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Development and Propagation of a Narrow Cold Frontal Rain Band in Northern California

This presentation explores the dynamics and thermodynamics of a precipitation event in the Truckee/Reno area on January 4, 2008. It analyzes observations and model-generated data to understand the development and propagation of the narrow cold frontal rain band.

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Development and Propagation of a Narrow Cold Frontal Rain Band in Northern California

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  1. Development and Propagation of a Narrow Cold Frontal Rain Band in Northern California KC King M.S. Student Desert Research Institute February 11, 2009

  2. Introduction • On January 4, 2008, the Truckee/Reno area experienced a precipitation event of heavy rain that turned to snow late in the day • Forecast from the National Weather Service missed the timing of the change from rain to snow and over forecasted the snowfall • This presentation will describe the dynamics and thermodynamics of the event on both synoptic and mesoscales using both observations and model generated data.

  3. Presentation Outline • Forecast for January 4, 2008 • Quick Dynamics Review • Observations of the event • Synoptic Scale • Mesoscale • Modeled Results • Hypotheses on Model Error • Conclusions

  4. Forecast for January 4, 2008 • On Jan 3, 2008 at 1:30 pm, the NWS issued a blizzard warning over the northern Sierras and Carson Range

  5. Blizzard Conditions: High Wind Warnings NWS Products

  6. The “Blizzard” of 2008Northstar Resort Weather Blog December 31, 2007 - 5:51 PM • “ Took a long hard look at the four forecast models. They are all telling the same story and it's not bobbling around: POTENTIALLY EPIC SNOW COMING TO TAHOE THIS WEEKEND.” • “Ok, let me see if I can explain it in a way that everyone can understand: • Short Term (New Years Day, Wednesday, Thursday) - BEAUTIFUL SKI/RIDE DAYS • Extended (This Weekend) - POWDER BEYOND YOUR WILDEST DREAMS”

  7. But it rained… and rained. So what happened?

  8. Jan. 4 Observations at Reno

  9. Narrow Cold Frontal Rain Bands(NCFR) • Narrow focused precipitation associated with a cold front • Characterized by large updrafts and intense precipitation • Strong cross-frontal gradients over a small area

  10. More on NCFRs • Caused by forced convection of the density current associated with the low-level leading edge of a cold front

  11. Jet Dynamics • Strong upper level jet off the west coast • Based on the 4-quadrant jet streak model: • Divergence/Rising motion (shown in blue) in the right exit region • Ageostrophic return branch circulation (shown in red) at lower levels

  12. Observations of Jan. 4 Event DIV CONV 250mb height/wind speed (m/s) 1/4/2008 at 00 Z

  13. Upper Air Observations Jan. 4, 2008 00Z 500 mb heights, absolute vorticity (x10-5/sec) 850 mb heights, winds (knots)

  14. Upper Air DynamicsJan. 4, 2008 12Z – 250 mb

  15. Upper Air ObservationsJan. 4, 2008 12Z 500 mb heights, absolute vorticity (x10-5/sec) 850 mb heights, winds (knots)

  16. Upper Air DynamicsJan. 4, 00Z

  17. IR Satellite View of Large ScaleJan. 5 00Z

  18. Surface ObservationsJan. 4, 2008 12Z 00Z

  19. Stratospheric Ozone Map High potential vorticity air masses from the stratosphere were brought down to the troposphere

  20. Observed Mesoscale Features • MSLP Map: 21Z • Mesolow forming by 21Z over the Sierras • Surface Temperature: 21Z • Pool of cold air over the CA/NV/OR border • Tongue of cold air is pushing south almost to Reno

  21. Progression of Mesoscale Features 23Z 00Z

  22. Radar Returns from Event • 1410Z – Jan 4, 2008 • Radar returns show precipitation occurring over most of Northern CA and into Reno by 14:10Z (6:10 am PST) • Red box highlights large amounts of precipitation over area near Bishop/Mono Lake

  23. KDAX Radar at 20Z • By 20Z we see the intensification of precipitation over the Sierras south of Tahoe • Strong returns over the foothills • At this point, it was still raining

  24. Truckee Wind Profiler Data • Strong low level winds (~10 mph) out of the south from 12Z-23Z to a height of at least 2.5 km • Veering of wind with height  warm advection through the column • 00Z on January 5 surface winds shift to southwesterly • Melt level also falls to near surface level at 0Z after rising for the 3 hours prior

  25. Surface Meteograms

  26. Summary of Observations • Upper level low pressure system associated with a straight jet streak approaching the west coast of the United States • Tropopause fold bringing an intrusion of high potential vorticity stratospheric air • Left jet exit region falling over Northern California and Nevada • Creates a transverse low level southerly jet • Low level jet advects warm air (due to latent heating) from the south keeping the North Tahoe and Reno areas warm and in rain rather than snow • Passage of the NCFR near 00Z leads to turning of the winds from southerly to westerly allowing cold air to be advected into the area and dropping the temperature • Snow begins falling near 00Z

  27. WRF Simulations • Simulations using the Weather Researching and Forecast Model (WRF) • Used North American Mesoscale Model (NAM) analysis data to initialize the model. • 27 km, 9 km, and 3 km grid size runs • Model initialized at 12Z and 18Z on 1/4/08 • Used to better understand the mesoscale dynamics

  28. Data from 9 km grid12Z initialization • MSLP and wind barbs at 20Z • Mesolow over the Tahoe area is shown • Winds turn too soon from the south to the southwest/west

