Yuichiro Oku

Estimation of Land Surface Heat Fluxes over the Tibetan Plateau Using GMS Data

A Surface Energy Balance System (SEBS) originally developed for the NOAA Advanced Very High Resolution Radiometer was applied to Geostationary Meteorological Satellite (GMS)-5 Visible/Infrared Spin-Scan Radiometer data that were supplemented with other meteorological data. GMS-5, which is a geostationary satellite, recorded continuous hourly information. Surface temperatures obtained from the GMS-5 data were entered into SEBS to estimate the hourly regional distribution of the surface heat fluxes over the Tibetan Plateau. The estimated fluxes are verified by using corresponding field observations. The diurnal cycle of estimated fluxes agreed well with the field measurements. For example, the diurnal range of the estimated sensible heat flux decreases from June to August. This reflects the change of dry to wet surface characteristics resulting from frequent precipitation during the summer monsoon. Over the Tibetan Plateau, the diurnal range of the surface temperature is as large as the annual range, so that the resultant sensible heat flux has a large diurnal variation. Thus, the hourly estimation based on the GMS data may contribute to a better understanding of the land surface–atmosphere interaction in this critical area.

Estimation of Land Surface Temperature over the Tibetan Plateau Using GMS Data

Geostationary Meteorological Satellite Visible/Infrared Spin-Scan Radiometer (GMS VISSR) images have been used to estimate diurnal variations of land surface temperature distributions over the Tibetan Plateau. The infrared split-window algorithm developed for NOAA Advanced Very High Resolution Radiometer (AVHRR) has been adapted for this purpose. Radiative transfer simulations are carried out to obtain the atmospheric transmittances and the difference temperatures that are involved in the internal coefficients of the split-window algorithm. Precipitable water distribution that is required by this algorithm is estimated from 6.7- mm brightness temperature utilizing spectral characteristics of the GMS water vapor channel. Cloud removal plays an important role in the surface temperature retrieval process. To identify convective cloud activity, many researchers use satellite infrared measurements with a fixed threshold technique. In this study, it is necessary to remove not only convective clouds but also warm clouds. For this purpose, a variable threshold technique is proposed. The threshold varies both seasonally and diurnally, and its value is determined on the basis of surface observations. With a variable threshold, it becomes possible to remove relatively warmer clouds in summer and detect colder ground surfaces at nighttime in the winter. The results of comparing estimated surface temperature from GMS data using this algorithm with in situ surface measurements show correlations around 0.8.

Recent Trends in Land Surface Temperature on the Tibetan Plateau

The diurnal, seasonal, and interannual variations in land surface temperature (LST) on the Tibetan Plateau from 1996 to 2002 are analyzed using the hourly LST dataset obtained by Japanese Geostationary Meteorological Satellite 5 (GMS-5) observations. Comparing LST retrieved from GMS-5 with independent precipitation amount data demonstrates the consistent and complementary relationship between them. The results indicate an increase in the LST over this period. The daily minimum has risen faster than the daily maximum, resulting in a narrowing of the diurnal range of LST. This is in agreement with the observed trends in both global and plateau near-surface air temperature. Since the near-surface air temperature is mainly controlled by LST, this result ensures a warming trend in near-surface air temperature.

Estimation of the cloud effective particle radius using MTSAT-1R data

The algorithm used to retrieve the cloud effective particle radius from the 3.7 μm band was adapted to the corresponding channel of the Japanese Advanced Meteorological Imager (JAMI) flown on board the Multi-functional Transport Satellite (MTSAT) geostationary platform. Snapshot comparisons with spatially well-resolved retrievals from the MODerate resolution Imaging Spectrometer (MODIS) instruments flown on the Terra polar platforms show qualitative agreement with MTSAT retrievals. The results of analysing daytime variation from eastern Asia to the northwest Pacific Ocean show that, not only is the effective particle radius smaller in continental clouds than in maritime clouds, but the daytime amplitude of the effective particle radius is also greater in continental clouds than in maritime clouds, where the effective particle radius value is approximately constant.

Environmental Stability for Convective Precipitation Under Global Warming

Severe convective weather induces various types of local-scale meteorological disasters, e.g., torrential rain, flush flooding, tornadoes, and damaging winds, which have significant impacts on our societal environments. Such convective weather is caused mostly by atmospheric deep convection that accompanies cumulonimbus clouds. With the explosive development and extension of densely populated, metropolitan areas worldwide, vulnerability of such areas to extreme convective weather is rapidly increasing. Therefore, accurate short-term forecasting and reliable long-term projections of extreme convective weather are critically important for the prevention and mitigation of induced disasters. As far as precipitation is concerned, cumulonimbus clouds primarily spawn extreme precipitation events. In a future climate under global warming, it is anticipated that convective weather including extreme precipitation is more prevalent and more intensified in various regions of the world. Previous studies indicated that extreme events producing heavy rainfall are projected to increase in a global warming climate (Kimoto et al., 2005; Kamiguchi et al., 2006). In the Fourth Assessment Report (AR4) from the Intergovernmental Panel on Climate Change (IPCC) (2007), it is summarized that under future global warming the intensity of precipitation events is projected to increase particularly in tropical and high latitude areas. Furthermore, the IPCC report says that in the warming climate the precipitation extremes are more enhanced than the precipitation means in most tropical and mid-/high-latitude areas. However, such future projections on precipitation characteristics are based on the numerical experiments by general circulation models (GCMs) that have horizontal resolutions of much coarser than the scales of individual cumulonimbus clouds, which means that the convective precipitation is not explicitly resolved but only represented by a parameterized way. Characteristics of extreme precipitation events due to deep convection under global warming, therefore, require further investigations in order to gain deeper understandings of convective precipitation under global warming. Even if we are not concerned with the long-term changes of convective precipitation but the short-term forecasts, we are still far from accurately predicting the times and locations of the occurrence of convective precipitation. Such difficulty in predicting the behaviour of cumulonimbus clouds not only in climatic timescales but also in daily-weather timescales is