Forecasting the Breakup of a Central Minnesota Cold-air Pool at High Temporal Resolution

From 28 September ¨C 5 October 2012, Saint Cloud State University was visited by a Mobile Integrated Sounding System (MISS) operated by NCAR's Earth Observing Laboratory. The deployment of MISS was the basis of a field campaign titled Boundary Structure Experiments with Central Minnesota Profiling (BaSE CaMP). The largest experiment was the first Intensive Observing Period on 29 September, labeled IOP Alpha.

During IOP Alpha, the MISS was deployed 30 km ESE of Saint Cloud near the town of Richmond. This site is located in a shallow (60 m) basin along the Sauk River valley where a series of lakes are formed. The synoptic setting for the event was very quiescent, directly underneath a 500 mb cutoff high. The first radiosonde launch from MISS was made at 1230 UTC, just before sunrise, with three more launches being made at ninety minute intervals thereafter, ending at 17 UTC. Additionally, the MISS wind profiler and surface station were run throughout this period, and half-hourly RASS profiles were made for the second half of the IOP. Between radiosonde launches, students collected more surface observations along the northern sidewall of the basin using handheld instrumentation.

The first sounding of the day showed a surface inversion with a temperature difference of 15 degrees Celsius approximately 150 m deep that was confirmed by the remote student observations. While the surface temperature increased at 1400 UTC, a surface mixed layer did not appear until 1530 UTC, though a remnant of the inversion remained just above this. By 1700 UTC, the inversion had completely eroded with a dry adiabatic layer extending up to 900 mb. An initial WRF simulation was run with 45 vertical levels and horizontal resolution decreasing down to 0.66 km. While the model soundings are much smoother and the inversion much weaker, the model does capture the change in structure quite well, though with a lag which reaches ninety minutes. In particular, the 1700 UTC model sounding matches the 1530 UTC MISS sounding quite well. Additional sensitivity tests were performed to determine which aspects of this setup were crucial for a quality forecast. Specific applications of forecasting this type of inversion breakup are also discussed. ->