Figure 1: Convective storm system in the desert during the North American monsoon season. (Image courtesy of Zack Guido, University of Arizona)
As we approach summertime we will discuss a topic about deserts, which are often associated with hot and dry conditions. In a desert region, we find large diurnal temperature differences, generally clear skies and a lack of significant plant and animal life due to little amounts of precipitation. Considerably deserts are found all over the globe from North America to Asia, and surprisingly even cold deserts exist, such as Antarctica. For the purpose of this article, we will focus on deserts that experience significant heat and what seems like an endless forecast of precipitation-free summer months
The main focus is a phenomenon that brings isolated rain and storms to deserts as a result of extreme heating, that being a Thermal Low-Pressure system, otherwise known as a heat low. Storm systems need a variety of ingredients to form, one of those is a heat source that generate rising air that will eventually give way to the formation of clouds. Figure 2 is a cross section graphic illustrating heights measured in millibars (mb) starting with 850 mb being closest to the surface. This shows the hottest area allowing for expansion as heat rises and eventual generation of clouds located at the red ‘L’ that symbolizes a low-pressure system.
Figure 2: Cross-section of Surface to 300mb height that shows low pressure formation in the hottest environment on the surface. (Image courtesy of the United States Navy)
Heat is a leading contributor to instability that aids in formation of a low-pressure system. As a matter of fact, we see similar mesoscale changes to weather systems in events like sea breezes (cold air from the sea meeting warm air over land) and lake-effect snow (cold air meeting warm lake water) both of which ultimately lead to rising air, instability and eventually low pressure systems that bring precipitation.
Multiple factors play a role in the formation of low-pressure systems that bring precipitation, but in the case of thermal lows, heat is the main lifting mechanism since heat rises. Ideally, we would want to see moisture and surface convergence that will add fuel to a low pressure system. While a low pressure can form from extreme heating, resultant precipitation does not always reach the surface. Due to the dryness of a desert region, precipitation can form, but may fall as virga — precipitation that evaporates before reaching the surface. Another factor that can hinder the production of precipitation during these events is a capping inversion. This means that parcels of air that rise to a certain level cannot rise further because the surrounding air is the same temperature or warmer.
Because thermal low pressure systems form during significant heating, we typically see them in the climatologically warmest times of the year. For example, the desert region extending through the southwestern United States and portions of Mexico experience a Monsoon season in mid-summer (July/August) as a result of this phenomenon.
While the desert regions of the United States are typically dry, the proximity to the surrounding bodies of water can allow for moisture transfer that feeds into the surrounding air, which ultimately allows for more precipitation. Below is a graphic that shows sources of moisture that transfer towards the low-pressure system.
Figure 3: The southwestern United States and portions of Mexico experiencing thermal lows with the aid of moisture transfer (highlighted in green). (Image courtesy of NOAA)
Topography can play a role in aiding storm development in thermal lows allowing for orographic lift, or forced lift due to higher elevation like mountains. Pollution can also be spread through these systems because of elevation differences along with mid-level pollutants like ozone that can be carried with thermal lows.
As a result of the above, forecasting for thermal lows can be complicated because of the above factors playing a role in which areas will receive precipitation or if the precipitation will reach the ground.
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© 2019 Meteorologist Jason Maska