An observational study of the evening transition boundary‐layer

Grant, A. L. M.

doi:10.1002/qj.49712353907

Cited by 70 publications

(35 citation statements)

References 37 publications

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“…Figure 6d shows the profiles for a "clear sky" period, from 18:30 to 23:00 on 6 November. As sunset was at 16:26, a transition period of 2 hours was estimated (Grant 1997) so periods from that time were excluded. A similar procedure was used to constrain mixing heights, i.e.…”

Section: Boundary Layer Types and Mixing Profilesmentioning

confidence: 99%

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

et al. 2011

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Abstract. Urban boundary layers (UBLs) can be highly complex due to the heterogeneous roughness and heating of the surface, particularly at night. Due to a general lack of observations, it is not clear whether canonical models of boundary layer mixing are appropriate in modelling air quality in urban areas. This paper reports Doppler lidar observations of turbulence profiles in the centre of London, UK, as part of the second REPARTEE campaign in autumn 2007. Lidar-measured standard deviation of vertical velocity averaged over 30 min intervals generally compared well with in situ sonic anemometer measurements at 190 m on the BT telecommunications Tower. During calm, nocturnal periods, the lidar underestimated turbulent mixing due mainly to limited sampling rate. Mixing height derived from the turbulence, and aerosol layer height from the backscatter profiles, showed similar diurnal cycles ranging from c. 300 to 800 m, increasing to c. 200 to 850 m under clear skies. The aerosol layer height was sometimes significantly different to the mixing height, particularly at night under clear skies. For convective and neutral cases, the scaled turbulence profiles resembled canonical results; this was less clear for the stable case. Lidar observations clearly showed enhanced mixing beneath stratocumulus clouds reaching down on occasion to approximately half daytime boundary layer depth. OnCorrespondence to: J. F. Barlow (j.f.barlow@reading.ac.uk) one occasion the nocturnal turbulent structure was consistent with a nocturnal jet, suggesting a stable layer. Given the general agreement between observations and canonical turbulence profiles, mixing timescales were calculated for passive scalars released at street level to reach the BT Tower using existing models of turbulent mixing. It was estimated to take c. 10 min to diffuse up to 190 m, rising to between 20 and 50 min at night, depending on stability. Determination of mixing timescales is important when comparing to physicochemical processes acting on pollutant species measured simultaneously at both the ground and at the BT Tower during the campaign. From the 3 week autumnal data-set there is evidence for occasional stable layers in central London, effectively decoupling surface emissions from air aloft.

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Section: Boundary Layer Types and Mixing Profilesmentioning

confidence: 99%

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

et al. 2011

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“…Studies published over the last several years have shown a renewed interest in the transitional PBL, with the bulk of this work focused on numerical simulation. General characteristics of the afternoon-to-evening transition (AET) time frame have been identified over many decades from surface data (Caughey et al 1979;Mahrt 1981;Grant 1997), Doppler wind profilers (Grimsdell and Angevine 2002;Angevine 2008), sodar (Busse and Knupp 2012), and an array of numerical simulation studies (e.g. Nieuwstadt and Brost 1986;Sorbjan 1997;Pino et al 2006;Nadeau et al 2011).…”

Section: Introductionmentioning

confidence: 99%

Multi-platform Observations Characterizing the Afternoon-to-Evening Transition of the Planetary Boundary Layer in Northern Alabama, USA

Wingo

Knupp

2014

Boundary-Layer Meteorol

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Observations from the University of Alabama in Huntsville campus and groundbased scanning radar for over 140 total spring, summer, and autumn cases are studied to contribute to the relative scarcity of long-term datasets documenting the afternoon-to-evening transition of the planetary boundary layer. A sunset relative frame of reference is employed, focusing on the period 3 h before to 2 h after astronomical sunset, and several findings are consistent with previous investigations. Fluctuating components of wind and temperature computed from nearly collocated surface, Doppler wind profiler, and vertically pointing Doppler lidar measurements show a consistent decline as turbulence intensity diminishes through the transition. When normalized by their initial values, a pattern emerges: temperature variances decline slowly at first then quite abruptly after about 90 min before sunset. After the temperature variances begin to wane, vertical velocity fluctuations decrease, and the rate of their decay increases as vigorous thermal structures diminish. The fastest decline of the horizontal wind variance occurs after an accelerated vertical wind variance decrease, and the horizontal wind fluctuations display the slowest rate of decrease among these quantities. Near-surface humidity measurements show increases in mean water vapour mixing ratio as a steady rise generally beginning about 80 min prior to sunset. Composites of mean lidar vertical motion show final convective-type towers of upward motion occur about an hour before sunset and are coherent through 800 m (all heights a.g.l.). Lidar vertical motion variance at 195 m decreases by more than an order of magnitude approaching sunset, then remains below 0.01 m 2 s −2 for the rest of the studied time frame. Subtle, but steady, increases in both horizontal wind speed and radar-derived horizontal wind convergence above the surface layer (at 300 m) span the entire 5-h time frame. While the convergence results show a broad range, an increase in the mean is clear and found to be statistically significant. Implications

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“…Hence, this decay period is very challenging to model and has profound implications on the dispersion of pollutants during nighttime. There has been an increasing interest in this transition period in recent years (Grant 1997;Sorbjan 1997Sorbjan , 2007Goulart et al 2003Goulart et al , 2010Beare et al 2006;Kleissl et al 2006;Pino et al 2006), with analyses involving numerical simulations, laboratory work and field experiments.…”

Section: Introductionmentioning

confidence: 99%

“…Two of the largest field campaigns designed to study the ABL convective decay were carried out by Grant (1997) over a shallow river valley in the UK and by Acevedo and Fitzjarrald (2001) using a sensor network around Albany airport, New York. Other studies have focused on the evening transition, but not specifically on the decay of convective turbulence.…”

Section: Introductionmentioning

confidence: 99%

A Simple Model for the Afternoon and Early Evening Decay of Convective Turbulence Over Different Land Surfaces

Nadeau

Pardyjak

Higgins

et al. 2011

Boundary-Layer Meteorol

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A simple model to study the decay of turbulent kinetic energy (TKE) in the convective surface layer is presented. In this model, the TKE is dependent upon two terms, the turbulent dissipation rate and the surface buoyancy fluctuations. The time evolution of the surface sensible heat flux is modelled based on fitting functions of actual measurements from the LITFASS-2003 field campaign. These fitting functions carry an amplitude and a time scale. With this approach, the sensible heat flux can be estimated without having to solve the entire surface energy balance. The period of interest covers two characteristic transition subperiods involved in the decay of convective boundary-layer turbulence. The first sub-period is the afternoon transition, when the sensible heat flux starts to decrease in response to the reduction in solar radiation. It is typically associated with a decay rate of TKE of approximately t −2 (t is time following the start of the decay) after several convective eddy turnover times. The early evening transition is the second sub-period, typically just before sunset when the surface sensible heat flux becomes negative. This sub-period is characterized by an abrupt decay in TKE associated with the rapid collapse of turbulence. Overall, the results presented show a significant improvement of the modelled TKE decay when compared to the often applied assumption of a sensible heat flux decreasing instantaneously or with a very short forcing time scale. In addition, for atmospheric modelling studies, it is suggested that the afternoon and early evening decay of sensible heat flux be modelled as a complementary error function.

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An observational study of the evening transition boundary‐layer

Cited by 70 publications

References 37 publications

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

Multi-platform Observations Characterizing the Afternoon-to-Evening Transition of the Planetary Boundary Layer in Northern Alabama, USA

A Simple Model for the Afternoon and Early Evening Decay of Convective Turbulence Over Different Land Surfaces

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