Study of the Positive Degree-day Factors on the Koxkar Baqi Glacier on the South Slope of Tianshan Mountains
The degree-day method is one of the simplest but sufficiently accurate schemes to estimate ablation on a glacier. It bases on linear correlation between ablation and sum of daily mean temperatures above melting point,which is called the positive degree-day sum ( PDD ) during a period. The factor linking the ablation to PDD is called the positive degree-day factor. Based on ablation and PDD,the degree-day factors are calculated. The degree-day factors range from 2.0 to 9.7 mm ℃ -1 d-1 and its average value is 5. 7 mm ℃-1 d-1 . There are some factors influencing the degree-day factors on the Koxkar Baqi Glacier,including altitude,surface condition of the glacier,albedo and terrain. The study shows that:1) The degree-day factor depends on altitude on the Koxkar Baqi Glacier;in general,the degree-day factor increases as altitude increases,then,the factor at higher altitude is larger than that at lower altitude,which is mainly due to the ablation caused by absorbed global radiation at the high altitude,where the PDD is low due to low summer air temperature. 2) The degree-day factor of the Koxkar Baqi Glacier decreases as the mean temperature increases,as what observed in Dongkemadi Glacier,July 1st Glacier,Yala glacier and AX010 Glacier 3) The surface condition of a glacier,including coverage of debris and snow,surface slope,and so on,is an important factor to the degree-day factor on the Koxkar Baqi Glacier. In the ablation area of the Koxkar Baqi Glacier,the zone below 3 900 m a. s. l. is covered with debris,and the zone above 3 900 m a. s. l,is debris-free;the depth of the debris varies from 0 to 250 cm in general. Debris has a strong influence on the surface energy balance and melting of the underlying ice. The surface debris layer works as a good insulator on the Koxkar Baqi Glacier. The factor in the debris-free area is larger than that in the debris-covered area,because of the main physical characteristics of the debris,such as the thermal resistance and albedo,which control the heat conduction to the ice-debris interface. It would be useful to understand the dependence of ablation on degree-day factor in the Koxkar Baqi Glacier,so that one can estimate the contribution of the Koxkar Baqi Glacier melting,using the degree-day method while calculating total discharge from the glacier basin.
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