Abrupt Seasonal Changes of Surface Climate Observed in Northern Mongolia by an Automatic Weather Station

Abstract

Continuous observations of surface meteorological elements have been carried out since September 30, 1993, to study the roles of land surface processes in seasonal climate variation. The observations were taken by the AANDERA Automatic Weather Station (AWS) at Baruunkharaa (48°55′N, 106°4′E) in northern Mongolia. This location is the center of the source region of the Siberian high, which is a major center of action for the winter climate and atmospheric circulation over the Eurasian Continent.
Abrupt changes of air temperature and specific humidity were detected during the course of seasonal change from October 1993 to September 1994. The annual cycle was divided into four seasons, based on the timing of these abrupt changes: i. e., winter (mid-November to early March), spring (mid-March to early June), summer (mid-June to later August) and autumn (early September to early November). Over several days in mid-November the air temperature decreased about 20°C, concurrent with an increase in the albedo from 0.5 to 0.9. A coreless winter was characterized by a near constant air temperature of about -15°C or less, along with a high albedo of more than 0.5: i. e., continuous snow cover except for the period from mid-February to early March. Winter ended with a sudden increase of air temperature of about 20°C during several days in mid-March. Spring was characterized by the development of the daytime mixed layer, as suggested from the afternoon decrease of specific humidity. An abrupt increase of specific humidity of about 5gkg^-1 -which occurred in mid-June- coincided with the onset of the summer season. During summer, the daytime increase of specific humidity attained its annual maximum, this is likely due to strong evaporation from the surface. In early September, the specific humidity dropped by about 5gkg^-1 over a period of several days. The characteristics of the autumn onset were nearly the same as spring, except for the absence of afternoon decrease in specific humidity, which implied that the diurnal growth of the mixed layer was not strong enough to create a strong entrainment.