Documentation of the changes from version 1 to version 2
Listed below are the points which changed in the ALMA convention from version 1 to version 2.
Correction of errors and extensions
The maximum rate of Snow Fall (Snowf) was increased from
0.002 to 0.0085 kg/m2/s. This change is based
on observed values in the Rhône basin.
The maximum depth of frozen soil and thawing is upgraded to 5 meters. The previous value of
2 meters was too small.
The ALMA standard now allows that only the wind speed is
provided. As it is straightforward to implement this flexibility
in you code we have provided
- The name and definition of the variable
DelSoilHeat were misleading. This variable is now labeled
DelSurfHeat and provides the change in energy content of the
snow free area. This should not only contain the energy change
in the soil but also take into account the energy in the canopy
if available in the scheme. It is now the true complementary of
the DelColdCont. Beware that if you have frozen water in your
soil you need to include the energy of the phase change as well
in the computation of DelSoilHeat.
- The units of
DelSurfHeat and DelColdCont were incompatible with the
"accumulated" instruction. The units are now corrected to J/m2. Another
solution would have been to replace the "accumulated"
instruction by "averaged" but this would have needed changes for
DelSoilMoist, DelSWE, DelSurfStore and DelIntercept for
- In order to ensure that the total amount of water present in the
soil is kept in only one variable two changes occurred in table
O.4 . The variable SoilMoist is now the total mass of
water present in the soil in the three phases. To separate
the phases two fractions were introduced : The fraction of the
moisture mass which is liquid (SMLiqFrac)
and the fraction of the mass in the solid phase (
SMFrozFrac). The units of these two variables are
"kg/m2/kg/m2" but for simplicity we will
consider this variable to be unit-less
Potential evapotranspiration has been introduced. The aim is to
obtain the potential evaporation which is coherent with the way
the scheme computes evapotranspiration. That is using the same
formulation as for the evapotranspiration but setting all
resistances, except the aerodynamic resistance, to zero. In most
scheme this will not yield a Penman-Monteith potential
evapotranspiration but exactly this discrepancy will be one of the
description of the averaging process to be used for the surface temperatures was
Table O.6 (renamed for the occasion) includes now the depth
of the water table as an optional variable for the model which
table is created for variables to be validated with remote sensed data.
Introduction of diagnostics related to the liquid water content in the snow pack
- Evaporation of the liquid water in the snow pack :
- To make sure that nothing is left out, the sublimation of
the snow free area has been added. This is usually a small flux
and the range for the quality control is still largely
- When liquid water is included in the snow pack the
DelColdCont needs to take into account the phase
- A new variable had to be added for the flux of water
flowing out of the snow-pack :
Qst . If the snow model does not include liquid water this
flux will be equal to snow melt flux. Else the difference
between Qst and the snow melt will provide an information on the
ability of the snow pack to retain liquid water.
- Snow melt (Qsm) has been completed with the
re-freezing flux (Qfz). It is the amount of liquid water
which re-freezes in the snow pack. It can not be a negative Snow
melt as both phase changes can occur at the same time in the snow pack.
The fraction of liquid water (SliqFrac) in the snow mass
has been introduced as a new diagnostic. This does not change
the definition of the snow mass
(SWE) , which remains the total water in the snow pack, but
allows to separate it into the liquid and frozen mass. The units
are "kg/m2/kg/m2" but for simplicity we
will consider this variable to be unit-less. For a multi-layer
scheme this will obviously be a 3D variable.
- As the distribution of rainfall and snow fall on the snow
covered and snow free fractions of the grid may change from one
model to the other, this information needs to be saved. Two new
variables have been introduced to record this :
RainfSnowFrac, SnowfSnowFrac .
Documenting the 3D nature of the snow pack
- Now that a 3D description of the snow pack is possible
SnowT needs to be redefined . It will only include the
snow temperature which interacts with the atmosphere. It can be
more complex than just the first layer temperature of a
multi-layer snow scheme. The temperature within the snow can now
be described with the variable
SnowTProf . For a simple one layer snow scheme this new
definition of SnowT variable does not change anything. SnowTProf
will contain the same values as SnowT.
SWE becomes a 3D variable. Obviously for simple snow
schemes all remains as it was.
SnowDepth and SliqFrac are now 3D variables. In the case of
simple one-layer schemes the vertical dimension becomes a
singleton. The sum over the 3rd dimension of SnowDepth will
yield the total depth of the snow pack.
This table now also includes the vertical profile of temperature
in the snow pack. In the most basic scheme with only one
snow temperature this variable will contain the same values as
DelSWE remain 2D. They are intended for verification of
conservation laws and not for process studies with the data. It
is believed that a verification of the energy and water conservation
of the integrated snow pack will be sufficient for the years to come.
Last modified: Wed Mar 28 13:51:34 GMT 2001