Characteristics of wind fields derived from multiple-Doppler synthesis and continuity adjustment technique (MUSCAT) and MUSCAT substituting mass flux conservation for the continuity equation are investigated mainly on the basis of the numerically simulated Doppler velocity data, assuming a ground-based dual-Doppler synthesis over a flat plane. The gain factor and the accuracy of the wind components are discussed for MUSCAT and the conventional synthesis. A modified formulation of data fit is tested, and found to be useful. The results of numerical experiments employing height-independent two-dimensional pure divergent wind fields show that the wind components around the surface exhibit similar accuracy, regardless of whether MUSCAT is used with the continuity equation or mass flux conservation. In both cases, the results are more accurate than those obtained from the conventional synthesis as far as the gain factors are close to unity. In a three-dimensional volume, in which the height-dependent two-dimensional wind fields are defined by their horizontal and vertical components, MUSCAT provides more accurate determination of wind components than the conventional method. Finally, MUSCAT is applied to a wind recovery from Doppler radar data collected for a shallow convective snow band over the Sea of Japan in 1993. The results indicate that the vertical wind fields are affected most extensively by the type of mass conservation adjustments and by the weighting factor applied to the filter term. This suggests that these should be appropriately chosen to ensure better wind recovery in certain cases.
The torrential-rainfall-associated tropical cyclone (TC) activity often causes flood disasters in China. Moreover, studies suggest that TC-related rainfall rates are likely to increase in a warmer climate. Using daily precipitation observations at 514 meteorological stations during 1965-2009, this study investigates the primary features of TC rainfall in China, with a particular focus on the contribution of TC rainfall to the changes in the precipitation climate and the average rainfall per TC. TC rainfall mainly affected eastern and southeastern China, accounting for more than 10% of the summer rainfall in South and Southeast China. TC rainfall trended upward in the lower reach of the Yangtze River and Southeast China, contributing to the “wetting in the south and drying in the north” pattern by enhancing the rainfall in Southeast China. This study suggests that the average rainfall per TC has significantly increased in Southeast China during 1965-2009. In the peak season (July-September), all significant changes are upward trends that occur south of the Yangtze River east of 110°E. This study finds that the increasing rainfall per TC was not accompanied with the enhanced TC intensity. In addition, no significant trend can be found in the translation speed of TCs that affected China during 1965-2009, suggesting that the increasing TC rainfall per TC in China was not due to the slowdown of TC movement.
This study focuses on longitudinal evolution of tropical cyclone genesis frequency (TCGF) anomaly over the western North Pacific (WNP) from July to October in the El Niño decaying (EDC) years and its possible mechanisms. Results show that TCGF anomaly exhibits a dipole pattern with a negative center in the eastern WNP and a positive center in the western WNP. In particular, the mean position of negative (positive) anomaly shifts eastward from 130°E (120°E) in July to 160°E (145°E) in October. Moreover, the evolving feature of negative TCGF anomaly is exhibited more clearly. This evolution of TCGF anomaly largely determines the distribution of climatological TC kinetic energy anomaly, which also shifts eastward and persists throughout the EDC fall. Tropical Indian Ocean warming and equatorial central eastern Pacific cooling jointly modulates the large-scale atmospheric circulation and environmental conditions over the WNP, contributing to the eastward shift of TCGF anomaly during the EDC summer and fall. A better understanding of the evolution of TCGF anomaly could be beneficial for the improvement of the seasonal TC prediction over the individual regions over the WNP for the EDC years.
