No abstract

Climatological monthly statistics of parameters that are widely used to characterize the environment for mesoscale convective systems in Japan is studied by using twice-daily (09 and 21 JST: Japan Standard Time) routine rawinsonde data from 18 stations of the Japan Meteorological Agency for the period between 1990 and 1999. The environmental parameters examined include CAPE (Convective Available Potential Energy), CIN (Convective Inhibition), precipitable water, SSI (Showalter Stability Index), and bulk Richardson number. The monthly medians of these parameters have the following characteristics: 1) Large values of CAPE generally occur in summer. Maximum of median CAPE is larger in the southwest than in the northeast of the Japan islands, and the month of the largest CAPE comes earlier in the southwest than it does in the northeast. At a majority of the stations, the medians of CAPE start to increase rapidly from June and remain large until September. 2) Minimum of SSI is generally found during the summer. SSI tends to be smaller for larger CAPE. The frequency that SSI becomes less than or equal to À2 K at 21 JST is significantly higher than that at 09 JST at all stations. In terms of the frequency that SSI becomes less than or equal to À2 K or 0 K, Tateno at 21 JST in August is found to be most unstable among the stations examined. 3) Most of the stations have large CIN in winter and small CIN in summer (July and August), indicating that convection may be triggered by a weaker forcing in summer. 4) Precipitable water shows a smooth seasonal variation with one peak during summer. Precipitable water at night (21 JST) is slightly higher than that at morning (09 JST) at most of the stations all the year round. 5) The largest Bulk Richardson number occurs during summer when CAPE is larger and vertical wind shear is smaller.

Strong phytoplankton blooms are occasionally observed around a recurvature point of typhoon tracks in the western subtropical Pacific. These are noteworthy events in subtropical regions where both nutrient concentrations and biological production are persistently low. We investigated the response of phytoplankton to typhoon passage using a numerical model with/without biogeochemical processes. The model reproduced the observed patch‐like phytoplankton bloom around a recurvature point of Typhoon Keith in 1997. The strong bloom is caused by the typhoon‐centered upwelling of nutrient‐rich water from below the euphotic layer, which supplies the nutrients required for phytoplankton growth, resulting in higher chlorophyll‐a concentrations. Biogeochemical processes then play essential roles in determining the response after the passage of typhoons in subtropical regions.

In this study, the authors focused on the seasonal variations of precipitation properties over the western Pacific, particularly those associated with the wind direction of the monsoon. An observational project over Peleliu Island in the Republic of Palau was carried out, and data on precipitation, equivalent cloud amount, and precipitable water were collected from 28 June 2001 to 30 April 2002. First, the monsoon season over Palau was defined as a period with 850-hPa zonal-wind sounding data with sustained winds exceeding 5 m s−1. The westerly wind regime continued until 25 November 2001, and the next westerly wind regime began on 18 May 2002. The equivalent cloud amount increased during the period when the westerly wind intensified. The precipitation had a diurnal variation in the active phase of the westerly wind regime, increasing from nighttime to early morning and decreasing in the afternoon. The diurnal variation was weak in the inactive phase and had a lesser afternoon maximum. Precipitation intensity was high and its duration was short during the westerly wind regime. The precipitable water decreased during the easterly wind regime when a dry period appeared, and precipitation was also suppressed during those days. However, there was little difference between the precipitation amounts of the westerly and easterly wind regimes. The equivalent cloud amount did not decrease as the zonal-wind direction changed to easterlies during the easterly wind regime. The authors noticed no diurnal variation of precipitation during the easterly wind regime. These differences in the precipitation properties during westerlies and easterlies may be related to the seasonal variation of humidity in the environment.

This paper describes the structure and evolution of northward propagating mesoscale convective systems (MCSs) observed over the tropical western Pacific on 15 June 2005. A wedge‐shaped convective cloud area, consisting of three groups of MCSs, was generated near the equator to the north of New Guinea at around 0000 LST on 15 June, and one of the groups of MCSs propagated toward Palau (7°–8°N). Dropsonde analysis revealed that the MCSs developed at the northern edge of a cold air mass that had a horizontal scale of more than 400 km. Global objective analysis data from the Japan Meteorological Agency showed that the cold air mass observed by the dropsonde analysis originated in a land breeze from New Guinea. The cold air mass was initially formed by cold advection from the south and was maintained by cooling with the MCSs. When the initial MCSs were generated along the land‐breeze front, an easterly wave, which was accompanied by low‐level southerly wind and a low‐pressure area, was located near Palau. The cold air mass associated with the land breeze between 132° and 137°E was locally extended toward the center of the low‐pressure area. The results suggest that the easterly wave located around Palau helped locally promote northward cold advection from New Guinea, inducing the long‐distance northward propagation of MCSs generated along the land‐breeze front.