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 (see below). T_rad is calculated using a simple energy balance at the surface of the snowpack, allowing for contributions from shortwave and long wave radiation, which includes the impacts of topography, cloud cover, vegetation, and 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.

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)

The main cards used in the RTI snow model include:

RTI_SNOW => Tells GSSHA to use the RTI snow model

NWSRFS_MF_CONSTANT 1.3 => Constant Melt Factor

NWSRFS_TIPM 1.0 => Same as TI Snow Method

NWSRFS_NMF 0.2 => Same as TI Snow Method

NWSRFS_FUA .5 => Same as TI Snow Method

NWSRFS_PLWHC 0.002 => Same as TI Snow Method

NWSRFS_FR_USE 0.0 => Same as TI Snow Method