Characteristics of secondary circulations over an inhomogeneous surface simulated with large-eddy simulation

Prabha, Thara V.; Karipot, Anandakumar; Binford, Michael W.

doi:10.1007/s10546-006-9137-6

Cited by 26 publications

(35 citation statements)

References 59 publications

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“…Furthermore, we find a spread in the development of cloud streets depending on the magnitude of the prescribed Q net , with correlation ratios ranging from 1 to 5. The fact that buoyancy-driven cloud street organization is favored in slightly unstable conditions (low sun) compared to stronger instabilities (high sun) agrees well with observations (e.g., Woodcock, 1942;Priestley, 1957;Grossman, 1982;Weckwerth et al, 1997).…”

Section: Without Wind: U = 0 M S −1supporting

confidence: 89%

“…Moving forward, we will examine whether the relationship between radiative transfer and convective cloud streets also applies to the real world with all the complexities of a diurnal cycle or static surface heterogeneities combined with complex wind fields. Several studies perform detailed analyses on the footprint of static surface heterogeneities in windy conditions, i.e., how upstream heterogeneities influence the characteristics of boundary layer dynamics (e.g., Raasch and Harbusch, 2001;Prabha et al, 2007;Courault et al, 2007;Maronga and Raasch, 2013;Chen et al, 2015;Gronemeier et al, 2016). It may very well be worth revisiting their analyses and particularly focus on static and dynamic (radiative) heterogeneities.…”

Section: Discussionmentioning

confidence: 99%

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The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

Jakub

Mayer

2017

Atmos. Chem. Phys.

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Abstract. The formation of shallow cumulus cloud streets was historically attributed primarily to dynamics. Here, we focus on the interaction between radiatively induced surface heterogeneities and the resulting patterns in the flow. Our results suggest that solar radiative heating has the potential to organize clouds perpendicular to the sun's incidence angle.To quantify the extent of organization, we performed a high-resolution large-eddy simulation (LES) parameter study. We varied the horizontal wind speed, the surface heat capacity, the solar zenith and azimuth angles, and radiative transfer parameterizations (1-D and 3-D). As a quantitative measure we introduce a simple algorithm that provides a scalar quantity for the degree of organization and the alignment. We find that, even in the absence of a horizontal wind, 3-D radiative transfer produces cloud streets perpendicular to the sun's incident direction, whereas the 1-D approximation or constant surface fluxes produce randomly positioned circular clouds. Our reasoning for the enhancement or reduction of organization is the geometric position of the cloud's shadow and its corresponding surface fluxes. Furthermore, when increasing horizontal wind speeds to 5 or 10 m s −1 , we observe the development of dynamically induced cloud streets. If, in addition, solar radiation illuminates the surface beneath the cloud, i.e., when the sun is positioned orthogonally to the mean wind field and the solar zenith angle is larger than 20 • , the cloud-radiative feedback has the potential to significantly enhance the tendency to organize in cloud streets. In contrast, in the case of the 1-D approximation (or overhead sun), the tendency to organize is weaker or even prohibited because the shadow is cast directly beneath the cloud. In a land-surface-type situation, we find the organization of convection happening on a timescale of half an hour. The radiative feedback, which creates surface heterogeneities, is generally diminished for large surface heat capacities. We therefore expect radiative feedbacks to be strongest over land surfaces and weaker over the ocean. Given the results of this study we expect that simulations including shallow cumulus convection will have difficulties producing cloud streets if they employ 1-D radiative transfer solvers or may need unrealistically high wind speeds to excite cloud street organization.

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Section: Without Wind: U = 0 M S −1supporting

confidence: 89%

Section: Discussionmentioning

confidence: 99%

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

Jakub

Mayer

2017

Atmos. Chem. Phys.

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“…The possible reason for this disagreement may arise from the different heterogeneity alignments with respect to wind directions used in those studies. Prabha et al (2007) showed that the heat flux heterogeneity and its alignment with respect to geostrophic wind affected the strength and orientation of organized thermals, and similar results were obtained by Kim et al (2004).…”

Section: Introductionsupporting

confidence: 72%

“…Land surface heterogeneities with different spatial scales also have different effects on the CBL; land surface heterogeneities at scales of tens of kilometres can induce mesoscale circulations (Gopalakrishnan et al, 2000;Baidya Roy et al, 2003;Kustas and Albertson, 2003;Kang et al, 2007;Taylor et al, 2007;Garcia-Carreras et al, 2010) while heterogeneities with scales (λ) on the order of the boundary-layer height (Z i ) mainly influence various CBL properties depending on values of λ/Z i (Prabha et al, 2007; and the references therein). Baidya showed that meso-γ -scale (2 ∼ 20 km) surface heterogeneity produces organized circulations with the same horizontal length-scale as that of the heterogeneity.…”

