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 (RWB) was found to be substantially delayed, with very deficient meltwater infiltration as compared to observations. Furthermore, most of the meltwater emerged as runoff, as opposed to soil moisture recharge. The deficiency emanated from the crudeness of snow-physics of the combined snow and ground layer of SSiB. In the current work, a new snow model employing a separate snow-laver was included. The snow-pack absorbs and transmits the incoming solar flux, thereby affecting the snow and ground temperatures through the winter 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 infiltrate more meltwater into the soil. The new-model produces a more realistic simulation of soil-moisture, as well as Volga river runoff in RWB evaluations. 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., neglecting the influence of snow aging on its thermal diffusivity, and simplifications in absorption of solar-flux in the snow cover, leading to an inadequate distinction between snow-pack surface and mean temperatures; and, (3) possible cold bias of the ISLSCP surface air temperature data.