6 research outputs found
Recommended from our members
Turbulence Regimes and Turbulence Intermittency in the Stable Boundary Layer during CASES-99
An investigation of nocturnal intermittent turbulence during the Cooperative Atmosphere Surface Exchange Study in 1999 (CASES-99) revealed three turbulence regimes at each observation height: 1) regime 1, a weak turbulence regime when the wind speed is less than a threshold value; 2) regime 2, a strong turbulence regime when the wind speed exceeds the threshold value; and 3) regime 3, a moderate turbulence regime when top-down turbulence sporadically bursts into the otherwise weak turbulence regime. For regime 1, the strength of small turbulence eddies is correlated with local shear and weakly related to local stratification. For regime 2, the turbulence strength increases systematically with wind speed as a result of turbulence generation by the bulk shear, which scales with the observation height. The threshold wind speed marks the transition above which the boundary layer approaches near-neutral conditions, where the turbulent mixing substantially reduces the stratification and temperature fluctuations. The preference of the turbulence regimes during CASES-99 is closely related to the existence and the strength of low-level jets. Because of the different roles of the bulk and local shear with regard to turbulence generation under different wind conditions, the relationship between turbulence strength and the local gradient Richardson number varies for the different turbulence regimes. Turbulence intermittency at any observation height was categorized in three ways: turbulence magnitude oscillations between regimes 1 and 2 as wind speed varies back and forth across its threshold value, episodic turbulence enhancements within regime 1 as a result of local instability, and downbursts of turbulence in regime 3.Keywords: Atmosphere, Fluxes, Velocity, Vertical variations, Gravity waves, Doppler lidar, Energy, Low level jet, Surface, MotionsKeywords: Atmosphere, Fluxes, Velocity, Vertical variations, Gravity waves, Doppler lidar, Energy, Low level jet, Surface, Motion
Recommended from our members
The Very Stable Boundary Layer on Nights with Weak Low-Level Jets
The light-wind, clear-sky, very stable boundary layer (vSBL) is characterized by large values of bulk
Richardson number. The light winds produce weak shear, turbulence, and mixing, and resulting strong
temperature gradients near the surface. Here five nights with weak-wind, very stable boundary layers during
the Cooperative Atmosphere–Surface Exchange Study (CASES-99) are investigated. Although the winds
were light and variable near the surface, Doppler lidar profiles of wind speed often indicated persistent
profile shapes and magnitudes for periods of an hour or more, sometimes exhibiting jetlike maxima. The
near-surface structure of the boundary layer (BL) on the five nights all showed characteristics typical of the
vSBL. These characteristics included a shallow traditional BL only 10–30 m deep with weak intermittent
turbulence within the strong surface-based radiation inversion. Above this shallow BL sat a layer of very
weak turbulence and negligible turbulent mixing. The focus of this paper is on the effects of this quiescent
layer just above the shallow BL, and the impacts of this quiescent layer on turbulent transport and numerical
modeling. High-frequency time series of temperature T on a 60-m tower showed that 1) the
amplitudes of the T fluctuations were dramatically suppressed at levels above 30 m in contrast to the
relatively larger intermittent T fluctuations in the shallow BL below, and 2) the temperature at 40- to 60-m
height was nearly constant for several hours, indicating that the very cold air near the surface was not being
mixed upward to those levels. The presence of this quiescent layer indicates that the atmosphere above the
shallow BL was isolated and detached both from the surface and from the shallow BL.
Although some of the nights studied had modestly stronger winds and traveling disturbances (density
currents, gravity waves, shear instabilities), these disturbances seemed to pass through the region without
having much effect on either the SBL structure or on the atmosphere–surface decoupling. The decoupling
suggests that under very stable conditions, the surface-layer lower boundary condition for numerical
weather prediction models should act to decouple and isolate the surface from the atmosphere, for example,
as a free-slip, thermally insulated layer.
