Tuesday, January 20, 2015

Try Your Hand at "Forecasting" Winter Precipitation

In my last post I described the different types of winter precipitation we are likely to encounter. Forecasting the type and amount winter precipitation can be tricky. I've already described the many challenges of forecasting snow amounts. Perhaps more challenging are those "borderline" situations where there is a transition zone between rain, freezing rain, sleet, and snow. What area is going to get what type of precipitation and for how long? Will a transition happen quickly or slowly?

I just became aware of a neat interactive applet that lets you explore how temperature and moisture profiles in the atmosphere affect the type of precipitation you see (thanks to NWS Chicago for the "heads up" on this). A second applet lets you "grow" ice crystals in different parts of a cloud to see what type of crystal is produced. These applets were developed by the Cooperative Institute for Meteorological Satellite Studies at the Space Science Engineering Center, University of Wisconsin, and are part of the Explore the Atmosphere page.

Screen shot of the Precipitation Type applet.
The Precipitation Type applet is fun to play with. You can adjust the temperature and wet bulb temperature (a measure of moisture in the air) at the surface and three additional points in the atmosphere. Symbols fall from the cloud indicating the type of precipitation. Snow accumulates on the ground as the simulation continues. If there is snow falling through a warm layer near the ground, the snowflakes change to rain drops. If the mid-level is above freezing and the surface is below freezing, freezing rain and drizzle will be depicted on the ground. Another neat feature of this applet is that it depicts evaporational cooling, which sometimes is a big consideration in a winter precipitation forecast. For example, if the surface temperature is 33°F and the air is dry, the air will cool as precipitation falls through it and some of the moisture is evaporated. This cooling will continue as long as the air is not saturated and there is no introduction of warm air from another source. Evaporative cooling is the same reason you feel cool or even cold getting out of the pool on a warm, dry day in the summer. To see this in action, do the following:
  1. Drag the temperature and wet bulb lines at the first level above the surface so they both read 1.8°C, that is, the air is saturated.
  2. Drag the wet bulb (yellow) line at the surface to about -7.0°C, and the temperature (green) line to 0.9°C (that's about 33°F).
Now, watch as precipitation begins to fall from the cloud as snow and change to rain in the near surface layer. Watch also how the temperature slowly falls to and below freezing (the number will change and the point will move lower). At this point freezing rain starts to accumulate. The temperature will continue to drop until eventually it reaches the wet bulb temperature, at which point it is saturated.

There are any number of scenarios you can set up. For example, if you separate the temperature and wet bulb at the very highest level, let's say, -10°C temperature and -19°C wet bulb, the cloud will disappear - it's too dry.

What's evident after playing with this enough is that it doesn't take much, a degree to two sometimes, to make a big difference in precipitation type. Add in other factors such as the depth of the warm or cold layers and you can see how forecasting winter precipitation can be very tricky.

Screen shot of the Grow Snow applet.
Often the type of snow crystals that form and fall can make forecasting snow amounts difficulty. A fluffy snow with large flakes and lots of air in between will accumulate rapidly. Snow crystals that are elongated and more compact will accumulate more slowly to to the greater density. The Grow Snow applet lets you grow your own ice crystals in different parts of the cloud to see what results.

Have fun with these!

1 comment:

  1. Can we count dew when it is heavy enough to show in the rain gauge? If so maybe we another "Co" to the name for Condensation!
    My ID CA-SC-7