New snow-physics to complement SSiB part I: Design and evaluation with ISLSCP initiative I datasets

In evaluation(s) as a part of the Global Soil Wetness Project using ISLSCP Initiative-I data, the snowmelt in SSiB in the Russian Wheat Belt region (R WB) was found to be substantially delayed, with very deficient meltwater in filtration as compared to observations. Furthermore, most of the meltwater emerged as runoff, as opposed to soil moisture recharge. The deficiency ema nated from the crudeness of snow-physics of the combined snow and ground la yer of SSiB. In the current work, a new snow model employing a separate sno w-laver was included. The snow-pack absorbs and transmits the incoming sola r flux, thereby affecting the snow and ground temperatures through the wint er and snow-melt periods. In the ISLSCP Initiative-I data evaluations, the snow-melt over the RWB region occurs 2-3 weeks sooner in the new model, and its soil thaws quite early in the snow-melt duration, which helps to infil trate more meltwater into the soil. The new-model produces a more realistic simulation of soil-moisture, as well as Volga river runoff in RWB evaluati ons. Some residual delay in the snow-melt (varying from 1-4 weeks) seems to be related to the following: (1) inaccuracies in the satellite retrievals of snow under dense forest canopies; (2) the modeling assumptions, e.g., ne glecting the influence of snow aging on its thermal diffusivity, and simpli fications in absorption of solar-flux in the snow cover, leading to an inad equate distinction between snow-pack surface and mean temperatures; and, (3 ) possible cold bias of the ISLSCP surface air temperature data.