Masanori Yoshizaki

Mismo field experiment in the equatorial Indian Ocean

The Mirai Indian Ocean cruise for the Study of the Madden-Julian oscillation (MJO)-convection Onset (MISMO) was a field experiment that took place in the central equatorial Indian Ocean during October–December 2006, using the research vessel Mirai, a moored buoy array, and landbased sites at the Maldive Islands. The aim of MISMO was to capture atmospheric and oceanic features in the equatorial Indian Ocean when convection in the MJO was initiated. This article describes details of the experiment as well as some selected early results. Intensive observations using Doppler radar, radiosonde, surface meteorological measurements, and other instruments were conducted at 0°, 80.5°E, after deploying an array of surface and subsurface moorings around this site. The Mirai stayed within this buoy array area from 24 October through 25 November. After a period of stationary observations, underway meteorological measurements were continued from the Maldives to the eastern Indian Ocean in early December. All observatio...

Numerical Study of OrograPhic-Convective Precipitation over the Eastern Arabian Sea and the Ghat Mountains during the Summer Monsoon

Abstract When the western coast of India lies in the path of the low-level west-southwest wind crossing the Arabian Sea during the summer monsoon season, deep convection frequently develops over the ocean off the coast. In such a situation, the maximum rainfall occurs near the coast, not over the Western Ghats. In order to study the physics underlying orographic-convective precipitation over this area, a two-dimensional compressible moist cloud model is applied. The model is written in terrain-following coordinates and includes the Coriolis force and a planetary boundary layer parameterization. The initial fields of thermodynamic variables are specified using observed data gathered upstream of the offshore precipitating systems over the Arabian Sea. Two wind profiles are considered: vertically uniform and nonuniform flows. The latter profile represents a monsoonal westerly jet at low levels and easterlies in the layer above 5 km. Three cases are considered for each wind profile by including or omitting mo...

Verification of precipitable water vapor estimated from shipborne GPS measurements

[1] Precipitable water vapor (PWV) was measured using a shipborne Global Positioning System (GPS) during a two month cruise in the equatorial Indian Ocean. More than 300 profiles were also observed by radiosondes released from the ship during the experiment. GPS atmospheric delay and PWV was estimated and compared to the radiosonde observations. The GPS-PWV is in good agreement with the radiosonde PWV (RS-PWV) with an rms error of 2.27 mm and a mean difference of less than 1 mm during the nighttime. In the daytime, the dry bias of RS-PWV becomes 3.63 mm. Thus GPS-PWV observed from a ship under way in the open ocean is sufficiently accurate. The ship based GPS-PWV data are accurate to be useful for numerical weather predictions as well as for the calibration of the satellite remote sensors.

Morning Precipitation Peak over the Strait of Malacca under a Calm Condition

Abstract This paper describes the formation mechanism of morning maximum in the diurnal cycle of precipitation, at the Strait of Malacca under a calm condition, with a nonhydrostatic mesoscale numerical model and ship-based observational data. The morning precipitation peak at the strait is induced by the convergence of two cold outflows that have been produced by the precipitation systems in the previous evening over Sumatra and the Malay Peninsula. The outflows converge at the Strait of Malacca around midnight; a new precipitation system is thus generated and reaches its peak in the early morning. Sensitivity experiments using the numerical model suggest that the timing and position of the morning precipitation peak are affected by the width of the strait. In the case of the Strait of Malacca, its width favors the formation of the morning precipitation system around the center of the strait, which explains why its diurnal cycle of precipitation can be observed much clearer than those in other coastal ar...

Structure and Formation Mechanism on the 24 May 2000 Supercell-Like Storm Developing in a Moist Environment over the Kanto Plain, Japan

Abstract The structure and formation mechanism of a supercell-like storm in a moist environment below a melting layer were investigated using dual-Doppler radar analysis and a cloud-resolving storm simulator (CReSS). The supercell-like storm developed over the Kanto Plain, Japan, on 24 May 2000. The environment of the supercell-like storm possessed large convective available potential energy (1000 J kg−1), strong vertical wind shear (4.2 × 10−3 s−1 between the surface and 5 km above sea level), and a moist layer (the relative humidity was 60%–90% below a melting layer at 3 km in height). The dual-Doppler radar analysis with a variational method revealed that the supercell-like storm had similar structures to those of a typical supercell in a dry environment below a melting layer, such as that in the Great Plains in the United States. The structures included a hook echo, an overhanging echo structure, and a strong updraft with strong vertical vorticity. However, some of the characteristics of the supercell...

