Difference between revisions of "Radiation-derived Temperature Index"

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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<sub>a</sub>, &deg;C) with a radiation-derived proxy temperature (T<sub>rad</sub>, &deg;C) in the melt equations.  T<sub>rad</sub> 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<sub>rad</sub> includes contributions from the snowpack, the need to calibrate tow melt factors (M<sub>f,min</sub> and M<sub>f,max</sub>) can be replaced with a constant melt factor (M<sub>f</sub>).
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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<sub>a</sub>, &deg;C) with a radiation-derived proxy temperature (T<sub>rad</sub>, &deg;C) in the melt equations (see below).  T<sub>rad</sub> 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<sub>rad</sub> includes contributions from the snowpack, the need to calibrate tow melt factors (M<sub>f,min</sub> and M<sub>f,max</sub>) can be replaced with a constant melt factor (M<sub>f</sub>).
  
 
The RTI snow model is based on Follum et al. (2015).
 
The RTI snow model is based on Follum et al. (2015).
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The RTI model has been tested at several locations, with the RTI model showing to more accurately capture spatial heterogeneity within the snowpack.
 
The RTI model has been tested at several locations, with the RTI model showing to more accurately capture spatial heterogeneity within the snowpack.
  
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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<sup>-1</sup> 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<sub>r</sub>, is assumed to be 0 &deg;C or the air temperature, whichever is greater.  Both melt routines only work when the air surface temperature is greater than 0 &deg;C and only one melt routine is run per time step.
 
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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<sub>TI</sub>) 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.
 
  
 
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| width=550 | ''M<sub>f</sub> = constant value defined by user || (16)
 
| width=550 | ''M<sub>f</sub> = constant value defined by user || (16)
 
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The main cards used in the RTI snow model include:
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:RTI_SNOW                  => Tells GSSHA to use the RTI snow model
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:NWSRFS_MF_CONSTANT 1.3    => Constant Melt Factor
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:NWSRFS_TIPM 1.0          => Same as TI Snow Method
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:NWSRFS_NMF 0.2            => Same as TI Snow Method
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:NWSRFS_FUA .5            => Same as TI Snow Method
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:NWSRFS_PLWHC 0.002        => Same as TI Snow Method
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:NWSRFS_FR_USE 0.0        => Same as TI Snow Method

Latest revision as of 19:21, 25 September 2018

The Radiation-derived Temperature-Index (RTI) snow model is based on the SNOW-17 snow model (TI method in GSSHA), but replaces Air Temperature (Ta, °C) with a radiation-derived proxy temperature (Trad, °C) in the melt equations (see below). Trad 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 Trad includes contributions from the snowpack, the need to calibrate tow melt factors (Mf,min and Mf,max) can be replaced with a constant melt factor (Mf).

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
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