Takehiko Satomura

Climatological Characteristics of the Intraseasonal Variation of Precipitation over the Indochina Peninsula

Abstract With the use of daily rain gauge data observed at 210 stations in the Indochina Peninsula (ICP) for the 26 yr from 1978 to 2003, this paper describes climatological characteristics of 2 types of intraseasonal variations (ISVs): the 30–60-day variation (30–60DV) and the 10–20-day variation (10–20DV). The authors find that these characteristics are quite different from place to place in the ICP. During the rainy season, variance of the 30–60DV is generally larger in coastal regions than over inland regions and it has two local maxima: one found in the coastal region of Myanmar (CMY) and the other in the southern Laos and central Vietnam region (SLCV). Wavelet analysis reveals that the 30–60DV in the CMY is active throughout the rainy season (May–October) and exhibits the maximum activity in May–June. In addition, its typical time scale shifts from 40 days in the early half of the rainy season to 50 days in the latter half. Cross-correlation analysis reveals that its signal propagates northward. On ...

Nonhydrostatic Atmospheric Modeling Using a Combined Cartesian Grid

Abstract A new method for representing topography on a Cartesian grid is applied to a two-dimensional nonhydrostatic atmospheric model to achieve highly precise simulations over steep terrain. The shaved cell method based on finite-volume discretization is used along with a cell-combining approach in which small cut cells are combined with neighboring cells either vertically or horizontally. A unique staggered arrangement of variables enables quite simple computations of momentum equations by avoiding the evaluation of surface pressure and reducing the computational cost of combining cells for the velocity variables. The method successfully reproduces flows over a wide range of slopes, including steep slopes where significant errors are observed in a model using conventional terrain-following coordinates. The advantage of horizontal cell combination on extremely steep slopes is also demonstrated.

An Observational Study of Intraseasonal Variations over Southeast Asia during the 1998 Rainy Season

Abstract In the present study, intraseasonal variations of precipitation over Thailand [Thailand daily rainfall (TDR)] during 1998 and the accompanying variations of the tropospheric circulation are analyzed, mainly with the use of rain gauge data and the Global Energy and Water Cycle Experiment (GEWEX) Asian Monsoon Experiment (GAME) reanalysis version 1.5. Wavelet analysis reveals that the amplitudes of the intraseasonal variations of the 1998 TDR exhibit two peaks. One peak is categorized as a 30–60-day variation that appeared in the early half of the rainy season, while the other peak is categorized as a 10–20-day variation that appears in the later half of the rainy season. The variation of atmospheric circulation associated with the latter peak of the TDR is analyzed. When the 10–20-day variation of the TDR is in the most active phase, a negative streamfunction anomaly at the 850-hPa level exists over the Bay of Bengal, and a positive anomaly is found in the Southern Hemisphere to the south of the n...

Westward generation of eastward-moving tropical convective bands in TOGA COARE

Mechanisms responsible for westward generation of eastward-moving tropical convective bands in the Tropical Ocean and Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) are investigated using a two-dimensional numerical cloud model. Sequential generation of new convective bands to the west of an old eastward-moving convective band is successfully simulated in an environment of a convectively active day during TOGA COARE, characterized by west winds at low levels and strong easterlies aloft. It is concluded that the westward generation of new convective bands is explained by a gravity wave mechanism. Two westward-propagating modes excited below the convective cells moving westward relative to the convective bands appear to play an important role. A slow-propagating mode (; 15 ms 21) excited by a shallow convective band is ducted in the troposphere under an unstable layer of small Richardson number containing its critical level. A fast-propagating mode ( ;25 m s21) excited by a deep convective band is ducted in the troposphere under the remaining region of the convective cell containing its critical level. These two modes propagate horizontally to the west and promote the growth of shallow convection into long-lived convective bands. A dry model with thermal forcing representing the convective cell showed that preferential excitation of westward-propagating waves below the convective cell is due to westward motion and ascension of the convective cell. A comparative simulation without the critical level confirms the proposed gravity wave mechanism.

Mechanisms of the Northward Movement of Submonthly Scale Vortices over the Bay of Bengal during the Boreal Summer

Abstract Mechanisms of the northward movement of submonthly scale vortices over the Bay of Bengal during the boreal summer (May–September) are studied with the use of a vorticity budget analysis applied to the ECMWF 40-yr Re-Analysis (ERA-40) data. To quantitatively evaluate the contribution from each term that constitutes the vorticity anomaly equation to the movement of the vortices, a vector measure, termed the forcing vector (FV), is used in the present study. Because the axis of the submonthly scale relative vorticity anomaly does not tilt meridionally below the 200-hPa level, the mechanisms of the northward movement of a composite submonthly scale vortex integrated from the surface to the 100-hPa level [the barotropic component (BTC)] are studied. The barotropic vortex moves northwestward, with northward speeds of 0.9° day−1. The meridional component of the FV (MFV), which represents the contribution to the meridional component of the movement, reveals that the primary and secondary terms that contr...