A multiday time series of ozone from an air quality campaign in Tennessee, which exhibited nocturnal
behavior typical of polluted air, showed the disappearance of ozone on weak low-level jets (LLJ) nights.
This behavior is consistent with the two-stratum structure of the vSBL, and with the nearly complete
isolation of the surface and the shallow BL from the rest of the atmosphere above, in contrast to cases with
stronger LLJs, where such coupling was stronger
Recommended from our members
Impact of model improvements on 80 m wind speeds during the second Wind Forecast Improvement Project (WFIP2)
During the second Wind Forecast Improvement Project (WFIP2; October 2015–March 2017, held in the Columbia River Gorge and Basin area of eastern Washington and Oregon states), several improvements to the parameterizations used in the High Resolution Rapid Refresh (HRRR – 3 km horizontal grid spacing) and the High Resolution Rapid Refresh Nest (HRRRNEST – 750 m horizontal grid spacing) numerical weather prediction (NWP) models were tested during four 6-week reforecast periods (one for each season). For these tests the models were run in control (CNT) and experimental (EXP) configurations, with the EXP configuration including all the improved parameterizations. The impacts of the experimental parameterizations on the forecast of 80 m wind speeds (wind turbine hub height) from the HRRR and HRRRNEST models are assessed, using observations collected by 19 sodars and three profiling lidars for comparison. Improvements due to the experimental physics (EXP vs. CNT runs) and those due to finer horizontal grid spacing (HRRRNEST vs. HRRR) and the combination of the two are compared, using standard bulk statistics such as mean absolute error (MAE) and mean bias error (bias). On average, the HRRR 80 m wind speed MAE is reduced by 3 %–4 % due to the experimental physics. The impact of the finer horizontal grid spacing in the CNT runs also shows a positive improvement of 5 % on MAE, which is particularly large at nighttime and during the morning transition. Lastly, the combined impact of the experimental physics and finer horizontal grid spacing produces larger improvements in the 80 m wind speed MAE, up to 7 %–8 %. The improvements are evaluated as a function of the model's initialization time, forecast horizon, time of the day, season of the year, site elevation, and meteorological phenomena. Causes of model weaknesses are identified. Finally, bias correction methods are applied to the 80 m wind speed model outputs to measure their impact on the improvements due to the removal of the systematic component of the errors.
</div
Coupled Air Quality and Boundary-Layer Meteorology in Western U.S. Basins during Winter: Design and Rationale for a Comprehensive Study.
Wintertime episodes of high aerosol concentrations occur frequently in urban and agricultural basins and valleys worldwide. These episodes often arise following development of persistent cold-air pools (PCAPs) that limit mixing and modify chemistry. While field campaigns targeting either basin meteorology or wintertime pollution chemistry have been conducted, coupling between interconnected chemical and meteorological processes remains an insufficiently studied research area. Gaps in understanding the coupled chemical-meteorological interactions that drive high pollution events make identification of the most effective air-basin specific emission control strategies challenging. To address this, a September 2019 workshop occurred with the goal of planning a future research campaign to investigate air quality in Western U.S. basins. Approximately 120 people participated, representing 50 institutions and 5 countries. Workshop participants outlined the rationale and design for a comprehensive wintertime study that would couple atmospheric chemistry and boundary-layer and complex-terrain meteorology within western U.S. basins. Participants concluded the study should focus on two regions with contrasting aerosol chemistry: three populated valleys within Utah (Salt Lake, Utah, and Cache Valleys) and the San Joaquin Valley in California. This paper describes the scientific rationale for a campaign that will acquire chemical and meteorological datasets using airborne platforms with extensive range, coupled to surface-based measurements focusing on sampling within the near-surface boundary layer, and transport and mixing processes within this layer, with high vertical resolution at a number of representative sites. No prior wintertime basin-focused campaign has provided the breadth of observations necessary to characterize the meteorological-chemical linkages outlined here, nor to validate complex processes within coupled atmosphere-chemistry models