## New Monthly Snow Scaling Correction

The first version of ACE had a method to calculate a correction factor for snow, but only for the time invariant case:

This is a simple calculation based on Equation 3.75 of Gosse and Phillips 2001 for changes in neutron flux resulting from mass shielding:

$\Phi_{f,cover} = \Phi_{f} e^{-Z_{cover} / \Lambda_f}$

Where $\Phi_{f}$ is the flux in the absence of snow cover, $\Phi_{f,cover}$ is the flux with a snow cover, $Z_{cover}$ is the thickness of snow in g cm-2 and $\Lambda_f$ is the attenuation coefficient for neutrons (default value of 160 g cm-2 in the above panel).  The panel above multiplies the snow thickness with the density to get $Z_{cover}$, divides by the attenuation coefficient $\Lambda_f$ and finds this number to the power of $e$.

A problem with this is that rarely is snow cover constant with time, and because of the exponential dependence on depth the snow shielding correction factor is not linear with the time history of snow cover.  As a result a more realistic snow cover correction term includes the temporal variability of snow cover (Gosse and Phillips 2001 Eq. 3.76):

$S_{snow} = \frac {1}{12} \sum_{i=1}^{12} e^{-Z_{cover,i} / \Lambda_f}$

where $S_{snow}$ is the snow correction factor and $-Z_{cover,i}$ is the snow cover during month $i$.  The snow cover has units of g cm-2 and is the product of snow depth and snow density. ACE now includes a monthly snow cover utility which is useful for users with monthly data comprising snow depth and snow density:

All variables and units are the same as for the time invariant mass shielding correction factor, but are monthly values instead of annual.