4.1.1 Resolution
Due to the small-scale characteristics of polar lows, the reduction
in spatial truncation errors provided by increased resolution is of primary
importance.
When dealing with gridpoint models, the forecaster should be aware that
the model is unable to adequately resolve any systems smaller than three
times the model grid spacing. That means a small-scale feature, such as
a polar low with a horizontal scale of around 100 km, would be on the
resolution limit of a typical 50-km resolution regional model. In a model
of that resoulution, the feature would most likely be crudely represented,
if at all.
However, it is important to keep in mind that all components of the
model function synergistically. This means that higher resolution works
best when the model also includes more realistic physics packages and
more detail in the surface specifications of fields such as soil, vegetation,
sea-surface temperature, and topography. High-resolution data must also
be available and the data assimilation system must handle those data
correctly at the resolution of the model.
For more information on high resolution and NWP, see the COMET module,
Ten Common NWP Misconceptions: High Resolution Fixes Everything. http://meted.ucar.edu/norlat/tencom/p02.htm
4.1.2 Representation
of Surface Geophysical Parameters
A correct characterization of surface geophysical fields is an important
factor in modeling mesoscale events. The sea ice boundary and the sea
surface temperature are used in the model as stationary forcing for the
entire integration period. Their accuracy contributes significantly to
the improvement of the forecast. Good quality surface fields increase
the probability of correct positioning and estimates of the latent and
sensible heat fluxes that trigger and feed polar low development.
4.1.3 Parameterization
of Condensation Processes
Condensation processes and, in particular, deep convection have been
shown to be crucial factors in the rapid development of polar lows. The
capacity of the model's convective parameterization is a key determinant
in predicting the development and evolution of a polar low. Convection
must be properly initialized to simulate the outburst of convection that
dominates the mature stage of a polar low.
Some deep convection schemes are more appropriate to certain time and
space scales than others. Therefore, as the model's resolution improves,
it is important to insure that the convective parameterization used is
adequate to simulate the very unique conditions in which polar lows develop.
4.1.4 Assimilation and
Upper Air Analysis
A determining factor for a numerical model to forecast polar lows successfully
is the capacity of the assimilation scheme. The assimilation scheme must
adequately capture and represent the conditions necessary for the onset
of polar low development. Potential vorticity anomalies and boundary
layer structure are of particular concern.
This can pose problems in the Arctic due to the scarcity of radiosonde
and surface data. Upper air data is particularly lacking in this region.
Though new analysis techniques make better use of the available satellite
and aircraft data, adequate coverage continues to be a problem.
References
Parker, Neil, 1997: Cold Air Vortices and Polar Low Handbook for
Canadian Meteorologists. Environment Canada