Mikiko Fujita

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...

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...

Characteristics of 3–4- and 6–8-Day Period Disturbances Observed over the Tropical Indian Ocean

Abstract A field observational campaign [i.e., the Mirai Indian Ocean cruise for the Study of the MJO-convection Onset (MISMO)] was conducted over the central equatorial Indian Ocean in October–December 2006. During MISMO, large-scale organized convection associated with a weak Madden–Julian oscillation (MJO) broke out, and some other notable variations were observed. Water vapor and precipitation data show a prominent 3–4-day-period cycle associated with meridional wind υ variations. Filtered υ anomalies at midlevels in reanalysis data [i.e., the Japan Meteorological Agency (JMA) Climate Data Assimilation System (JCDAS)] show westward phase velocities, and the structure is consistent with mixed Rossby–gravity waves. Estimated equivalent depths are a few tens of meters, typical of convectively coupled waves. In the more rainy part of MISMO (16–26 November), the 3–4-day waves were coherent through the lower and midtroposphere, while in the less active early November period midlevel υ fluctuations appear le...

Observed behaviours of precipitable water vapour and precipitation intensity in response to upper air profiles estimated from surface air temperature

Extremely heavy precipitation affects human society and the natural environment, and its behaviour under a warming climate needs to be elucidated. Recent studies have demonstrated that observed extreme precipitation increases with surface air temperature (SAT) at approximately the Clausius–Clapeyron (CC) rate, suggesting that atmospheric water vapour content can explain the relationship between extreme precipitation and SAT. However, the relationship between atmospheric water vapour content and SAT is poorly understood due to the lack of reliable observations with sufficient spatial and temporal coverage for statistical analyses. Here, we analyse the relationship between atmospheric water vapour content and SAT using precipitable water vapour (PWV) derived from global positioning system satellites. A super-CC rate appears in hourly PWV when the SAT is below 16 °C, whereas the rate decreases at high SAT, which is different from the precipitation-SAT relationship. The effects of upper air temperature and water vapour can consistently explain the super-CC rate of PWV relative to SAT. The difference between moist and dry adiabatic lapse rates increases with SAT, in consequence of more ability to hold water vapour in the free atmosphere under higher SAT conditions; therefore, attainable PWV increases more rapidly than the CC rate as SAT increases.