  29. Evolution of MSLP and Wind Fields9 km run, 12Z initialization 21Z 22Z

  30. 9 km Simulation, 18Z Initialization • To try to better simulate the southerly winds seen in the observations the model was initialized at 18Z • Hoped that the later initialization would lead to a more accurate simulation of the ageostrophic low level return jet 21Z: 250 mb heights, winds

  31. 9 km Simulation, 18Z Initialization • By 00Z on Jan. 5, the shortwave trough over the Sierras has deepened and extended southward • Winds continue out of the SW 00Z: 250 mb heights, winds

  32. 9 km Simulation, 18Z InitializationMSLP, Winds 20Z 21Z 22Z 23Z

  33. 3 km Simulation, 18Z Initialization 21Z • To get an even closer view, we used a 3 km grid over the Reno/Tahoe region • Wanted to capture the mesoscale dynamics that kept the area warm and the passage of the NCFR • MSLP, winds shown to the right • Mesolow just to east of Truckee as seen in obs 22Z

  34. 3 km Simulation, 18Z InitializationMSLP, Winds 00Z • Winds begin to turn SW and then W from 22Z – 0:40Z • Mesolow just to the east of Truckee, California deepens • Surface convergence likely leading to vertical motions, condensation, precipitation • Flow is out of the south, supporting the hypothesis that warm advection of air from the south keeps temperatures relatively high leading to rain, not snow 00:40Z

  35. 3 km Simulation, 18Z InitializationSurface Temperature 21Z 00Z In general, surface temperatures are too low in comparison to the observations by 3-5 C

  36. 3 km Simulation, 18Z Initialization20 Min Precipitation • Well defined band of precipitation extending from west of Sacramento to northwest of Truckee • This band is a manifestation of the NCFR ahead of the cold front 22Z

  37. 3 km Simulation, 18Z Initialization20 Min Precipitation 23Z 00Z Distinct, thin line of high rainfall rate (NCFR) moves SE and combines with another band of precipitation forming to the south over the next four hours.

  38. Hypotheses on Model Performance • WRF had large errors in both wind direction and surface temperature • These errors were lessened by initializing the model closer to the time of interest (12Z versus 18Z) • The secondary circulation set up in the jet exit region that sustains the southerly winds was very sensitive to the initial conditions • This is likely due to the influence of terrain and the barrier jet. Without proper forcing and initial state, the southerly winds seem to be overwhelmed by westerlies / barrier jet. • This may be due to insufficient data from the area over the ocean. • Important for forecasting because the simulations were so sensitive to the initial state/time. • Very difficult to forecast an event like this due to the complex interactions between the larger scale circulation, mesoscale features, and the terrain.

  39. Conclusions • The precipitation from the January 4 storm was likely due to a intense rising motion in the left exit region of an upper level jet streak and intensified by orographic lifting. The snowfall seems to be associated with a narrow cold frontal rain band. • Both observations and model results support the conclusion that ahead of the cold front a NCFR developed that brought intense precipitation (snow). • The jet dynamics likely led to intensification of the front and sustained southerly flow over the Northern Sierras. • The southerly wind brought warm air from latent heating due to intense precipitation in the Sierras near Mammoth Lakes and Yosemite, keeping the melt level high and preventing formation of snow. • As the winds turned from out of the south to southwesterly effectively cutting off the source of warm air, the cold front progressed and the precipitation changed from rain to snow. • In order to effectively capture this event with the WRF, the initial state was crucial to developing the correct wind structure.

  40. References Browning, K.A., Reynolds, R., 1994: Diagnostic Study of a Narrow Cold-Frontal Rainband and Severe Winds Associated with a Stratospheric Intrusion. Quarterly Journal of the Royal Meteorological Society, 120, p. 235-257. Hobbs, P.V., Persson, O.G., 1982: The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. Part V: The Substructure of Narrow Cold-Frontal Rainbands. Journal of the Atmospheric Sciences, 39, p. 280-295. Kaplan, M., 2008, Class Notes from Synoptic Meteorology Class. Koch, S.E., Kocin, P.J., 1991: Frontal Contraction Processes Leading to the Formation of an Intense Narrow Rainband. Meteorology and Atmospheric Physics, 46, p. 123-154. Lin, Y.L., 2007. Mesoscale Dynamics. Cambridge University Press. Parsons, David B., 1992: An Explanation for Intense Frontal Updrafts and Narrow Cold-Frontal Rainbands. Journal of the Atmospheric Sciences, 49, p. 1810-1825. Rutledge, S.A., Hobbs, P.V., 1984: The Mesoscale and Microscale Structure and Organization of Clouds and Precipitation in Midlatitude Cyclones. Part XII: A Diagnostic Modeling Study of Precipitation Development in Narrow Cold-Frontal Rainbands. Journal of the Atmospheric Sciences, 41, p. 2949-2972.

  41. References Con’t National Climatic Data Center, http://www.ncdc.noaa.gov/oa/ncdc.html. Cable News Network, http://www.cnn.com. Reno Gazette Journal, http://www.rgj.com. Northstar Ski Resort, Unofficial Weather Blog, http://northstarsnow.blogspot.com/. Utah State Meteorology, Mesowest, http://www.met.utah.edu/jhorel/html/mesonet/. Plymouth State Weather Center, http://vortex.plymouth.edu/make.html.

  42. Thank you! Special thanks to Michael Kaplan and Chris Smallcomb for their time and assistance. Any questions?

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