Radiation-derived Temperature Index

The Radiation-derived Temperature-Index (RTI) snow model is based on the SNOW-17 snow model (TI method in GSSHA), but replaces Air Temperature (T_a, °C) with a radiation-derived proxy temperature (T_rad, °C) in the melt equations. T_rad is calculated using a simple energy balance at the surface of the snowpack, allowing for contributions from shortwave radiation, shading, cloud cover, and the atmosphere. All of the calculations are internal within GSSHA, limiting the need for additional inputs. Because T_rad includes contributions from the snowpack, the need to calibrate tow melt factors (M_f,min and M_f,max) can be replaced with a constant melt factor (M_f).

The RTI snow model is based on Follum et al. (2015).

The RTI model has been tested at several locations, with the RTI model showing to more accurately capture spatial heterogeneity within the snowpack.

method of estimating snowfall accumulation and melting is based on the National Weather Service River Forecasting System (NWSRFS) SNOW-17 model. This method takes into consideration the time of year, melt due to temperature, melt due to precipitation, and Heat Deficits within the snow pack. Two equations are used to calculate the amount of melt during a time step. Equation 7 is used in precipitation dominated time spans when the average precipitation over the previous 6 hours has exceeded 0.25 mm hr^-1 and precipitation is occurring during the current time step. Equation 8 is used in all other times when the melt is considered temperature-dominated. The precipitation temperature, T_r, is assumed to be 0 °C or the air temperature, whichever is greater. Both melt routines only work when the air surface temperature is greater than 0 °C and only one melt routine is run per time step.

The TI method also keeps track of melt water being stored in the snowpack and water released from the snow pack. Although Equations 1-5 help simulate the “ripeness” of the snow pack by accounting for the heat deficit, Equations 18-21 are included in the TI method to help determine how much liquid water is being stored within the snow pack. The amount of melt that leaves the snow pack (M_TI) is calculated in Equation 20 and is representative of the overall melt from the pack after the heat deficit and water storage capacity of the pack are accounted for.

Mr = σ * dt * [(Trad + 273)4 - 2734] + 0.0125 * Px * fruse * Tr + 8.5 * fua * (dt/6) * [(0.9 * esat - 6.11) + 0.00057 * Pa * Trad

(14)

Mnr = Mf * (Trad - MBASE) + 0.0125 * Px * fruse * Tr

(15)

Mf = constant value defined by user

(16)