Meteorological Modeling for the August 2000 Houston-Galveston Ozone Episode: PBL Characteristics, Nudging Procedure, and Performance Evaluation Institutional Repository Document uri icon

abstract

  • This report describes evaluations of the performance of various configurations of the MM5 modeling system, as compared to planetary boundary layer (PBL) structure and profiler winds. Soundings from the three sounding sites are grouped by time of day and by regime. Systematic differences between the different model runs are found; the differences between the models and observations vary from site to site. Higher vertical resolution did not produce improved boundary layer structure. The MCNC runs had well-mixed PBLs, even at night, but were too shallow during the day. Most of the runs with the MRF PBL were similar and performed fairly well. One area of possible concern is the systematic underestimate of the strength of the sea breeze inversion, an error which may lead to too much diffusion of constituents into and out of the advancing marine air. The Gayno-Seaman PBL scheme appeared to be more realistic, but its sea breeze inversion was too strong. Wind errors were computed at a variety of heights, grouped by weather regime, and with 24-hour running means and departures from running means. Most of the model error was associated with the departures from the running means. The MRF PBL schemes tended to perform best overall. All model runs except MCNC developed large biases at heights above 1 km. The MCNC run was worst during Regime 1 but was best during Regime 2 when other model runs produced only a small fraction of the observed low-level jet speeds. Based on these and previously-reported evaluations of various model configurations, a particular configuration was chosen. This configuration uses the MRF PBL with 43 vertical levels and one-way nesting. The soil moisture availability is specified to decrease during the model integration, to simulate evaporation of rain that fell just prior to the ozone episode. A new subroutine was added to the MM5 to permit model restarts with updated soil moisture. The nudging strategy is then outlined. The approach followed here uses a large time window for nudging so as to effectively average out possibly erroneous hour-to-hour variations in low-level winds that were introduced during the quality assurance process. The default value for nudging strength is used. No nudging is performed prior to August 25 because the convection on the previous days are not resolved by the profiler network and any attempt at nudging would cause aliasing in the model fields. The final model run, called the driver run, also utilizes lower-tropospheric nudging of water vapor on the 12 km grid. This nudging is designed to suppress a robust outflow boundary which sweeps through Houston on August 31 in the model forecasts but not in the observations. The nudging successfully prevents convection from developing on the 4 km grid and reinforcing the outflow, but a weak wind surge does reach Houston during the evening. The thermodynamic performance of the driver run is very similar to its predecessor runs, since no nudging is applied to the temperature field. The wind field is dramatically improved, at least by comparison to the profiler data. Since this data set was used to nudge the model in the first place, further objective verification of the improvement due to nudging is necessary. The wind and temperature fields during the high ozone days of the episode are examined in detail. On two of the days, August 30 and 31, the model wind and 3 temperature fields have realistic large-scale and small scale features and further improvement is unlikely. Three other days, August 26, August 29, and September 1, are generally accurate but have wind errors which are likely to lead to position errors in the simulation of ozone by a photochemical model. The remaining day, August 25, had erroneous or too-light surface winds. On this day, high values of ozone are likely to be simulated by a photochemical model, but it is possible that the mixture of ozone precursors that leads to the high ozone will be fundamentally different due to the transport errors. In summary, the driver model run produces generally accurate daytime lowertropospheric temperatures and winds. On most days of the episode, the meteorological fields appear to be adequate for driving the particular combination of mixing and chemical processes that lead to high ozone on each of those days.

author list (cited authors)

  • Nielsen-Gammon, J.

complete list of authors

  • Nielsen-Gammon, John

publication date

  • February 2002