Development of convective clouds on August 24, 2009 in northern Kanto, Japan, was investigated using stereo photogrammetric analysis and Ka-band and X-band radars, in order to clarify the behavior and structure of convective clouds developing around a mountainous area. Convective activity was divided into three stages based on the spatial distribution of X-band radar echoes: stage 1 with no echo (1006-1200 JST), stage 2 with echoes limited to the mountainous area (1200-1400 JST), and stage 3 with echoes developing over the plain (1400-1600 JST). During stages 1 and 2, the convective clouds (echoes) initiated over the mountains, and then moved toward the foot of mountains and dissipated, repeatedly. During stage 3, convective echoes formed over the mountains moved to the plain without dissipating at the foot of the mountains. In stage 2, new convective echoes tended to form to the rear (upshear) side of pre-existing echoes, while in stage 3, some new cellular echoes formed to the front (downshear) side of pre-existing echoes. Specific humidity in the mountains increased in stage 1, while GPS-derived precipitable water at the foot of the mountains increased during stages 2 and 3. The relationship between the behavior of convective clouds and the transport of water vapor by local wind circulation is discussed.
Accurate estimation of evapotranspiration from remotely sensed data typically requires accurate estimation of net radiation, sensible heat flux, and soil heat flux (G). The focus of this paper is on evaluating the accuracy of commonly used empirical soil-heat-flux-estimating equations. The equations were applied to estimate the ratio (Γ) of G to net radiation using 250-m resolution Moderate Resolution Imaging Spectroradiometer (or MODIS) vegetation index data from the Terra and Aqua satellites and ground measurements at three sites in China that represent humid, semi-arid, and arid climates. The results revealed the following: equations that determine Γ on the basis of the vegetation index alone give highly inaccurate results; information on soil moisture condition is critical to obtain more reliable Γ estimates; soil moisture condition of bare soil is related to its albedo in a log-linear way. A new equation has been developed that considers the effect of soil moisture condition through the albedo of bare soil; this equation has been shown to produce highly accurate results over a wide range of climate and land cover types. It is expected that the proposed G-estimating equation could be embedded in remote sensing evapotranspiration algorithms in future studies.
We investigated the impact of the sea surface temperature (SST) over the Sea of Japan on precipitation and snowfall on the Sea of Japan side of Honshu, Japan, using precipitation and snowfall data from Automated Meteorological Data Acquisition System (AMeDAS) stations, records of sea level pressure from meteorological stations, wind fields from the Japanese 25-year reanalysis (JRA25) dataset, and multiple SST datasets. We examined the data on various timescales from daily to monthly because SST over the Sea of Japan also varies on timescales shorter than a month. The results showed that the impact of SST over the Sea of Japan on precipitation on the Sea of Japan side was strongly timescale-dependent. On the shorter intraseasonal timescales of several days to a few weeks, SST had a clear impact on precipitation, while the impact was indistinct when the 15-day and monthly averaged precipitation and SST values were used. On the other hand, cold surges over the Sea of Japan primarily accounted for the amount of precipitation on the Sea of Japan side. The timing and strength of cold surges controlled precipitation on all timescales. We clearly found the impact of cold surges on SST on 15-day mean values, compared with the pentad mean values. To understand detailed impacts of SST on precipitation, including air-sea interaction processes over the Sea of Japan, synoptic and shorter intraseasonal timescales should be examined in addition to seasonal or interannual timescales.
Wind tunnel experiments and large-eddy simulations for stable stratification are performed to specify flux Richardson number Rf and turbulent Prandtl number Pr as a function of gradient Richardson number Ri. We attempted to avoid self-correlation by using independent samples for the variables commonly contained in these nondimensional numbers and confirmed the dependence of Rf and Pr on Ri for 10−3 < Ri < 5. We found that Rf could exceed unity in a stable boundary layer under a developing stage, while the assumption of local energy balance violates for Rf > 1, which corresponds to negative production of turbulent kinetic energy. Nevertheless, the analysis of the TKE budget shows that the third-order term in the prognostic equation of TKE, which plays a role in the TKE transfer, can contribute to increase TKE despite negative TKE production. Therefore, TKE cannot be determined locally and the effects of TKE transfer must be taken into account in the region satisfying Rf > 1.
