Geographical variations in seasonal characteristics of nocturnal cooling in central japan

Geographical variations in nocturnal cooling are investigated using statistical analyses of routine observational data in central Japan. Seasonal variations in the relationships between nocturnal cooling intensity and topographic features are investigated in terms of synoptic atmospheric conditions and derived mesoscale circulation. Cluster analysis demonstrates that nocturnal cooling types can be classified into six main categories. Seasonal patterns of each cooling type are associated with the origin of weak wind conditions. Weak winds in inland areas are of down-slope winds that drain cold air, whereas those in coastal areas are weak synoptic winds. Nocturnal cooling is strengthened with altitude up to around 700 m during spring and summer, likely caused by the effect of water vapor gradient.