COLPEX: Field and Numerical Studies over a Region of Small Hills

Price, J. D.; Vosper, Simon; Brown, Andrew R.; Ross, Andrew; Clark, Peter A.; Davies, F.; Horlacher, V.; Claxton, B. M.; McGregor, James; Hoare, J. S.; Jemmett-Smith, Bradley; Sheridan, Peter F.

doi:10.1175/2011bams3032.1

Cited by 72 publications

(93 citation statements)

References 20 publications

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“…This may in part be attributable to the site's location on a ridge top. Such locations are susceptible to slow drainage flows into nearby valleys at night in light winds and subsidence conditions, thereby preventing the formation of a deep SBL on the high ground (Price et al 2011).…”

Section: Discussionmentioning

confidence: 99%

“…1b). There is no large-scale orography nearby, however, and so the site topography could be described as similar to the region of small hills studied in COLPEX (Price et al 2011). ANSTO itself is atop a broad ridge, with significant gullies close by to the east and south.…”

Section: Site Characteristicsmentioning

confidence: 97%

“…The failure of numerical weather, climate and dispersion models to represent SBL characteristics adequately within their finite grid structure leads to a range of problems, including inadequate predictions of surface temperature, fog, dew/frost and pollution levels (e.g. Mahrt 1998;Cuxart et al 2006;Price et al 2011).…”

Section: Turbulent Mixing In the Nocturnal Stable Boundary Layer (Sbl)mentioning

confidence: 99%

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Bulk Mixing and Decoupling of the Nocturnal Stable Boundary Layer Characterized Using a Ubiquitous Natural Tracer

Williams

Chambers

Griffiths

2013

Boundary-Layer Meteorol

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Vertical mixing of the nocturnal stable boundary layer (SBL) over a complex land surface is investigated for a range of stabilities, using a decoupling index (0 < D rb < 1) based on the 2-50 m bulk gradient of the ubiquitous natural trace gas radon-222. The relationship between D rb and the bulk Richardson number (R ib ) exhibits three broad regions: (1) a well-mixed region (D rb ≈ 0.05) in weakly stable conditions (R ib < 0.03); (2) a steeply increasing region (0.05 < D rb < 0.9) for "transitional" stabilities (0.03 < R ib < 1); and (3) a decoupled region (D rb ≈ 0.9-1.0) in very stable conditions (R ib > 1). D rb exhibits a large variability within individual R ib bins, however, due to a range of competing processes influencing bulk mixing under different conditions. To explore these processes in R ib -D rb space, we perform a bivariate analysis of the bulk thermodynamic gradients, various indicators of external influences, and key turbulence quantities at 10 and 50 m. Strong and consistent patterns are found, and five distinct regions in R ib -D rb space are identified and associated with archetypal stable boundary-layer regimes. Results demonstrate that the introduction of a scalar decoupling index yields valuable information about turbulent mixing in the SBL that cannot be gained directly from a single bulk thermodynamic stability parameter. A significant part of the high variability observed in turbulence statistics during very stable conditions is attributable to changes in the degree of decoupling of the SBL from the residual layer above. When examined in R ib -D rb space, it is seen that very different turbulence regimes can occur for the same value of R ib , depending on the particular combination of values for the bulk temperature gradient and wind shear, together with external factors. Extremely low turbulent variances and fluxes are found at 50 m height when R ib > 1 and D rb ≈ 1 (fully decoupled). These "quiescent" cases tend to occur when geostrophic forcing is very weak and subsidence is present, but are not associated with the largest bulk temperature gradients. Humidity and net radiation data indicate the presence of low cloud, patchy fog or dew, any of which may aid decoupling in these cases by preventing temperature gradients from increasing sufficiently to favour gravity wave activity. The largest temperature gradients in our dataset are actually associated with smaller values of the decoupling index (D rb < 0.7), indicating the presence of mixing. Strong evidence is seen from enhanced turbulence levels, fluxes and submeso activity at 50 m, as well as high temperature variances and heat flux intermittencies at 10 m, suggesting this region of the R ib -D rb distribution can be identified as a top-down mixing regime. This may indicate an important role for gravity waves and other wave-like phenomena in providing the energy required for sporadic mixing at this complex terrain site.