This study investigates the natural relationships between the carbon cycle and land surface processes in the Northern Hemisphere, related to recent El Niño events by a general circulation model with a Biosphere-Atmosphere Interaction Model Version 2 (BAIM2). Two cases are simulated; the periods of Case 1 and Case 2 are from 1996 to 2000 and from 2001 to 2005, respectively. Case 1 included the El Niño event from 1997 to 1998, and Case 2 included that from 2002 to 2003 in each simulation period. The physical and biological mechanisms of the relationship between the changes of land surface processes with climate changes and variations of CO2 exchange between the atmosphere and terrestrial ecosystem are systematically examined using the results of direct simulation with a terrestrial biosphere-atmosphere fully coupled global climate model. In 1998 and 2003, high surface temperature and low soil wetness in the Eurasian Continent and North America occurred in the warm season in the Northern Hemisphere. These climate conditions are thought to have induced relatively low gross primary production and net ecosystem production values and contributed to anomalously high atmospheric CO2 concentrations in these years. The results of this study suggest that there is a common feature in the relationship between the carbon cycle and land surface processes in the Northern Hemisphere, related to recent El Niño events. Specifically, an anomaly of the atmospheric circulation in the warm season in the Northern Hemisphere in the year El Niño event ends, which is most likely induced by the global teleconnection associated with the tropical sea surface temperature anomaly, causes an anomaly in the land surface processes; this in turn induces anomalously low carbon absorption by the terrestrial ecosystem, which affects the variation of atmospheric CO2 concentration growth rate.
A new cloud simulation chamber was built at the Meteorological Research Institute (MRI) to investigate the details of the fundamental processes of cloud formation. The MRI cloud chamber was designed as an adiabatic-expansion-type cloud chamber covering temperatures from 30 to −100°C, pressures from 1030 to 30 hPa, and an evacuation rates corresponding to ascent rates from 0 to 30 m s−1. Improvements to the cooling system and cloud characterization instrumentation distinguish the new facility from past devices of this type that are no longer functional (e.g., the Colorado State University dynamic cloud chamber), and the capabilities exceed those of any other active facility (e.g., the Aerosol Interactions and Dynamics in the Atmosphere (AIDA) chamber) for covering a range of atmospheric conditions while reproducing approximately adiabatic parcel conditions. Results from the preliminary experiments demonstrate the accuracy of coordinated pressure and temperature controls to reproduce cloud formation processes (both dry and wet adiabatic expansion processes) and the ability of the chamber's instrumentation to measure aerosol, cloud droplet, and ice crystal characteristics. Performance tests demonstrate the chamber's usefulness as a facility to investigate cloud droplet and ice crystal formation processes through the activation of various types of aerosol particles.
A method to assimilate slant total delays derived from global positioning system (GPS) data using the four-dimensional variational data assimilation technique was developed and applied to a line-shaped, local heavy rainfall event that formed on 19 August 2009 over Okinawa Island, Japan. First, to identify the primary factors affecting rainband initiation, we performed impact tests using the Japan Meteorological Agency non-hydrostatic model (JMANHM) with 5-km horizontal grid spacing. Simulations in which the orography of Okinawa was removed successfully reproduced the rainband over southern Okinawa, which showed that the primary factor leading to rainband initiation was land surface heating. However, the timing of rainband initiation in these experiments was delayed, and the rainfall intensities were weaker than those observed. To reduce these discrepancies, we first conducted a high-resolution numerical experiment using JMANHM with 2-km horizontal grid spacing (NODA) followed by data assimilation experiments with GPS observations (i.e., GPS zenith total delay (GPS-ZTD), GPS precipitable water vapor (GPS-PWV), and GPS slant total delay (GPS-STD)) at the same resolution. As a result, increasing the horizontal resolution improved the simulation of the rainfall intensity. Generally, compared with NODA, the assimilations of GPS-ZTD and GPS-PWV are known to slightly improve the timing of the subsequent rainband initiation. However, the GPS-STD assimilation significantly improved the water vapor and temperature fields over a wide area and yielded a clearly improved forecast in terms of both rainfall timing and intensity.