temperature, a perturbation method divided into the mean and the perturbation part was used with a different correction method being applied to each part. The mean was corrected by a weighting function while the perturbation was corrected by the self-organizing maps method, which is one of the artificial neural networks method. The results with correction agree well with the observed pattern compared to those without correction, improving the spatial and temporal correlations as well as the root mean square error. In addition, they represented detailed spatial features of temperature including topographic signals, which cannot be expressed properly by gridded observation. Through comparison with in-situ observation with gridded values after objective analysis, it was found that the detailed structure correctly reflected topographically diverse signals that could not be derived from limited observation data.
We evaluated change in flood risk under global warming using the output from the latest version of the Model for Interdisciplinary Research on Climate (MIROC5), an atmosphere-ocean general circulation model. River discharge for the 21st century were simulated for the two Representative Concentration Pathway (RCP4.5 and RCP8.5) scenarios and converted to the Discharge Probability Index (DPI) to evaluate future flood risk. The occurrence of flood events corresponding to various DPI categories was calculated for each continental region. The results show a significant increase in the risk of massive flood incidents during the 21st century in Asia, Africa, Oceania, and South America, with relatively large differences between the two scenarios. In contrast, both scenarios showed only slight increases in massive flood risk in North America and almost no change in Europe. For the RCP8.5 scenario in particular, the risk of massive flood occurrence will increase approximately ten times in Africa, seven times in Asia, and five times in South America by the end of the current century. Further analyses indicated that these projected flood increases will occur mainly due to the increases in the number of rainy days and the annual maximum daily precipitation, and the decrease in snowmelt in high latitudinal regions will play an important role on the unchanged risk in Europe in spite of the projected increase in precipitation.
We analyzed long-term trends in temperature for urban areas in Japan, after classifying observations according to coincident wind speed and precipitation. Trends were defined as departures from regional temperatures at nearby rural sites with population density of less than 100 people per square kilometer, using a 30 year data set from the Automated Meteorological Data Acquisition System (AMeDAS) of Japan. “High-wind” and “low-wind” cases were defined by the upper and lower one-thirds of the geostrophic wind speed (GWS) calculated from sea-level pressure gradients at surrounding stations. “Rain” days were defined as those having a precipitation rate greater than or equal to 1 mm per six hours, while “no-rain” cases had less than this. Temperature trends were significantly higher in low-wind conditions than in high-wind conditions at stations where the population density was more than 3000 km-2, and trends were higher in no-rain conditions than in rain conditions even for slightly urbanized areas with a population density from 100 to 300 km-2. Analysis using surface wind speed instead of GWS yielded a result similar to that described above, although differences in trends between high- and low-wind conditions were smaller than those obtained from an analysis using GWS. At night, the differences in temperature trends between high-wind and low-wind conditions, and between rain and no-rain conditions, were greater than they were when daily mean temperatures were used. These results agree with our understanding that the urban heat island effect is more pronounced at night under clear skies and low-wind conditions. This study provides convincing evidence of urban-induced warming, not only in large cities but also at slightly urbanized sites in Japan.
This work investigates the teleconnection patterns over the North Pacific/North America sector and regional rainfall variability over the southwestern USA during boreal autumn, associated with two types of El Niño. These two types, called cold tongue (CT) and warm pool (WP) El Niños, have an opposing impact on atmospheric circulation over the eastern North Pacific. When CT El Niño occurs, a strong cyclonic anomaly tends to appear over the North Pacific, and the associated southwesterly winds bring unusually moist air and thereby enhance rainfall over the southwestern USA. However, during WP El Niño autumns, a tripolar anomaly develops over the North Pacific. The associated northerly and northeasterly winds transport unusually dry air to the southwestern USA causing a reduction in rainfall. In this region, the rainfall response to WP El Niño is similar to that of La Niña, but opposite to that of CT El Niño. Since the early 1990s, the WP El Niño event has occurred more frequently, while the CT El Niño events has become less. The La Niña events remain roughly unchanged in terms of the zonal location. Autumn rainfall deficits over the southwestern USA have also been more frequent after the 1990s. The El Niño regime change thus appears to contribute to a decadal difference in the regional autumn rainfall.
A typical Northeast China cold vortex (NCCV) that lasted for approximately 51 hours (from 0000 UTC 18 May to 0300 UTC 20 May, 2010) and produced several heavy rainfall events over Northeast China was successfully simulated using the Weather Research and Forecasting Model (WRF). Based on this successful simulation, the NCCV was analyzed in detail. Synoptic analyses revealed that the NCCV developed downward from the top and finally stretched to a maximum vertical extension of 900-200 hPa. The NCCV was colder than its surroundings in the middle-lower troposphere, whereas in the upper troposphere, it was warmer than its surroundings. There were significant warm and cold advections for the duration of the NCCV, favoring the enhancement of available potential energy (APE) and conducive to sustaining the vortex. The results of the quasi-Lagrange-form eddy flux circulation (EFC) and eddy kinetic energy (EKE) budget analyses indicate that different factors dominated the NCCV in various stages of its evolution. As for the EFC budget, the interactions between the NCCV and background circulations were more important than the interactions between the vortex and other synoptic systems; in contrast, for the EKE budget, the interactions between the NCCV and other synoptic perturbations were more important. The pattern of environmental circulations was very important to the NCCV because the vortex intensified as it moved from areas of low vorticity or EKE to areas with higher values. The baroclinic energy conversion was much greater than the barotropic energy conversion during the life cycle of the NCCV, implying that the NCCV was a typical baroclinic cyclone. Moreover, during the decaying stage, the attenuation of the NCCV appeared to be closely related to the frequency dispersion processes of the baroclinic Rossby wave.
In general, a change in the total cloud amount (TCA) affects the solar irradiance at the surface as a result of on-the-way reflection of solar radiation. The amount of global solar radiation (GSR) increased globally after c.a. 1990, and this increase resulted from a decreases in the TCA and an increases in atmospheric transparency. Here we present the TCA and GSR trends categorized by the TCA for 33 years in Japan (from 1974 through 2006) on the basis of simultaneous observation from 53 weather stations. These trends are calculated using data obtained on a daily basis. The results averaged across Japan show that both GSR and TCA are increasing at the rate of 2.2% and 1.5% per decade, respectively. In most of Japan (regions further north than Kyushu), a high GSR increase rate mostly appears with a large TCA. This fact cannot be accounted for by the increase in the atmospheric transparency alone. There are two possible causes for increases in GSR and TCA in Japan: a change in cloud appearance frequency against the solar zenith angle and a decrease in cloud optical thickness.