meltpdd_simple

 PURPOSE:  simple script to calculate melt for a DEM based on a temperature index model
 using positive degree days (PDD).
 -------------------------------------------------------------------
 USAGE: 
 where: [dem] is the input topography grid
        [T] is the mean annual air temperature MAAT at sealevel in degC
        [minalt] is the minimum altitude to calculate melt for (default
        1000)
        [params] is a vector containing the 3 parameters ddf_ice, lrate, a_seas
        and tau_a (degree day factor for ice[m/d degC], temperature lapse rate in deg/100m 
        annual temp fluctuation in degrees and length of the year in days. 
        Default is [8.0,0.6,5.0,365];
        
 -------------------------------------------------------------------------
 OUTPUTS:
        
 -------------------------------------------------------------------
 NOTE:  only pdd_ice used atm, no seasonal perturbation etc
        Melt for areas lower than [minalt] is set to 0 instead of NaN so
        subgrids can be added!

 See also:
 Functions used:

 Felix Hebeler, Geography Dept., University Zurich, Jan 2007.

0001 function m = meltpdd_simple(dem,T,minalt,params)
0002 % PURPOSE:  simple script to calculate melt for a DEM based on a temperature index model
0003 % using positive degree days (PDD).
0004 % -------------------------------------------------------------------
0005 % USAGE:
0006 % where: [dem] is the input topography grid
0007 %        [T] is the mean annual air temperature MAAT at sealevel in degC
0008 %        [minalt] is the minimum altitude to calculate melt for (default
0009 %        1000)
0010 %        [params] is a vector containing the 3 parameters ddf_ice, lrate, a_seas
0011 %        and tau_a (degree day factor for ice[m/d degC], temperature lapse rate in deg/100m
0012 %        annual temp fluctuation in degrees and length of the year in days.
0013 %        Default is [8.0,0.6,5.0,365];
0014 %
0015 % -------------------------------------------------------------------------
0016 % OUTPUTS:
0017 %
0018 % -------------------------------------------------------------------
0019 % NOTE:  only pdd_ice used atm, no seasonal perturbation etc
0020 %        Melt for areas lower than [minalt] is set to 0 instead of NaN so
0021 %        subgrids can be added!
0022 %
0023 % See also:
0024 % Functions used:
0025 %
0026 % Felix Hebeler, Geography Dept., University Zurich, Jan 2007.
0027 
0028 
0029 % default parameter values
0030 ddf_ice  = 0.008; % DDF for ice in m/d degC
0031 %ddf_snow = 0.003;
0032 lrate = 0.6;  % temperatur lapse rate in deg/100m
0033 a_seas   = 5.0;   % annual amplitude of temperature fluctuations in degrees
0034 tau_a    = 365;   %length of the year in days used in PDD
0035 
0036 if ~exist('minalt','var')
0037     minalt=1000;
0038 end
0039 
0040 if ~exist('params','var')
0041     params=[ddf_ice,lrate,a_seas,tau_a];
0042 end
0043 
0044 
0045 % initialise
0046 m = T-(dem*params(2)/100); % get surface temperature for the DEM
0047 %melt=nan(12,size(dem,1),size(dem,2)); for monthly values
0048 %figure, imagesc(m)
0049 %title('Temperature')
0050 % calc melt
0051 m = m * params(1)*params(4);
0052 m(m<0)=0;
0053 m(dem<minalt)=0;
0054 %figure, imagesc(m)
0055 %title('Melt')