How Snow Accumulates

Before we go into the assumptions used by NWP models, ensembles, and snow accumulation products, we need to go over some important physical details on how snow accumulates. Multiple factors can modulate how snow will accumulate as it falls from the cloud and reaches the ground. These factors are described below.

Crystal Type(s):

Given the same liquid water equivalent, snow accumulation characteristics at the surface depend heavily on the type of snow crystal(s) present. The type(s) of crystals are governed by the temperature and humidity characteristics of the environment as shown in the figure at right. Snow crystals can be broken down into two major types: plates and columns.

Plates include dendrites (high humidity environment) and solid plates (lower humidity environment). Dendrites are composed of many branches with air trapped between the barbs. The larger the dendrites/aggregates of dendrites, the larger the air-to-snow ratio and the greater potential snow depth upon accumulation at the surface. Solid plates, on the other hand, have no branches with trapped air and will pile up to less depth at the surface. Therefore, according to the figure, knowledge of the atmospheric moisture content is important to determine whether solid plates or dendrite are more likely.

Columns, including needles, tend to settle on the ground with little air between adjacent crystals, particularly as they are compressed, so the snow depth is generally less compared to dendrites and solid plates.

Aggregation:

Aggregation refers to the collection of two or more snow crystals which collide and join together during development in the cloud and/or descent to the surface. This can result in very large particles. A heavy fall of large dendrite aggregates can pile up to great depths quickly due to all of the air that becomes trapped between the branches of the aggregates. On the other hand, solid plates, needles, and other columns do not aggregate as readily when they collide since they don't have branches/barbs.

Riming:

Riming is a specific type of aggregation. Snow crystals can accrete supercooled liquid during descent through a warm cloud layer. Since rime ice results in less air in the branches of dendrites, the snow accumulation will be less than would be expected with the same crystals not subjected to riming.

Wind/Turbulence:

Complex dendrite crystals with many branches and barbs are quite fragile. In windy and/or turbulent conditions, the flakes are apt to collide as they form and fall from their source. The branches and barbs of the dendrites tend to break off during these collisions. Since these fractured crystals will pile up with less air between the branches than more pristine dendrites, the result is less accumulation in windy or turbulent conditions than in calm conditions with comparable thermodynamic profiles, as there is less air caught in the accumulated pile of snow.

Even if it is not windy as the snow falls, after accumulating on the surface, blowing snow along the ground can break off branches and barbs on dendrites, causing the snow to settle on the ground at less depth than before the wind started.

Wind Drift:

As the snow falls from its source, strong winds can blow the crystals downwind in their journey between the cloud and the ground. In a large storm system with mostly uniform precipitation, this effect would generally be negligible. But when the snow falls in a narrow band or in convective cells, strong winds in the subcloud layer can displace the location of resulting accumulation on the ground. This will generally not be captured in NWP output.