Wednesday, April 16, 2014

Monsoon Convection

The North American Monsoon is a thermally driven circulation that develops as a result of strong ocean-land temperature contrasts (Higgins, et al., 2006). In summer months, monsoon circulation is characterized by ascent over the continent and descent over the ocean (Climate Prediction Center, 2004). This circulation transports moist, tropical air from the Gulf of California and the Gulf of Mexico inland towards northwest Mexico and the southwest United States. Monsoon moisture contributes to as much as 70% of precipitation in the Southwest (Adams & Comrie, 1997). The strength of the monsoon has a large societal and economic impact. If the annual monsoon is too weak agriculture and residents suffer from drought. On the other hand, if the monsoon is too strong, residents will frequently experience severe thunderstorms, dust storms, and floods. Thus, long and short-term prediction of monsoon strength, duration, and timing is necessary for drought prediction and for warning the public of severe weather. (Grantz, Rajagopalan, Clark, & Zagona, 2007).
IR satellite imagery. NWS
The monsoon develops in May and June as the storm track weakens and moves north. As the season progresses, an upper-level ridge develops over the western United States causing a low-level southerly jet transporting moisture from the Gulf. Strong surges of moisture caused by synoptic features such as cyclones, easterly waves, and inverted troughs are responsible for the more severe weather events (Higgins, et al., 2006).

In July and August the monsoon circulation reaches its peak and extends as far north as Utah. However, precipitation patterns fade and become irregular towards the northern extent of the monsoon (Grantz, Rajagopalan, Clark, & Zagona, 2007). These areas may experience dry periods between surges of moisture. As shown in Adams and Comrie’s review, there is not a strong monsoon signal at Salt Lake City. The southern parts of Utah, however, are more likely to experience weak and infrequent monsoon related precipitation events during the monsoon season (Adams & Comrie, 1997). The monsoon begins to decay in September and October as the ridge over the western states and moisture transport weaken (Higgins, et al., 2006; Yao & Wang, 1997).

Monsoon related precipitation is most severe as CAPE increases with intense surface heating and advection of moisture. Localized instabilities are strongest in the afternoon and result in thunderstorm development (Higgins, et al., 2006). Thunderstorms can merge together to produce severe weather such as dust storms, lightning, flash floods, and high winds. Lightning in storms with low relative humidity can initiate wild fires (Climate Prediction Center, 2004). As storms begin to precipitate, surrounding temperatures will begin to cool as rain water evaporates (Higgins, et al., 2006).

Complex topography makes forecasting convection difficult. Numerical models in the past have done a poor job of predicting precipitation during monsoons because they cannot resolve localized convection and terrain. Convection tends to develop over mountain ranges, but terrain induced confluence also contributes to thunderstorm development (Adams & Comrie, 1997).

Monsoon strength can vary each year due to changes in the land’s surface characteristics. One example is the negative correlation between spring snowpack depth and summer monsoonal strength (Grantz, Rajagopalan, Clark, & Zagona, 2007). For winters with a deep snow pack, more energy is required to melt the snow and evaporate spring runoff. In addition, widespread snow cover has a higher albedo and reflects solar radiation. These conditions effectively reduce the surface heating over land and delay or weaken the monsoon circulation.

Future Research
Most of the current research on Monsoon circulation and precipitation is focused in Arizona and northwest Mexico where the monsoon signal is strongest. However, much like atmospheric rivers, the inland penetration of the North American Monsoon is not well understood. Future documentation and research on how monsoon circulation penetrates inland into Utah would improve our ability to predict the infrequent, but significant, monsoon convection in Utah.

Reading List
Adams, D. K., & Comrie, A. C. (1997). The North American Monsoon. Bulletin of the American Meteorological Society, 2197-2213.
Climate Prediction Center. (2004, August). The North Amercian Monsoon. Retrieved from Reports to the Nation on our Changing Planet:
Grantz, K., Rajagopalan, B., Clark, M., & Zagona, E. (2007). Seasonal Shifts in the North American Monsoon. Journal of Climate, 1923-1935.
Higgins, C. V., Amador, J., Ambrizzi, T., Garreaud, R., Gochis, D., Gutzler, D., . . . Zhang, C. (2006). Toward a Unified View of the Americna Monsoon Systems. Journal of Climate- Special Section, 4977-5000.
Yao, Y., & Wang, X. L. (1997). Influence of the North American Monoon System on the U.S. Summer Precipitatino Regime. Journal of Climate, 2600-2622.

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