Seasonal characteristics of nocturnal cooling in relation to the downward long-wave radiation in a mountainous area, Central Japan

Nocturnal cooling is one of the distinctive topoclimates that occur in a mountainous area with complex terrain. We have found the seasonal differences in frequency and intensity of nocturnal cooling between summer and autumn in a mountainous area of Central Japan (Iijima and Shinoda, 1998, 2000). An observational study was made of the effect of downward long-wave radiation on the seasonal differences in nocturnal cooling in the study area during the warm season of 1999. An observation of downward longwave radiation was carried out at a subalpine site (2,230m) of Yatsugatake Range (36‹18'N, 138‹22'E) from 21 July through 4 December, 1999, using precision infrared radiometer (PIR, Eppley Lab Inc.). Other climatological elements, such as air temperature, relative humidity and solar radiation, were also observed during the same period. The downward long-wave radiation under the fair-weather conditions exhibits a seasonal maximum (ca. 350W/m²) in mid-August and a minimum (210W/m²) in early-December, along with an abrupt decrease during mid-September. We calculated an intensity of potential nocturnal cooling (PNC) from the air temperature and long-wave radiation data, assuming no sensible and latent heat fluxes (Kondo, 1994). The seasonal change in PNC indicates that the nocturnal cooling is likely suppressed (enhanced) during summer (autumn) related to increased (decreased) downward long-wave radiation. In addition, we defined an intensity of actual nocturnal cooling (ANC) as a difference between the temperature around sunset and nocturnal minimum temperature. The ratio of ANC to PNC is ca. 0.5 and 0.3 for summer and autumn, respectively. The small ratio for autumn suggests that the nocturnal cooling is weakened under the synoptic conditions with strong westerly, although decreased downward long-wave radiation is favorable for the radiative cooling.