Characterizing Summertime Wind Systems in the Complex Terrain of the Columbia River Basin during WFIP2, and Validating HRRR Model Skill in Simulating These Flows

In a recent study we used three Doppler lidars to investigate summertime diurnal flow systems in the arid interior of Washington and Oregon, in the Columbia River Basin to the east of the Cascade Mountain Range (Banta et al. 2020). The Basin was the site of the Second Wind Forecast Improvement Project (WFIP-2), an 18-month field-deployment and NWP-modeling study to improve quantitative predictions of wind properties, such as speed, direction, and turbulence, for wind-energy applications. Goals of this project included better understanding and modeling of wind-flow patterns related to wind-energy generation. Detailed, precise measurements of the wind profile were available at 15-min intervals from the Doppler lidars at three locations in the Basin separated by 70 km, as part of a comprehensive deployment of remote-sensing and in-situ instrumentation.

The flows described in Banta et al. (2020) are an important source of wind-generated electrical power during the warm season by the numerous wind farms located over this region. These recurrent, thermally forced wind systems reach maximum wind speeds in the lowest 400 m at midnight local time and minimum speeds at noon. They result from a regional sea-breeze forcing modified by mountains and complex-terrain effects, and thus represent an intrusion of marine air that originated west of the Cascades. The study also validated the ability of the operational HRRR model to simulate these flows. Significant model errors were identified due to mistiming of the onset of these flows and a premature demise of the strong westerly intrusion flow after midnight. The study was based on analyses of eight days during summer 2016, when the flow evolution was found to be characteristic of the marine intrusions. As pointed out in that paper, however, almost all summertime days saw conditions favorable to sea-breeze formation—strong heating inland to the east of the Cascades, contrasting with cool upwelling ocean temperatures to the west—yet not all days experienced marine intrusions. In this presentation we investigate flow evolution on other days during summer 2016 when classic marine intrusions did not occur, and we evaluate HRRR model errors in simulating these flows.

Here we use diurnal analyses of data from the three Doppler lidars (time-height cross sections, time series, profiles of wind speed, etc.) to characterize and, where appropriate, group the days of June, July, and August 2016 according to the structure and diurnal evolution of the wind flow. One group included many days found to have a distinct diurnal progression similar to (and including) the eight marine-intrusion days of the previous study. Another group included several weak-wind days that also exhibited a diurnal trend but displaced later in time. Many days had strong westerly gap flow all day due to a strong large-scale pressure gradient driving westerly flow through the Cascade-Mountain gaps, often for a day or more following a cold-frontal passage. This group gave rise to another group of days when the strong synoptic pressure-gradient winds spun down, which often occurred at similar times of the day—declining wind speeds through the early-morning hours and minimum speeds during midday hours. Here we present composite time-height cross sections and time series of the mean speed, standard deviation, and other properties of the winds, to reveal the mean behavior of each group and when the behavior exhibited the most variability. We then relate these to aspects of the basic forcing, such as the surface radiation and energy budget, horizontal pressure gradients, and regional cloudiness. This dataset is then used to evaluate the ability of NOAA’s HRRR forecast model to simulate these composite flows, and their day-to-day variability, by calculating model-error statistics of the winds themselves and the basic forcing mechanisms studied.

Reference:

Banta, R.M., Y.L. Pichugina, et al., 2020: Characterizing NWP model errors using Doppler-lidar measurements of recurrent regional diurnal flows: Marine-air intrusions into the Columbia-River Basin. Mon. Wea. Rev., 148, 929-953; doi.org/10.1175/MWR-D-19-0188.1