Steven K. Krueger — University of Utah
Andrew Thomas Lesage — University of Utah
Category
Atmospheric State & Surface
Description
Example of front location determined from mesonet data (left) and radar image at the same time (right). Red and blue dots indicate mesonet triangles with strong surface convergence (red) or surface divergence (blue).
We have implemented an objective method to locate cold pool boundaries (fronts) using mesonet station time series and have applied the method to four summer months of 5-minute data from the Oklahoma Mesonet. Our method is similar to that used by Engerer, Stensrud, and Coniglio (2008).
To locate a front, we first calculate a cold pool “score”, which (currently) combines three indicators of frontal passages: surface pressure rise, surface temperature drop, and surface wind vector change, over 30-minute time intervals. A frontal passage occurs at a station if (1) the cold pool score exceeds a threshold and (2) the cold pool score has reached its largest value during the 40-minute interval centered on this time. If a front eventually reaches all three stations that define a mesonet triangle, the front can be tracked as it traverses the triangle.
As expected, frontal passages are characterized by strong surface convergence ahead of the front and strong surface divergence behind the front. The number of frontal passages during June–August 1997 varied substantially across the mesonet. The number generally decreased from northwest to southeast by a factor of 1/2 to 1/4. The median surface temperature drop associated with frontal passages was 7 K. The largest drops were 13 K. The median surface pressure rise associated with frontal passages was 1.5 mb. The largest rises were 7 mb. There was only a weak correlation between temperature drop and pressure rise. This indicates that the vertical structure of the temperature perturbation in the cold pools varies from case to case.
Future work will include further refining the cold pool score formula, attempting to define cold pool area, and compiling a cold pool properties data set for a multi-year time period. Such a data set could be used to evaluate parameterizations, such as for cold pools in a cumulus parameterization, or for microphysics in cloud-resolving models.
