P1.7

The Real-Time Mesoscale Analysis (RTMA)• An operational objective surface analysis

for the continental United States at 5-km resolution developed by the National Centers for Environmental Prediction (NCEP)• Incorporates land synoptic, METAR,

various mesonets, and satellite based wind observations• Analyzed fields: temperature, dewpoint,

pressure, and wind • Relies on 2D-Var analysis using NCEP’s

Gridpoint Statistical Interpolation (GSI) Analysis System• Background is 5-km downscaled Rapid

Update Cycle (RUC) 1h forecast• See Pondeca et al. (2007; 18WAF/

22NWP-4A.5) or the COMET training module (Jascourt 2007; www.meted. ucar.edu/nwp/RTMA/) for more info

RTMA Background Error Covariances• RTMA in data-rich locations depends

primarily on assumptions regarding the relative magnitudes of observation and background error• RTMA in data voids depends primarily on

assumptions regarding how the errors in the background field relate from one location to another• RTMA assumes that the background

error covariance depends locally on terrain gradients• The covariance is quasi-Gaussian except

that errors along (perpendicular to) smoothed terrain contours are more strongly (weakly) related to one another• The background error covariance in

complex terrain during some synoptic situations may be more directly related locally to the actual terrain than the smoothed terrain

Contact: dan.tyndall@utah.edu

Synoptic Situation: Strong Radiational Inversions in Mountain Valleys• Temperature analyses and forecasts in

mountain valleys are difficult during clear nights as temperature profile becomes inverted over short distances with higher (lower) temperature on the slopes (valley floors) (Myrick et al. 2005: Wea. Forecasting, 149-160)• The RUC 1h forecast valid 0900 UTC 22 Oct.

2007 downscaled to RTMA 5-km grid did not capture the strong inverted temperature profile observed in Rush Valley• Observations in the Rush Valley lower the

RTMA temperature analysis relative to the background over a broad area in vicinity of the Rush Valley

• Root-mean squared differences between experimental analysis and observations largely unchanged from those computed between control analysis and observations when computed over continental-scale grid or over northern Utah subdomain• Hence, removal of terrain smoothing

has small impact in vicinity of observations used in the analysis• Analysis in regions with limited data,

such as to the west of the Rush Valley, strongly affected by terrain smoothing

Temperature observations (°C) near the top of the hour at 0900 UTC 22 Oct. 2007 available from MesoWest (www.met.utah.edu/mesowest) do not necessarily match those used in the RTMA that are plotted in the other figures. Temperature increases from below -6°C to near 0°C from the Rush Valley floor to its eastern sidewalls.

Sensitivity to Terrain Smoothing• Control experiment:

Aspect tensor that defines background error covariance relies on smoothed terrain field• Sensitivity experiment:

Terrain smoothing omitted entirely in order to constrain background errors more closely by the 5-km terrain

Future Work• Data denial tests are required to

objectively quantify the impact of reducing or removing the terrain smoothing• A parallel version of the RTMA is

currently available for which the terrain smoothing has been reduced• Alternatively, define background error

covariance in terms of boundary layer stability gradients

Summary

More info: 4 PM Thu. Talk 16.3

Sensitivity of the Real-Time Mesoscale Analysis to Background Field CharacteristicsD.P. Tyndall1, M.S.F.V. de Pondeca2, and J. Horel1

Department of Meteorology, University of Utah1 and the National Centers for Environmental Prediction2

Rush

Valley

20 km

Background temperature analysis (shading; °C) downscaled from RUC 1 hr forecast valid at 0900 UTC 22 Oct. 2007. RTMA terrain (m) is contoured at an interval of 200 m. Observations (values; °C) used in the RTMA analysis are shown by gray values. The background field is roughly -3°C in the Rush Valley, which is warm relative to the basin observations and cold relative to those on the slopes.

20 km

RTMA temperature analysis (shading; °C) and temperature observations (values; °C) valid at 0900 UTC 22 Oct. 2007. The RTMA analysis is 1-5°C cooler than the background in the Rush Valley and within 1-3°C of the available observations in the Rush Valley.

20 km

Control analysis using smoothed terrain (shaded; °C) and differences between observations and the control analysis (values; °C).

20 km Experimental analysis with unsmoothed terrain (shaded; °C); differences between observations and experimental analysis (values; °C).

20 km

Experimental temperature analysis (shading; °C) and temperature observations (values; °C). Strong inverted temperature profile between valley floor and eastern sidewall of Rush Valley is well defined with weaker inverted profile on western side of valley.

20 km

Difference between control analysis and background (shaded; °C) and differences between observations and background (values; °C). Control analysis is cooled over a broad region around the Rush Valley with the greatest differences erroneously in the data void to the west of the Valley as a result of the characteristics of the smoothed terrain.

20 km

Difference between experimental analysis and background (shaded; °C) and differences between observations and background values (values; °C). Large negative adjustments to the background are constrained to the Rush Valley.

20 km