Two-and Three-Dimensional Modelling Studies of the Big Thompson Storm

Abstract The Big Thompson storm occurred on 31 July–1 August 1976 over Big Thompson Canyon, Colorado, when a secondary cold frontal surge was accelerated and reached the foothills of the Front Range. Two- and three-dimensional moist compressible cloud models developed by Ogura and Yoshizaki are applied to this storm event. Adopting highly simplified terrain shapes, this study addresses two aspects of the storm. One is the distinct characteristics of the storm structure, as schematically depicted by Carasena et al.; the other is that heavy precipitation occurred in the basin area rather than over the mountain peak area. When the model was initialized in such a way that moisture-rich, low-level, strong easterlies impinged upon the orography, the model predicted the development of a storm that not only caused heavy precipitation at the right location relative to the mountain peak, but also reproduced the observed storm in many aspects, both in two- and three-dimensional (2D and 3D) simulations. The major qua...

Performance of Long-Term Integrations of the Japan Meteorological Agency Nonhydrostatic Model Using the Spectral Boundary Coupling Method

The spectral boundary coupling (SBC) method, which is an approach used to couple a limited-area model with a large-scale model, was introduced into a nonhydrostatic model. To investigate whether the SBC method works well in a long-term integration of a high-resolution nonhydrostatic model, two numerical experiments were conducted with a model having a horizontal grid interval of 5 km. In one experiment, the SBC method was employed, while it was not in the other experiment. The time integration in both experiments was over a 40-day period. The nonhydrostatic model was nested into objectively analyzed fields, instead of the forecasts from an extended-area model. Predicted patterns of sea level pressure and precipitation were compared with objective analyses, and data provided by the Global Precipitation Climatology Project (GPCP), respectively. The predicted rainfall amounts and surface temperature over the Japanese islands were statistically evaluated, making use of the analyzed rainfall and surface data observed by the Japan Meteorological Agency (JMA). All results examined in the present study exhibited better performances with use of the SBC method than those without the SBC method. It was found that the SBC method was highly useful in long-term simulations by a high-resolution nonhydrostatic model.

Numerical Simulations of the Formation of Melting-Layer Cloud

Abstract A number of previously published observational studies have reported the common occurrence of cloudy layers at around 5-km elevation in the tropics. There are two candidate processes that are able to explain the occurrence of cloudy layers in the middle level: cloud detrainment promoted by the stable layer and enhanced condensation to compensate for melting cooling. In the present study, the authors used a cloud-resolving nonhydrostatic model and conducted numerical simulations of a squall line to clarify the process responsible for the formation of midlevel thin cloud, especially the cloud at the 0°C level. In a two-dimensional control experiment thin cloud was simulated in the middle level, and cloud coverage showed a notable peak just below the 0°C level for environments without a stable layer in the initial temperature profile. Enhanced and weakened stability layers simultaneously appeared above and below the peak level of the cloud coverage. The formation of midlevel thin cloud is associated...

A Numerical Study of the Mesoscale Convective System Observed over Okinawa Island in June 1987

Abstract A mesoscale convective system (MCS) observed over Okinawa Island in Japan during the Baiu season is studied numerically by using a two-dimensional compressible model with warm-rain cloud physics. The initial formation of the system on the leeside of the mountain and two stages of the evolution (stationary and propagating stages) are simulated. The stationary stage, which is made by the piling of the cold pool in the lower layer for the system to move against the flow as a density current, is more distinguished by the orographic effects of forced lifting to the MCS in the upwind side of the mountain and damming the outflow spreading in the leeside. Once cold-air overflows the mountain crest, the system moves fast upwindward and the propagating stage starts. The simulated MCS has a vertical structure similar to those observed in midlatitude-tropical squall lines.

Simulations of Forecast and Climate Modes Using Non-Hydrostatic Regional Models

Two applications with a cloud-resolving model are shown utilizing the Earth Simulator. The first application is a case in the winter cold-air outbreak situation observed over the Sea of Japan as a forecast mode. Detailed structures of the convergence zone (JPCZ) and formation of mechanism of transverse convective clouds (T-modes) are discussed. A wide domain in the horizontal (2000 × 2000) was used with a horizontal resolution of 1 km, and could reproduce detailed structures of the JPCZ as well as the cloud streets in the right positions. It is also found that the cloud streets of T-modes are parallel to the vertical wind shears and, thus, similar to the ordinary formation mechanism as longitudinal convective ones. The second application is changes in the Baiu frontal activity in the future warming climate from the present one as a climate mode. At the future warming climate, the Baiu front is more active over southern Japan, and the precipitation amounts increase there. On the other hand, the frequency of occurrence of heavy rainfall greater than 30 mm h-1 increases over the Japan Islands.