A theory for surface-layer scaling of thermally-driven slope flows

Mapping Intimacies ◽

10.5194/ems2021-396 ◽

2021 ◽

Author(s):

Dino Zardi

Keyword(s):

Boundary Layer ◽

Surface Layer ◽

Sensible Heat Flux ◽

Viscous Sublayer ◽

Transport Processes ◽

Heat Fluxes ◽

Night Time ◽

Slope Winds ◽

Thermally Driven ◽

Momentum And Heat Fluxes

Sloping terrains of any inclination favour the development, under the daily cycle of day time surface heating and night time cooling, of thermally-driven organised flows, displaying peculiar boundary layer structures, and eventually triggering the development of atmospheric convection.The ubiquitous occurrence over the Earth of variously tilted surfaces - from gently sloping plains to steep cliffs, or valley and basin sidewalls &#8211; makes the understanding of such flows of utmost importance in view of the appropriate forecasting of the associated boundary layer transport processes. Also, they display a highly conceptual relevance, as they represent a prototypal situations for many other thermally driven-flows over complex terrain. &#160;&#160;An appropriate surface-layer scaling for slope wind is derived extending the classical analysis for flat horizontal terrain situations to the cover inclines. In the former, momentum and heat fluxes at the surface are two independent quantities, and vertical profiles of velocity and temperature can only be connected to them by means &#160;of similiarity relationships, as fluxes are nearly invariant with height.Instead, equations governing slope winds show that the mean wind and temperature profiles are closely connected to the flux structure normal to the slope, as this is not constant. Also, surface values of momentum flux and sensible heat flux are shown to be proportional to each other.Based on the above relationships, suitable expressions are derived for the slope-normal profiles of velocity and temperature, both in the viscous sublayer and in the fully turbulent surface layer, as well as for the appropriate scaling factors in the two regions.

Download Full-text

Sensible Heat Fluxes in the Nearly Neutral Boundary Layer: The Impact of Frictional Heating within the Surface Layer

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-18-0158.1 ◽

2019 ◽

Vol 76 (4) ◽

pp. 1039-1053

Author(s):

J. M. Edwards

Keyword(s):

Boundary Layer ◽

Surface Layer ◽

Frictional Heating ◽

Sensible Heat Flux ◽

Surface Fluxes ◽

Heat Fluxes ◽

Sensible Heat ◽

Simple Modification ◽

Neutral Boundary Layer ◽

The Impact

Abstract The effect of frictional dissipative heating on the calculation of surface fluxes in the atmospheric boundary layer using bulk flux formulas is considered. Although the importance of frictional dissipation in intense storms has been widely recognized, it is suggested here that its impact is also to be seen at more moderate wind speeds in apparently enhanced heat transfer coefficients and countergradient fluxes in nearly neutral conditions. A simple modification to the bulk flux formula can be made to account for its impact within the surface layer. This modification is consistent with an interpretation of the surface layer as one across which the flux of total energy is constant. The effect of this modification on tropical cyclones is assessed in an idealized model, where it is shown to reduce the predicted maximum wind speed by about 4%. In numerical simulations of three individual storms, the impacts are more subtle but indicate a reduction of the sensible heat flux into the storm and a cooling of the surface layer.

Download Full-text

Land-surface processes and summer-cloud-precipitation characteristics in the Tibetan Plateau and their effects on downstream weather: a review and perspective

National Science Review ◽

10.1093/nsr/nwz226 ◽

2020 ◽

Vol 7 (3) ◽

pp. 500-515 ◽

Cited By ~ 8

Author(s):

Yunfei Fu ◽

Yaoming Ma ◽

Lei Zhong ◽

Yuanjian Yang ◽

Xueliang Guo ◽

...

