Countergradient heat flux observations during the evening transition period

Blay-Carreras, E.; Pardyjak, Eric R.; Pino, David; Alexander, Dennis C.; Lohou, Fabienne

doi:10.5194/acp-14-9077-2014

Cited by 31 publications

(22 citation statements)

References 27 publications

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“…An average shear production for this group is about 0.7 as observed for the near-neutral data during the afternoon transition just before stable stratification has started. As discussed in Blay-Carreras et al (2014b), at this site, there is a delay period between when the buoyancy flux becomes zero and when the vertical virtual potential temperature gradient becomes zero. Therefore, this group of…”

Section: Normalization Of the Tke Budget Termsmentioning

confidence: 99%

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

et al. 2016

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Abstract. The decay of turbulence kinetic energy (TKE) and its budget in the afternoon period from midday until zerobuoyancy flux at the surface is studied in a two-part paper by means of measurements from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign for 10 intensive observation period days. Here, in Part 1, near-surface measurements from a small tower are used to estimate a TKE budget. The overall boundary layer characteristics and mesoscale situation at the site are also described based upon taller tower measurements, radiosoundings and remote sensing instrumentation. Analysis of the TKE budget during the afternoon transition reveals a variety of different surface layer dynamics in terms of TKE and TKE decay. This is largely attributed to variations in the 8 m wind speed, which is responsible for different amounts of nearsurface shear production on different afternoons and variations within some of the afternoon periods. The partitioning of near-surface production into local dissipation and transport in neutral and unstably stratified conditions was investigated. Although variations exist both between and within afternoons, as a rule of thumb, our results suggest that about 50 % of the near-surface production of TKE is compensated for by local dissipation near the surface, leaving about 50 % available for transport. This result indicates that it is important to also consider TKE transport as a factor influencing the near-surface TKE decay rate, which in many earlier studies has mainly been linked with the production terms of TKE by buoyancy and wind shear. We also conclude that the TKE tendency is smaller than the other budget terms, indicating a quasi-stationary evolution of TKE in the afternoon transition. Even though the TKE tendency was observed to be small, a strong correlation to mean buoyancy production of −0.69 was found for the afternoon period. For comparison with previous results, the TKE budget terms are normalized with friction velocity and measurement height and discussed in the framework of Monin-Obukhov similarity theory. Empirically fitted expressions are presented. Alternatively, we also suggest a non-local parametrization of dissipation using a TKE-length scale model which takes into account the boundary layer depth in addition to distance above the ground. The non-local formulation is shown to give a better description of dissipation compared to a local parametrization.

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Section: Normalization Of the Tke Budget Termsmentioning

confidence: 99%

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

et al. 2016

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“…7a), when the heat flux at the same level has already become downward. Blay-Carreras et al (2014) reported on a sensible heat flux switching sign before the vertical temperature gradient switched sign near the surface, indicating the occurrence of countergradient heat fluxes at those levels. This phenomenon can be investigated with the present data in great detail by looking at individual cases.…”

Section: The Effect Of the Power Plant On The Local Atmospheric Boundmentioning

confidence: 99%

Monitoring the Micrometeorology of a Coastal Site next to a Thermal Power Plant from the Surface to 140 m

Acevedo

Degrazia

Puhales

et al. 2018

Bulletin of the American Meteorological Society

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“…Generally, stratification begins to develop as soon as the sensible heat flux reverses to downward. In contrast, Blay-Carreras et al (2014) and Sandeep et al (2015) find that the surface inversion did not begin to develop until about 30 min after the sensible heat flux reversed sign, possibly due to the lingering influence of large convective eddies.…”

Section: Temperature Decrease Near the Surfacementioning

confidence: 84%

The near‐surface evening transition

Mahrt

2017

Quart J Royal Meteoro Soc

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Understanding of the evening boundary‐layer transition has increased substantially in the past few years although such progress has led to new questions particularly regarding the evolution of the wind field. Results from the literature and analyses of measurements from three contrasting sites in our study suggest that the near‐surface temperature evolution of the evening transition begins with a late afternoon period of slow cooling. Subsequent reversal from upward heat flux to downward heat flux is followed by a period of rapid cooling. This rapid cooling typically occurs over a 60–90 min period, mostly before sunset, and accounts for much of the total ‘nocturnal cooling’. The rapid cooling subsequently transitions into slow cooling that can last for the rest of the night. The details of the temperature evolution vary substantially between sites and vary between nights at a given site. The evolution of the wind and temperature fields during the evening transition are coupled, but both are characterized by unique features. The near‐surface wind usually decelerates in the late afternoon sometimes reaching temporary near‐calm conditions. The wind‐directional shear close to the surface can account for most of the total vector shear for part of the evening transition. Even weak slopes can generate local drainage flows that are sometimes eliminated by formation of regional drainage flows. Our study also examines other aspects of the evening transition that do not necessarily fit into this simple framework. For example, at one site, an intermediate flow develops between the elimination of a larger‐scale upslope flow and the formation of local cold‐air drainage.

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Countergradient heat flux observations during the evening transition period

Cited by 31 publications

References 27 publications

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

Turbulence kinetic energy budget during the afternoon transition – Part 1: Observed surface TKE budget and boundary layer description for 10 intensive observation period days

Monitoring the Micrometeorology of a Coastal Site next to a Thermal Power Plant from the Surface to 140 m

The near‐surface evening transition

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