Model Simulations of Extreme Orographic Precipitation in the Sierra Nevada during the New Year's Holiday Flood of 2005-06 Phillip J. Marzette, Michael L. Kaplan, Christopher Adaniya Division of Atmospheric Sciences, Desert Research Institute, Reno, Nevada James Wallman and Rhett Milne National Weather Service, Reno, Nevada

Outline Overview of observations of 2005 event Operational Multiscale Environment model with Grid Adaptivity (OMEGA) is a numerical model that uses an unstructured, adaptive grid. Features of second heavy precipitation event with observations and model comparisons 250 hPa jet streak Mean sea level pressure and surface winds Cross sections Vertical Velocity Relative humidity and θe Precipitation amounts Summary and Conclusions Acknowledgments

Overview of observations of 2005 event 2005 event was very warm and moist for late December. Two heavy precipitation episodes for 2005 event: First peak ~ 0600 UTC 31 December 2005. Second peak ~ 1500 UTC 31 December 2005. How does spillover precipitation on the lee side occur and how can we forecast it? Reno Truckee Carson City (Above) 1200 UTC 30 December 2005 infrared satellite image. (Below) Multi-sensor analysis of 30 December 2005-1 January 2006 NEXRAD total precipitation (inches). Blue Canyon South Lake Tahoe

OMEGA overview OMEGA Features Uses an unstructured, triangular grid to adapt to terrain, clouds and plumes. Highest resolution used for this experiment is 1 kilometer. Sensitivity Studies Aviation Quasi-Regular Octant grid vs. NCEP/NCAR reanalysis data. Both simulations used static grid and Kuo-Anthes convective scheme initialized on 1200 UTC 30 December 2005. Reno Truckee Carson City

Features of second heavy precipitation event with observations and model comparisons Schematic of jet streak depicting ageostrophic motions in the entrance and exit regions (e.g., Uccellini and Johnson 1979).

Features of second heavy precipitation event with observations and model comparisons 1500 UTC 31 December 2005 (27 hour forecast) OMEGA 250 hPa isotachs and wind barbs (knots; 1 m s-1 ≈ 1.944 knots). The simulation on the left is from the AVN and the right is from the NCEP/NCAR reanalysis.

Features of second heavy precipitation event with observations and model comparisons North American Regional Reanalysis (NARR) mean sea level pressure (MSLP) and surface wind barbs (m s-1). The low-level return branch circulation is a response due to jet streak coupling. The transverse ageostrophic circulation and latent heat induced contribute to the development of the meso low on the lee side. 1500 UTC 31 December 2005 1500 UTC 31 December 2005

Features of second heavy precipitation event with observations and model comparisons Reno Reno Truckee Truckee Carson City Carson City 1500 UTC 31 December 2005 (27 hour forecast) OMEGA MSLP and surface winds barbs for the (left) AVN and (right) NCEP/NCAR reanalysis simulations. Terrain height (m; base - 1000 m) is in the background.

Features of second heavy precipitation event with observations and model comparisons 1500 UTC 31 December 2005 (27 hour forecast) OMEGA vertical velocity cross section (m s-1) along Mt. Rose Highway (NV-431) between foothills and US-395 (Left-AVN, Right-NCEP/NCAR).

(Arrow: Approximate location of Reno, NV.)‏ Features of second heavy precipitation event with observations and model comparisons NARR relative humidity (yellow and green color filled contours) and θe (red lines) cross section between 39.5ºN, 123ºW and 39.5ºN, 117ºW. Latent heat release, high relative humidity and moist neutral profiles (∂θe/∂z ≈ 0) are present at the crest of the Sierra Nevada. 1500 UTC 31 December 2005 (Arrow: Approximate location of Reno, NV.)‏

Features of second heavy precipitation event with observations and model comparisons 1500 UTC 31 December 2005 OMEGA relative humidity (black contours with color fill) and θe (orange contours) cross section along Mt. Rose highway (NV-431) between foothills and US-395 (Left-AVN, Right-NCEP/NCAR).

Features of second heavy precipitation event with observations and model comparisons Reno Reno Truckee Truckee Carson City Carson City OMEGA 36 hour total precipitation amount (mm) at 0000 UTC 1 January 2006 (Left-AVN, Right-NCEP/NCAR).

Features of second heavy precipitation event with observations and model comparisons Precipitation amounts for the event were 1.7” for Reno, NV; 8.46” for Blue Canyon/Emigrant Gap, CA and 4.5” for South Lake Tahoe, CA. The spillover precipitation that is simulated is a few kilometers to the east of the observed data. Multi-sensor analysis of 30 December 2005-1 January 2006 NEXRAD total precipitation (inches).

Summary and Conclusions The dynamics of the second flooding rainfall event were due to: The low-level return branch circulation is an isallobaric response consistent with the coupling of the upper and lower level jet streaks. The meso low within the subsynoptic trough forms due to the transverse ageostrophic circulations modified by latent heating. Upward vertical motion, mid-level moisture advection, and instability generation lead to latent heat release accompanying the spillover precipitation on the lee side. The major differences in the two data sets were: More southerly momentum was present in the NCEP/NCAR reanalysis vs. the AVN. More moisture was also present in the NCEP/NCAR reanalysis vs. the AVN.

Acknowledgments The credit for the OMEGA model goes to the Science Applications International Corporation (SAIC), whom we would like to thank for letting us use OMEGA for our research purposes. This material is based upon work supported by the National Science Foundation under Grant No. 0447416 as well as UCAR/COMET Grant No. S06-58387, DOD/Army Grant No. N61339-04-C-0072, and a DRI/IPA Grant.