THE TURBULENT STRUCTURE AND SEASONAL VARIATION OF NOCTURNAL COLD DRAINAGE FLOW INTRUDING A PLAIN AND COASTAL AREA.

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The observational case study of the nocturnal drainage cold flow on 26 April 1997 shows the characteristics of the gravity current which intduded into land breeze occurred in the Kochi Plain. From the soder measurements the depth and speed of the drainage cold flow is about 75-80m and about 3.5m/s, respectively.The three points for investigating the feature of the cold flow are settled. The observations from April 1997 to the present (March 1998) at Saoka primary school, the points existed nearer the mountainous area, indicate the turbulent characteristics and the seasonal variations as follows :1.the simultaneous abrupt increase of air temperature and large fluctuation of mixing ratio before onset of the upslope flow (valley wind) are found. The phenomena are associated with the thermal convection along the side slope of mountain.2.The cold flows consisted of the air mass intruded along the side slope and along the valley bottom drained into the plain at around the sunset time of Kochi.3.From the histograms of the two thermal meteorogical factors (air temperature [T], mixing ratio [q] in the calm condition), each modes are 24ﾟC in T,16 (g/kg) in q during summer season, and 7ﾟC in T,3 (g/kg) during winter season.