Climatology of Environmental Parameters for Mesoscale Convections in Japan

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 −2 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 −2 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.