Keyword(s):

Boundary Layer ◽

Tibetan Plateau ◽

Land Surface ◽

Sensible Heat Flux ◽

Vertical Direction ◽

Heat Fluxes ◽

Surface Processes ◽

Convective Precipitation ◽

The Tibetan Plateau ◽

Land Surface Processes

Abstract Correct understanding of the land-surface processes and cloud-precipitation processes in the Tibetan Plateau (TP) is an important prerequisite for the study and forecast of the downstream activities of weather systems and one of the key points for understanding the global atmospheric movement. In order to show the achievements that have been made, this paper reviews the progress on the observations for the atmospheric boundary layer, land-surface heat fluxes, cloud-precipitation distributions and vertical structures by using ground- and space-based multiplatform, multisensor instruments and the effect of the cloud system in the TP on the downstream weather. The results show that the form drag related to the topography, land–atmosphere momentum and scalar fluxes is an important part of the parameterization process. The sensible heat flux decreased especially in the central and northern TP caused by the decrease in wind speeds and the differences in the ground-air temperatures. Observations show that the cloud and precipitation over the TP have a strong diurnal variation. Studies also show the compressed-air column in the troposphere by the higher-altitude terrain of the TP makes particles inside clouds vary at a shorter distance in the vertical direction than those in the non-plateau area so that precipitation intensity over the TP is usually small with short duration, and the vertical structure of the convective precipitation over the TP is obviously different from that in other regions. In addition, the influence of the TP on severe weather downstream is preliminarily understood from the mechanism. It is necessary to use model simulations and observation techniques to reveal the difference between cloud precipitation in the TP and non-plateau areas in order to understand the cloud microphysical parameters over the TP and the processes of the land boundary layer affecting cloud, precipitation and weather in the downstream regions.

Download Full-text

Countergradient heat flux observations during the evening transition period

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-9077-2014 ◽

2014 ◽

Vol 14 (17) ◽

pp. 9077-9085 ◽

Cited By ~ 18

Author(s):

E. Blay-Carreras ◽

E. R. Pardyjak ◽

D. Pino ◽

D. C. Alexander ◽

F. Lohou ◽

...

Keyword(s):

Boundary Layer ◽

Heat Flux ◽

Surface Layer ◽

Transition Period ◽

Potential Temperature ◽

Sensible Heat Flux ◽

Buoyancy Flux ◽

Horizontal Shear ◽

Evening Transition ◽

Virtual Potential Temperature

Abstract. Gradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several intensive observational periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower instrumented with high-speed temperature and velocity sensors. The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are ~ 30–80 min. The existence of the delay and its duration can be explained by considering the convective timescale and the competition of forces associated with the classical Rayleigh–Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turnover timescale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover timescale.

Download Full-text

The Modification of Bottom Boundary Layer Turbulence and Mixing by Internal Waves Shoaling on a Barrier Reef

Journal of Physical Oceanography ◽

10.1175/2011jpo4344.1 ◽

2011 ◽

Vol 41 (11) ◽

pp. 2223-2241 ◽

Cited By ~ 55

Author(s):

Kristen A. Davis ◽

Stephen G. Monismith

Keyword(s):

Boundary Layer ◽

Internal Waves ◽

Fast Response ◽

Heat Fluxes ◽

Bottom Boundary Layer ◽

Mixing Efficiency ◽

Florida Shelf ◽

Bottom Boundary ◽

Boundary Layer Turbulence ◽

Momentum And Heat Fluxes

Abstract Results are presented from an observational study of stratified, turbulent flow in the bottom boundary layer on the outer southeast Florida shelf. Measurements of momentum and heat fluxes were made using an array of acoustic Doppler velocimeters and fast-response temperature sensors in the bottom 3 m over a rough reef slope. Direct estimates of flux Richardson number Rf confirm previous laboratory, numerical, and observational work, which find mixing efficiency not to be a constant but rather to vary with Frt, Reb, and Rig. These results depart from previous observations in that the highest levels of mixing efficiency occur for Frt < 1, suggesting that efficient mixing can also happen in regions of buoyancy-controlled turbulence. Generally, the authors find that turbulence in the reef bottom boundary layer is highly variable in time and modified by near-bed flow, shear, and stratification driven by shoaling internal waves.

Download Full-text

Influence of along-valley terrain heterogeneity on exchange processes over idealized valleys

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-6589-2015 ◽

2015 ◽

Vol 15 (12) ◽

pp. 6589-6603 ◽

Cited By ~ 18

Author(s):

J. S. Wagner ◽

A. Gohm ◽

M. W. Rotach

Keyword(s):