Geographical Distribution of Variance of Intraseasonal Variations in Western Indochina as Revealed from Radar Reflectivity Data

Abstract This study reveals remarkable differences in the geographical distribution of variance between two types of intraseasonal variations in daily-mean radar reflectivity data in the western part of the Indochina Peninsula. In this region, the Downa Range lies parallel to the coast and separates the inland region from the coastal region. The 30–60-day variation of reflectivity factor dominates most of the coastal region, while its variance in the inland region is less than that of background red noise with the same frequency band. Horizontal gradients in the variance are largest over the range, implying that the mountain range plays a significant role in the geographical contrast. Correlation analysis with reanalysis data shows that the variation only in the coastal region is associated with a synoptic-scale zonal wind anomaly with the same time scale, suggesting the importance of an orographic rainfall process that brings a large amount of precipitation only to the windward side of the Downa Range. I...

Flow Regimes of Nonlinear Heat Island Circulation

Abstract Previous laboratory and numerical experiments show that the nonlinear heat island circulation has two different flow regimes: One has two maximums of updraft at both edges of the heat island (type E), while the other has a single maximum of updraft at the center of the island (type C). Our theoretical consideration shows that the heat island circulation is principally governed by two nondimensional parameters: a nonlinear parameter eN = Δθ/(Γδ) and a Prandtl number Pr = ν/κ, where Δθ is the surface temperature anomaly of the heat island, Γ the vertical gradient of the basic potential temperature, δ the thickness of the thermal boundary layer, ν the kinematic viscosity, and κ the temperature diffusivity. For a given fluid, the Prandtl number is fixed, so that the flow regime is principally determined by the nonlinear parameter eN. Numerical experiments are performed to confirm the theoretical prediction. The steady-state flows obtained from the numerical experiments are categorized into either of ...

Three-dimensional cut-cell modelling for high-resolution atmospheric simulations

Owing to the recent rapid development of computer technology, the resolution of atmospheric numerical models has increased substantially. With the use of next-generation supercomputers, atmospheric simulations using horizontal grid intervals of O(100) m or less will gain popularity. At such high resolution, more of the steep gradients in mountainous terrain will be resolved, which may result in large truncation errors in those models using terrain-following coordinates. In this study, a new three-dimensional (3D) Cartesian coordinate non-hydrostatic atmospheric model is developed. A cut-cell representation of topography based on finite-volume discretization is combined with a cell-merging approach, in which small cut cells are merged with neighbouring cells either vertically or horizontally. In addition, a block-structured mesh-refinement technique is introduced to achieve a variable resolution on the model grid, with the finest resolution occurring close to the terrain surface. The model successfully reproduces a flow over a 3D bell-shaped hill that shows a good agreement with the flow predicted by the linear theory. The ability of the model to simulate flows over steep terrain is demonstrated using a hemisphere-shaped hill. The advantage of a locally refined grid around the hill, with cut cells at the terrain surface, is also demonstrated. The model reproduces smooth mountain waves propagating over varying grid resolution without introducing large errors associated with the change of mesh resolution. At the same time, the model shows a good scalability on the locally refined grid.

Categorizing precipitating clouds by using Radar and Geostationary satellite

The classification of precipitating cloud systems over Thailand was attempted by using radar reflectivity and Multifunctional Transport Satellites (MTSAT) infrared brightness temperature (TBB) data. The proposed method can classify the convective rain (CR) area, stratiform rain (SR) area and non-precipitation area such as cumulus and cirrus cloud by applying an integrating analysis of rain gauge, ground-based radar and geostationary satellite data. Since the present study focuses on precipitation, the classified results of precipitation area are used to estimate quantitative precipitation amount. To merge different rainfall products, the bias between the products should be removed. The bias correction method is used to estimate spatially varying multiplicative biases in hourly radar and satellite rainfall using a gauge and radar rainfall product, respectively. An extreme rain event was selected to obtain the multiplicative bias correction and to merge data set. Correlation coefficient (CC), root mean square error (RMSE) and mean bias are used to evaluate the performance of bias correction method. The combined radar-MTSAT method is a simple and useful method. This method has been successfully applied to merge radar and gauge rainfall for hydrological purpose.