Today I want to discuss and explain the concept of thermally direct circulations. How do they work and how does the circulation impact the development of fronts? Let’s try and answer these questions.
A thermally direct circulation is a circulation that can exist along a frontal boundary. A typical directly thermal circulation is initiated by solar heating just ahead of a cold front. When air is heated, the air rises. Rising air is useful; after all, rising air is needed to generate lift and convection. The rising air creates a region of lower air pressure near the surface and a higher pressure at the top of the circulation. That air must go somewhere, so a second piece must exist in order to generate and complete the circulation. Often, that second piece is sinking air behind the cold front. Behind the front, there is often clouds and precipitation which cools and compresses the atmosphere. This compression creates a lower pressure aloft and a higher pressure at the surface from the sinking motion in the atmosphere.
So, we have rising motion ahead of the front, and sinking motion behind the front. In order for the circulation to become complete, there must be a complete motion. The rising air ahead of the front will want to move towards the region of lower pressure aloft behind the front.. And the sinking air will want to move towards the region of surface low pressure ahead of the front. (figure 1).
Thermally direct circulations can aid in the development of cold fronts. A key to the strengthening of a cold front is an increase in the temperature difference ahead and behind a front. This increasing of the temperature gradient is a key to frontogenesis. Additionally, the motion near the surface aids in the movement of the low level cold air mass into the warmer air mass ahead of the front.
One caveat, thermally direct circulations is only one way for fronts to develop or increase in strength. Other factors can either aid or inhibit frontal development. But thermally direct circulations can give us insight into one of the processes that are behind the development and strengthening of fronts.
Figure 1. An example of a thermally direct circulation. Here, the purple colors represent rising air and the grey colors represent sinking air. A. Expansion of the atmosphere, in this case, induced by heating of the air near the surface, generates a rising motion in the atmosphere. B. The rising air then moves towards the region of sinking air behind the front. C. Behind the front, the rain has cooled, and clouds have limited solar heating of the air. This cooling causes a contraction of the air, which is then filled by the air moving in via B. D. The sinking air reaches the ground and moves towards the region of lower pressure that has been created by the rising air from A. (source https://www.tropicaltidbits.com GFS Model Run).
References
Markowski, P and Richardson, Y., Mesoscale Meteorology in Midlatitudes. 2016, Wiley Blackwell.
Tropical Tidbits webpage. https://www.tropicaltidbits.com/