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One of the more critical and more difficult winter forecast problems is determination of precipitation type. Will the precipitation be solid, liquid, or freezing? Where will the rain-snow line be located? This lesson focuses on the general determination of solid versus liquid precipitation. Freezing precipitation is important but is discussed in a separate lesson. For a thorough discussion of winter precipitation, complete the first five lessons in Part VIII of this course.
For determination of rain versus snow you need to know more than just the surface temperature. One study of the frequency of rain and snow as a function of temperature shows the following:
|equal probability of rain and snow:||36.5oF / 2.5oC|
|95 percent probability of snow:||34oF / 1.1oC|
|95 percent probability of rain:||42oF / 5.6oC|
|no snow observed:||43oF / 6.1oC or more|
Whether rain or snow occurs at the Earth's surface depends upon the low level thermal structure. That is, the temperature profile in the lowest eight thousand feet above ground level (AGL) determines what happens to precipitation (which we assume forms as snow) as it falls toward the ground. To form rain, you need an above-freezing layer deep enough to melt snow to rain. Studies have found that the melting depth, that is, the depth of the above freezing layer, varies from 750 feet to 1500 feet AGL, depending upon snowflake type, melted drop size, and lapse rate.
Operationally you have no way to determine snowflake type or melted drop size. You may have some feeling for the lapse rate. In general, the depth necessary for melting snow to rain is less when the temperature increases more rapidly toward to surface, i.e., steeper lapse rate. On average, assume that the freezing level must be at least 1200 feet AGL in insure that snow will melt to rain.
Another study summarized the chance of snow as a function of the height of the freezing level:
|Freezing Level Height (AGL)||Probability|
|35 mb / 920 feet||50 percent|
|25 mb / 660 feet||70 percent|
|12 mb / 315 feet||90 percent|
The key to determining precipitation type is anticipating changes in the low level temperature structure. The temperature change equation can be used to assess physical factors that create these changes. In the temperature change question, the local change in temperature is a function of three terms:
Horizontal temperature advection is usually the dominant factor influencing the local temperature change. If there is warm air advection, temperatures increase. If there is cold air advection, temperatures decrease. The best example of cold air advection impacting a location is the very strong cold air advection that follows a cold front passage during the winter.
The vertical displacement term is usually opposite in sign and about half the magnitude of the horizontal advection term. The omega equation tells us that warm air advection (WAA) is associated with upward vertical movement while cold air advection (CAA) is associated with downward vertical movement. Rising air cools at the adiabatic rate while sinking air warms are the dry adiabatic rate. Thus, as a general rule, the combination of WAA (warming) and rising air (cooling) results in a net warming but at about half the magnitude expected from WAA alone.
Consider the special case of a cold core upper low where there is no thermal advection but upward vertical motion. In this case the main thermal affect is from the vertical displacement term. This is a case where snow will tend to stay as snow or where rain may change to snow as cooling occurs.
The non-adiabatic term can impact the vertical temperature profile in a number of ways. Two specific examples are discussed below.
Consider what happens when precipitation falls through an unsaturated layer, particularly a very dry layer. As rain falls through an unsaturated layer, it evaporates, eventually saturating the layer and cooling it to its wet-bulb temperature.
What happens if the temperature of a layer is initially above freezing, but the wet-bulb temperature of the layer is below freezing? In this case, what is initially rain changes to snow as the layer cools. The non-adiabatic term becomes the dominant factor as long as evaporation is occurring. Once the layer becomes saturated and evaporation stops, thermal advection or vertical displacement will again become the dominant factor in determining changes in the local temperature profile.
Using the physical processes just described, you can image a situation where rain turns to snow due to evaporational cooling, then as the layer saturates and WAA reasserts itself, the snow turns back to rain. Not a simple situation nor a simple forecast.
Melting snow to rain requires latent heat. This heat is taken from the surrounding air. In order to obtain substantial temperature change due to melting, it is necessary to have rather heavy amounts of precipitation falling with little or no warm advection. If this occurs, you can have heavy rain turn to heavy snow as the freezing level sinks downward due to cooling by latent heat absorption.
Although this situation can occur, cases of substantial lowering of the freezing level due to melting are relatively rare because the combination of heavy precicpitation without warm air advection is rare.
Radiation and surface sensible heat exchange are non-adiabatic effects that are often important in temperature forecasting. Experience has shown that both of these factors usually have a minor impact on the rain-snow forecast problem. Their effect is typically close to the surface and does not influence the vertical temperature profile above the boundary layer. There are a couple of exceptions, however.
The type of surface, i.e., snow versus water, may be important in some situations. This factor is a combination of sensible heat transfer, advection and vertical mixing in the boundary layer. For example, low level on-shore flow over Norfolk, Virginia, tends to keep the lower levels of the atmosphere warm enough for liquid precipitation. The flow of low level air over the warm off-shore water keeps temperatures above freezing. As a result, snow is not a frequent occurrence on the Norfolk peninsula. On the other hand, warm air flowing over a snow field can be cooled and moistened as it helps evaporate (sublimate) the surface snow. If there is turbulence mixing in the boundary layer, cloudy may form.
A situation where radiational effects can influence precipitation type occurs when temperatures are around the freezing point and the precipitation is liquid. If the temperature is above freezing, rain will occur. If the temperature is below freezing, freezing rain will occur. Even on a cloudy day, there will be some diurnal temperature change due to radiation heating during the day and radiational cooling at night. Assuming other effects such as advection are minor, the diurnal temperature change may be sufficient to move temperature across the freezing point and modify the precipitation type from freezing rain to rain, or vice versa.
The impact of non-adiabatic factors can often be difficult to judge. Use your basic knowledge of phase changes for water in combination with advective and radiational effects when evaluating these situations.
Many of the factors discussed in this lesson are difficult to evaluate operationally. If your forecast area contains an upper air station, twice a day you will have a picture of the low level temperarture structure. You can use this observation in combination with the three modifying effects described above (horizontal advection, vertical displacement, non-adiabatic effects) to anticipate short-term changes in the low level thermal structure.
If you do not have a local upper air station, you will likely depend upon model soundings. Nota bene, be sure that the model sounding implications correlate with what is observed at the location of interest. For example, if the model sounding implies rain, but the surface observation shows snow, use the model sounding with caution.
Forecasting precipitation type is a challenge. Even though the physical processes involved in changing snow to rain are well understood, you have to judge each situation on its own merits. Combine as much surface and upper level information as possible and look at the influence of the three basic factors that were discussed above.
Instructions: Place the cursor over the answer of your choice. If you are correct, it will be highlighted in green; if you are incorrect, it will be highlighted in red.
How high should the freezing level be to insure that snow will melt to rain?
In most situations, the dominant factor influencing changes in the low level thermal structure is:
Vertical displacement of air reduces the magnitude of horizontal temperature advection by how much?