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Section: Discussionmentioning

confidence: 99%

Section: Site Characteristicsmentioning

confidence: 97%

Section: Turbulent Mixing In the Nocturnal Stable Boundary Layer (Sbl)mentioning

confidence: 99%

See 1 more Smart Citation

Bulk Mixing and Decoupling of the Nocturnal Stable Boundary Layer Characterized Using a Ubiquitous Natural Tracer

Williams

Chambers

Griffiths

2013

Boundary-Layer Meteorol

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“…Several field campaigns have recently increased interest in these thermal circulations at different scales: e.g. METCRAX 2006 (Whiteman et al, 2008), COLPEX (Price et al, 2011), PCAPS (Lareau et al, 2013 or METCRAX II (Lehner et al, 2015b).…”

Section: Introductionmentioning

confidence: 99%

Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study

et al. 2015

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Abstract. The interactions among several stable-boundarylayer (SBL) processes occurring just after the evening transition of 2 July 2011 have been analysed using data from instruments deployed over the area of Lannemezan (France) during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The near-calm situation of the afternoon was followed by the formation of local shallow drainage flows (SDFs) of less than 10 m depth at different locations. The SDF stage ended with the arrival of a stronger wind over a deeper layer more associated with the mountain-plain circulation, which caused mixing and destruction of the SDFs. Several gravity-wave-related oscillations were also observed on different time series. Wavelet analyses and wave parameters were calculated from high resolution and accurate surface pressure data of an array of microbarometers. These waves propagated relatively long distances within the SBL. The effects of these phenomena on turbulent parameters (friction velocity and kinematic heat flux) have been studied through multi-resolution flux decomposition methods performed on high frequency data from sonic anemometers deployed at different heights and locations. With this method, we were able to detect the different time-scales involved in each turbulent parameter and separate them from wave contributions, which becomes very important when choosing averaging-windows for surface flux computations using eddy covariance methods. The extensive instrumentation allowed us to highlight in detail the peculiarities of the surface turbulent parameters in the SBL, where several of the noted processes were interacting and producing important variations in turbulence with height and between sites along the sloping terrain.

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“…The COLd air Pooling Experiment (COLPEX) was an investigation of the formation of cold air pools in such small-scale United Kingdom orography (Price et al, 2011). It combined high-resolution modeling and observations for a field campaign in the Clun Valley, Shropshire, UK, from January 2009 to April 2010.…”

Section: Introductionmentioning

confidence: 99%

Assessment of valley cold pools and clouds in a very high-resolution numerical weather prediction model

et al. 2015

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Abstract. The formation of cold air pools in valleys under stable conditions represents an important challenge for numerical weather prediction (NWP). The challenge is increased when the valleys that dominate cold pool formation are on scales unresolved by NWP models, which can lead to substantial local errors in temperature forecasts. In this study a 2-month simulation is presented using a nested model configuration with a finest horizontal grid spacing of 100 m. The simulation is compared with observations from the recent COLd air Pooling Experiment (COLPEX) project and the model's ability to represent cold pool formation, and the surface energy balance is assessed. The results reveal a bias in the model long-wave radiation that results from the assumptions made about the sub-grid variability in humidity in the cloud parametrization scheme. The cloud scheme assumes relative humidity thresholds below 100 % to diagnose partial cloudiness, an approach common to schemes used in many other models. The biases in radiation, and resulting biases in screen temperature and cold pool properties are shown to be sensitive to the choice of critical relative humidity, suggesting that this is a key area that should be improved for very high-resolution modeling.

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COLPEX: Field and Numerical Studies over a Region of Small Hills

Cited by 72 publications

References 20 publications

Bulk Mixing and Decoupling of the Nocturnal Stable Boundary Layer Characterized Using a Ubiquitous Natural Tracer

Bulk Mixing and Decoupling of the Nocturnal Stable Boundary Layer Characterized Using a Ubiquitous Natural Tracer

Interactions among drainage flows, gravity waves and turbulence: a BLLAST case study

Assessment of valley cold pools and clouds in a very high-resolution numerical weather prediction model

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