In atmospheric dynamics, a negatively tilted upper-level trough tends to be stronger and much more dynamic, particularly if there is a closed upper low
Both rising air and heavy precipitation (especially snow) work to cool the atmosphere from the top down. This is called dynamic cooling and it is extremely efficient at turning an otherwise rainy system into a snow producer in the storm’s western/northwestern quadrants. Snowfall in those quadrants tends to be wet with large flakes, but the snow only lasts for the period of time that the intense dynamics are directly overhead.
The dynamic cooling causes the freezing level to drop to near ground level quickly, albeit for a short period of time. Once the forcing that causes the tropospheric cooling moves off to the east and/or weakens (weakening is inevitable after the system occludes), the lower levels begin to warm again. In fact, dynamically-induced snow tends to change back to rain before the precipitation ends. In other words, this type of storm system creates its own environment favorable for snow via complex physics.
Factors that Contribute to Dynamic Cooling
- Narrow zone of rapidly rising air associated with the strong dynamics
- Air cools as it rises causing the freezing level to drop
- Precipitation changes to snow aloft as the freezing level drops to lower altitudes
- Snow aloft initially melts because temperatures are still near freezing
- Melting snow takes in an enormous amount of latent heat (required for melting)
- The latent heat of melting causes the temperature to fall further, accelerating the drop in the freezing level
- Cooling aloft creates instability allowing for embedded convective elements
- Elevated convection further cools the atmosphere via enhanced precipitation rates and updraft cooling
- Freezing level continues to fall closer to the surface in a feedback loop of all processes above
Animation of a Trowal and the Formation of Very Heavy Snow
In this computer animation, notice how the trowal and subsequent heavy snow develop after the trough’s orientation changes and the system occludes. These features may not be easy to spot for most, but you can certainly see changes taking place.
Credit: Animated GIFs created from Vis5D Movies at http://marrella.aos.wisc.edu/trowal.html
Tracer Trajectory of Air Rising in a Trowal
Credit: Animated GIFs created from Vis5D Movies at http://marrella.aos.wisc.edu/trowal.html
Surface temperatures may be mild initially, then drop to the low-mid 30s with a subsequent changeover to wet snow within a narrow corridor directly beneath and along the path of the deformation axis. Snow that is generated exclusively through dynamic processes, in the absence of a preexisting, sufficiently favorable thermodynamic profile for snow is referred to as a “warm snowstorm” (we are no stranger to them in the South). Although the tropospheric cooling necessary for snow is established dynamically by the storm itself, these events are generally unsustainable; once the vigorous forcing mechanisms responsible for the rapid cooling begin to weaken, so too will the other factors that I listed above. As the intensity of the precipitation weakens or move away, the snow typically changes back to light rain before ending, followed by a rise in surface temperatures.
Example of Dynamic Cooling
To illustrate dynamic cooling, consider this animated gif created using model-simulated radar, mean sea level pressure, and surface temperatures from a past event. Notice that the surface temperatures across southeastern Georgia and southern South Carolina drop rapidly at the roughly the same time that the surface low east of Charleston is rapidly intensifying. The drop in temperature is due to dynamic cooling, and the physical processes responsible for the dynamic cooling are also responsible for the surface low’s rapid deepening/intensification.
Example Case: Model-Simulated Thermodynamic Profile
Animation of Model-Simulated Soundings for Savannah
Note: We are adding subsections and illustrations as time permits.