Section: Introductionmentioning

confidence: 99%

Properties of a simulated convective boundary layer over inhomogeneous vegetation

Wang

Tian

Parker

et al. 2010

Quart J Royal Meteoro Soc

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Using a high-resolution boundary-layer model, effects of heterogeneity over a mesoscale flat domain are investigated through a series of idealised model simulations. In these simulations two different land surface types (bare soil and vegetation) are arranged in two different patterns. It is found that the effect of heterogeneity remains significant with geostrophic winds of 10 m s −1 . However, when the geostrophic wind direction is perpendicular to the alignment of the surface heterogeneities (with alignment here defined as the direction along the edges of a sequence of surface patches), higher winds tend to weaken the coherent circulations caused by the surface heterogeneities. The vertical winds generated by the mesoscale circulations associated with the surface heterogeneities are on the order of 0.5 cm s −1 . When the geostrophic wind direction is perpendicular to the alignment of the surface heterogeneities over a three-strip surface type, the mesoscale pattern in horizontal velocity is also pronounced, with significant fluctuations at the interfaces between two different surface patches. The heights at which the heterogeneity effects on potential temperature and winds vanish are well above the convective boundary layer top and reach at least 3.3 Z i under light winds, but depend on the wind speed and directions as well as the orientation of surface heterogeneities.Finally, the implications of the surface heterogeneity for initiation of deep convection have been explored for a surface consisting of two-dimensional strips of alternating soil and vegetated surfaces aligned in the north-south direction. For this surface pattern, the interaction between westerly background winds and the secondary circulation sets up conditions which favour the initiation of deep convection at the eastern, downwind edge of the soil strip.

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“…However, as discussed by Prabha et al (2007), the orientation of surface heterogeneity with respect to the geostrophic wind is important. Stationary circulations may develop, which can enhance entrainment and down-wind scalar diffusion.…”

Section: Introductionmentioning

confidence: 99%

Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning

Brunsell

Mechem

Anderson³

2011

Atmos. Chem. Phys.

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Abstract. The role of land-atmosphere interactions under heterogeneous surface conditions is investigated in order to identify mechanisms responsible for altering surface heat and moisture fluxes. Twelve coupled land surface -large eddy simulation scenarios with four different length scales of surface variability under three different horizontal wind speeds are used in the analysis. The base case uses Landsat ETM imagery over the Cloud Land Surface Interaction Campaign (CLASIC) field site for 3 June 2007. Using wavelets, the surface fields are band-pass filtered in order to maintain the spatial mean and variances to length scales of 200 m, 1600 m, and 12.8 km as lower boundary conditions to the model (approximately 0.25, 1.2 and 9.5 times boundary layer height). The simulations exhibit little variation in net radiation. Rather, there is a pronounced change in the partitioning of the surface energy between sensible and latent heat flux. The sensible heat flux is dominant for intermediate surface length scales. For smaller and larger scales of surface heterogeneity, which can be viewed as being more homogeneous, the latent heat flux becomes increasingly important. The simulations showed approximately 50 Wm −2 difference in the spatially averaged latent heat flux. The results reflect a general decrease of the Bowen ratio as the surface conditions transition from heterogeneous to homogeneous. Air temperature is less sensitive to variations in surface heterogeneity than water vapor, which implies that the role of surface heterogeneity may be to maximize convective heat fluxes through modifying and maintaining local temperature gradients. More homogeneous surface conditions (i.e. smaller length scales), on the other hand, tend to maximize latent heat flux. The intermediate scale (1600 m) this does not hold, and is a more complicated interaction of scales.Correspondence to: N. A. Brunsell (brunsell@ku.edu) Scalar vertical profiles respond predictably to the partitioning of surface energy. Fourier spectra of the vertical wind speed, air temperature and specific humidity (w,T andq) and associated cospectra (wT ,wq andTq), however, are insensitive to the length scale of surface heterogeneity, but the near surface spectra are sensitive to the mean wind speed.

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Characteristics of secondary circulations over an inhomogeneous surface simulated with large-eddy simulation

Cited by 26 publications

References 59 publications

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

The role of 1-D and 3-D radiative heating in the organization of shallow cumulus convection and the formation of cloud streets

Properties of a simulated convective boundary layer over inhomogeneous vegetation

Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning

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