Boundary Layer ◽

Cross Sections ◽

Trajectory Analysis ◽

Layer Structure ◽

Volume Effect ◽

Transport Processes ◽

Valley Floor ◽

Mass Fluxes ◽

Exchange Processes ◽

Thermally Driven

Abstract. Idealized numerical simulations of thermally driven flows over various valley–plain topographies are performed under daytime conditions. Valley floor inclination and narrowing valley cross sections are systematically varied to study the influence of along-valley terrain heterogeneity on the developing boundary layer structure, as well as horizontal and vertical transport processes. Valley topographies with inclined valley floors of 0.86° increase upvalley winds by a factor of about 1.9 due to smaller valley volumes (volume effect) and by a factor of about 1.6 due to additional upslope buoyancy forces. Narrowing the valley cross section by 20 km per 100 km along-valley distance increases upvalley winds by a factor of about 2.6. Vertical mass fluxes out of the valley are strongly increased by a factor between 1.8 and 2.8 by narrowing the valley cross sections and by a factor of 1.2 by inclining the valley floor. Trajectory analysis shows intensified horizontal transport of parcels from the foreland into the valley within the boundary layer in cases with inclined floors and narrowing cross sections due to increased upvalley winds.

Download Full-text

An Extremum Solution of the Monin–Obukhov Similarity Equations

Journal of the Atmospheric Sciences ◽

10.1175/2009jas3117.1 ◽

2010 ◽

Vol 67 (2) ◽

pp. 485-499 ◽

Cited By ~ 18

Author(s):

Jingfeng Wang ◽

Rafael L. Bras

Keyword(s):

Surface Layer ◽

Atmospheric Surface Layer ◽

Turbulent Transport ◽

Asymptotic Properties ◽

Field Measurements ◽

Surface Heat ◽

Heat Fluxes ◽

Obukhov Length ◽

Surface Heat Fluxes ◽

Momentum And Heat Fluxes

Abstract An extremum hypothesis of turbulent transport in the atmospheric surface layer is postulated. The hypothesis has led to a unique solution of Monin–Obukhov similarity equations in terms of simple expressions linking shear stress (momentum flux) and heat flux to mean wind shear and temperature gradient. The extremum solution is consistent with the well-known asymptotic properties of the surface layer. Validation of the extremum solution has been made by comparison to field measurements of momentum and heat fluxes. Furthermore, a modeling test of predicting surface heat fluxes using the results of this work is presented. A critical reexamination of the interpretation of the Obukhov length is given.

Download Full-text

Local momentum and heat fluxes in transient transport processes and inhomogeneous systems

Physical Review E ◽

10.1103/physreve.94.053309 ◽

2016 ◽

Vol 94 (5) ◽

Cited By ~ 16

Author(s):

Youping Chen ◽

Adrian Diaz

Keyword(s):

Transport Processes ◽

Heat Fluxes ◽

Transient Transport ◽

Inhomogeneous Systems ◽

Momentum And Heat Fluxes ◽

Local Momentum

Download Full-text

Parameterizations of Sea-Spray Impact on the Air–Sea Momentum and Heat Fluxes

Monthly Weather Review ◽

10.1175/mwr-d-11-00007.1 ◽

2011 ◽

Vol 139 (12) ◽

pp. 3781-3797 ◽

Cited By ~ 54

Author(s):

J.-W. Bao ◽

C. W. Fairall ◽

S. A. Michelson ◽

L. Bianco

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Weather Prediction ◽

Heat Fluxes ◽

Sea Spray ◽

Surface Boundary ◽

Near Surface ◽

The Mean ◽

Momentum And Heat Fluxes ◽

The Impact

Abstract This paper focuses on parameterizing the effect of sea spray at hurricane-strength winds on the momentum and heat fluxes in weather prediction models using the Monin–Obukhov similarity theory (a common framework for the parameterizations of air–sea fluxes). In this scheme, the mass-density effect of sea spray is considered as an additional modification to the stratification of the near-surface profiles of wind, temperature, and moisture in the marine surface boundary layer (MSBL). The overall impact of sea-spray droplets on the mean profiles of wind, temperature, and moisture depends on the wind speed at the level of sea-spray generation. As the wind speed increases, the mean droplet size and the mass flux of sea-spray increase, rendering an increase of stability in the MSBL and the leveling-off of the surface drag. Sea spray also tends to increase the total air–sea sensible and latent heat fluxes at high winds. Results from sensitivity testing of the scheme in a numerical weather prediction model for an idealized case of hurricane intensification are presented along with a dynamical interpretation of the impact of the parameterized sea-spray physics on the structure of the hurricane boundary layer.

Download Full-text