Trends in Estimated Mixing Depth Daily Maximums R. L. Buckley, A. Dupont, R. J. Kurzeja, and M. J. Parker Atmospheric Technologies Group Savannah River National Laboratory Palmetto American Meteorological Society Mini-Technical Meeting, 03 April 2009

Background Daily weather forecasts provided for worker safety Prescribed fires conducted at the Savannah River Site (SRS) on routine basis to reduce potential fire hazards Fire-weather forecasts are part of daily forecast package Mixing depth is an important factor in determining altitude and dilution of smoke plumes

Description of Work This work examines trends in average maximum mixing depths at SRS for ~5 years using mesoscale model (RAMS) Compare results between two versions of the model and examine seasonal trends Compare simulations with specific days in which special balloon soundings were released

Standard Regional Simulations Horizontal grid spacing 20 km. Lowest level above ground (~25 m AGL). Initialize model with ETA (NAM) and nudge to lateral BCs every 3 hours Simulate 48 hours (keeping final 36 hours), updating twice per day Basis for fire weather forecasts

Standard Meteorogram Forecast Product

Assumptions to Determine Trends Use potential temperature gradient derivation Limit to earliest 12-hr daylight period (i.e. 12Z to 00Z starting 07 or 08 LST) Discard periods when RAMS did not run properly, or when windy and overcast or foggy conditions were predicted Average maximums determined monthly Time period considered (Jan. 2003Sep. 2007)

Results (Over 1700 simulations considered in each line depicted).

Comments on Mixing Depth Trends Mixing depths for RAMS43 greater than RAMS3a +RAMS43 tends to predict higher surface temperature daily maximums +Surface parameterization differences Maximum averages lower in 2003 than later years +SRS measurements support this trend

Observed Quantities Averaged over Summer (May-Sep) P : Total Precipitation S : Incoming solar radiation (average every 15-min from 12 to 00 UTC) T : Temperature (average every 15-min from 12 to 00 UTC) HS 3 : Number of 15-min periods with heat stress category 3

Comparison with Specific Observed Soundings ~50 SRS balloon-borne soundings released from 2003–2007 Simulated mixing depth within ±20% of observed mixing depth roughly 40% of all times Better agreement with RAMS43 than RAMS3a Difficulties seen in estimating mixing depth during mid- morning (transition)

Sample Comparison (20-Apr-2007, RAMS43-vs-Observation) Obs: RED Sim: 00Z Blue Sim: 12Z Green 00Z better than 12Z (shorter forecast lead time)

Conclusions Simulated mixing depths calculated for ~5 year period. Daily maximums recorded using two versions of RAMS. Monthly averages indicate expected seasonal trends. Lower summer maxima predicted in 2003 agree with SRS observations of temperature, precipitation, solar radiation, and heat stress. RAMS43 mixing depths tend to be higher than RAMS3a mixing depths due to higher surface temperature predictions from differences in surface parameterizations. Comparison with ~50 balloon-borne soundings released over SRS indicate better overall agreement with RAMS43 simulations.