21st Conf. on Severe Local Storms and 19th Conf. on Weather Analysis and Forecasting/15th Conf. on Numerical Weather Prediction

Monday, 12 August 2002: 11:15 AM
An examination of the variability of snowfall associated with terrain-induced circulations in central New Mexico using the workstation ETA and MM5 models
Deirdre Kann, NOAA/NWS, Albuquerque, NM; and E. Ritchie
Poster PDF (309.8 kB)
Research over the past decade has illustrated that meso-scale models can be successful in modeling the effects of terrain. Although numerical guidance from meso-scale models has not routinely been available to NWS forecast offices, forecasters have learned to consider the combination of synoptic scale regimes with local terrain-induced influences. Such “rules-of-thumb” can result in forecasts with improved verification much of the time, even when all the mechanisms associated with the pattern are not completely understood. The focus of this study is to determine if a meso-scale model can accurately depict variations in the snowfall associated with a common winter storm pattern and, if so, result in an improved understanding of the associated terrain-induced circulations.

Albuquerque is the largest population center of New Mexico and is situated in an area regularly influenced by nearby terrain. Located along the Rio Grande, the Sandia and Manzano Mountains form a north-south barrier to the east and are separated by the Tijeras Pass, which has an elevation decrease of ~360 meters. East gap winds develop frequently in the Tijeras Pass, which is located 20 km east of the Albuquerque International airport. East winds are measured 13% of the time at the airport, and the strongest winds at the airport (>20 mph) are generally from the east.

Major snow events in New Mexico are frequently the result of a closed upper level low in a positively tilted trough with a surface high pressure surge into the eastern plains. Upslope flow along the central mountain chains can result in locally heavy snowfall. However, the east gap wind in Albuquerque has a downslope component, which can often inhibit precipitation across the Albuquerque metro area. In January of 2001, two winter storms associated with this synoptic pattern resulted in widespread snow across much of the state. In the first, Albuquerque received no snow and the void, as documented by visible imagery, was a near-circular area of about 20 miles. Ten days later, when a similar but stronger system developed, east winds of greater than 20 mph were recorded at the airport for 24 continuous hours. In this case, however, two inches of snow was measured at the airport with higher amounts across the metro area.

In initial simulations of these cases using the workstation Eta model run at a resolution of 15-km, observed variation in snowfall across the Rio Grande valley and other terrain-influenced areas were not depicted. Forecasts are now being produced using the PSU/NCAR nonhydrostatic model (MM5) using horizontal resolutions of 3 km for the state and 1 km for the central valley/mountains. For both models, model precipitation is verified using NWS station observations as well as cooperative measurements and snotel data. It is anticipated that if an improved precipitation field can be produced in the Albuquerque metro area, the model output will lead to an improved understanding of the meso-scale precipitation processes and thus improved forecasts of future events.

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