Wednesday, March 11, 2015

Great Salt Lake Breeze and Lake Level

A proposal for my statistics class.

Great Salt Lake Breeze and Lake Level
Wind patterns at Salt Lake International Airport are often characterized by a thermally driven lake-breeze circulation. A pressure gradient between the Great Salt Lake (GSL) and the adjacent land causes surface winds to blow from the cool lake towards the warm land during the day. Winds reverse at night and blow from the cooled land towards the lake. Past studies and modeled simulations suggest that the strength of a lake breeze is partially dependent on the lake’s dimensions. Small lakes have weaker lake breezes than large lakes. Furthermore, lake breezes tend to occurs less frequently for small lakes because of interference from terrain or synoptic forced winds (Crosman and Horel 2010).
Lake dimensions can change over time. In the last century, fluctuations in the GSL level has caused the lake area to grow and shrink. For example, archived MODIS images show a larger lake on May 31, 1987 (left) when lake levels where high and a smaller lake on March 10, 2015 (right) when lake levels were low.

The purpose of this project to compare historical lake level measurements with radiosonde and pilot balloon wind observations to confirm the relationship between lake breeze strength and lake level. For instance, it is expected that the lake breeze will be stronger during years when Farmington Bay contains water. The results of this work may have implications for understanding how the lake area influences air quality in Salt Lake City.
Zumpfe and Horel (2007) have shown that winds at the airport generally shift to the north in the late morning or afternoon on days without precipitation. These wind shifts occur most frequently between April and October. They also show that the occurrence of lake breeze fronts in the Salt Lake Valley was more frequent when the lake level was high.
In previous publications, lake breeze strength has been quantified by the distance it penetrates inland, its vertical depth, and the on-shore wind speed. While lake breezes are influenced by several factors, numerical simulations suggest that they are especially sensitive to changes in lake dimension (Crosman, 2012). Other studies found that lakes larger than 100 kilometers across behave similarly to a sea breeze, but lake breezes for different sizes of smaller lakes behave non-linearly. For instance, a small change in lake area for a small lake will have a higher increase in wind speed than a small change in lake area for a larger lake (see figure 7d in Crosman and Horel 2012). Part of the reason of this non-linear behavior between lake size and wind speed is caused by changes in the curvature of the lake. Large curvature is associated with more diverging on-shore winds and thus weaken the lake-breeze.
The data that will be used for this project will be from weather balloons and lake level measurements. Daily, low level wind observations from balloons date from 1926 to present. That data set included 22 years not used in Zumpfe and Horel’s study. Measurements of GSL levels are available for the same time period with only one measurement per month.
We want to see if on-shore wind seed can be predicted from lake level. To analyze the sea breeze only wind observations below 1500 meter elevation (approximately 200 meters above ground level) will be used. The first thing to look as will be a linear regression between afternoon wind speed and lake level. As lake dimension sensitivity studies suggest, this relationship should be non-linear. However, the non-linear relationship may not be apparent if changes in lake area for the GSL during this time period are not large enough. Also, since lake breeze strength is dependent on synoptic and seasonal influences, I expect the relationship will be weak. Additional work will be needed to filter the data and remove the seasonal component of the lake breeze. It may be best to break the analysis up by season or month. I’ll also need to filter out days that are obviously not lake breezes. For example, afternoon winds from the south should not be considered. Other statistical methods may be used out of necessity or suggestion.

Crosman ET, Horel JD (2012) Idealized Large-Eddy Simulations of Sea and Lake Breezes: Sensitivity to Lake Diameter, Heat Flux and Stability. Boundary-Layer Meteorol 144:309-328
Crosman ET, Horel JD (2010) Sea and Lake Breezes: A Review of Numerical Studies. Boundary-Layer Meteorology 137:1-29
Zumpfe DE, Horel JD (2007) Lake-Breeze Fronts in the Salt Lake Valley. J Appl. Meteor. Climatol. 46:196-211

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