Horizontal Convective Rolls: Determining the Environmental Conditions Supporting their Existence and Characteristics

Weckwerth, Tammy M.; Wilson, James W.; Wakimoto, Roger M.; Cṙook, N. Andrew

doi:10.1175/1520-0493(1997)125<0505:hcrdte>2.0.co;2

Cited by 185 publications

(235 citation statements)

References 49 publications

Supporting

Mentioning

175

Contrasting

Order By: Relevance

“…Roll or streak-like structures in a shear-buoyancy-driven boundary layer are well-described phenomena (e.g. Moeng and Sullivan 1994;Weckwerth et al 1997;Drobinski et al 1998;Drobinski and Foster 2003). The locations of updrafts and downdrafts in the instantaneous snapshots at time t 1 in Fig.…”

Section: Vertical Transport In the Early-morning Valley Atmospherementioning

confidence: 97%

Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

Brötz

Eigenmann

Dörnbrack

et al. 2014

Boundary-Layer Meteorol

View full text Add to dashboard Cite

We investigate the evolution of the early-morning boundary layer in a low-mountain valley in south-western Germany during COPS (convective and orographically induced precipitation study) in summer 2007. The term low-mountain refers to a mountainous region with a relief of gentle slopes and with an absolute altitude that remains under a specified height (usually 1,500 m a.s.l.). A subset of 23 fair weather days from the campaign was selected to study the transition of the boundary-layer flow in the early morning. The typical valley atmosphere in the morning hours was characterized by a stable temperature stratification and a pronounced valley wind system. During the reversal period-called the low wind period-of the valley wind system (duration of 1-2 h), the horizontal flow was very weak and the conditions for free convection were fulfilled close to the ground. Ground-based sodar observations of the vertical wind show enhanced values of upward motion, and the corresponding statistical properties differ from those observed under windless convective conditions over flat terrain. Large-eddy simulations of the boundary-layer transition in the valley were conducted, and statistical properties of the simulated flow agree with the observed quantities. Spatially coherent turbulence structures are present in the temporal as well as in the ensemble mean analysis. Thus, the complex orography induces coherent convective structures at predictable, specific locations during the early-morning low wind situations. These coherent updrafts, found in both the sodar observations and the simulation, lead to a flux counter to the gradient of the stably stratified valley atmosphere and reach up to the heights of the surrounding ridges. Furthermore, the energy balance in the surface layer during the low wind periods is closed. However, it becomes unclosed after the onset of the valley wind. The partition into the sensible and the latent heat fluxes indicates that missing flux components of sensible heat are the main reason for the unclosed energy balance in the considered situations. This result supports previously published investigations on the energy balance closure.

show abstract

Section: Vertical Transport In the Early-morning Valley Atmospherementioning

confidence: 97%

Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

Brötz

Eigenmann

Dörnbrack

et al. 2014

Boundary-Layer Meteorol

View full text Add to dashboard Cite

show abstract

“…LeMone, 1973;Atkinson and Zhang, 1996;Weckwerth et al, 1997Weckwerth et al, , 1999Bennett et al, 2010). Weckwerth et al (1999) stressed that well-defined horizontal convective rolls occur only while the ratio-z i /L is less than ∼ 25, where z i is the CBL height and L is the Monin-Obukhov length, and that cellular cells are the dominant convection as −z i /L increases.…”

Section: Characteristics Of Convective Cellsmentioning

confidence: 99%

Daytime urban breeze circulation and its interaction with convective cells

Ryu

Baik

Han

2012

Quart J Royal Meteoro Soc

View full text Add to dashboard Cite

The structure and evolution of daytime urban breeze circulation (UBC) and its interaction with convective cells in two dimensions are examined through highresolution numerical model simulation. The UBC is the thermally forced, solenoidal circulation that results from the difference in surface energy balance between the urban and rural areas. As the temperature excess in the urban area increases, the UBC becomes larger and stronger with time and the two urban-breeze fronts (the leading edges of the UBC) that initially form at the urban-rural boundaries in the morning move toward the urban centre. Meanwhile, due to strong surface heating in the daytime, a number of convective cells form in both the rural and urban areas and the different characteristics between rural and urban convective cells are identified. The aspect ratio of the urban cells is smaller than that of the rural cells, which is partly attributed to the deeper urban boundary layer. The cell updrafts originating from the urban area are stronger, warmer and drier than those from the rural area. As the UBC develops, the convective cells that form in the rural area are advected toward the urban area by the UBC. Under the influence of the UBC, the cell updrafts originating from the rural area weaken and the water vapour mixing ratio in the updrafts decreases. As the cell updrafts originating from both the rural and urban areas merge with the urban-breeze front, the front intensity increases and the water vapour mixing ratio at the front is modulated.

show abstract

“…Note that roll-cell patterns are often referred to as rolls in the literature, because they produce well-defined cloud lines if the updrafts on the crosswind sides of the cells merge into clearly-defined bands of updraft air. Indeed, careful analysis of roll+cell radar signatures by Weckwerth et al (1997) indicate strong two-dimensional signatures, justifying this characterization. The structure could be verified with a combination of cross-wind aircraft legs and legs at a small angle to the expected roll direction, the latter revealing either longwavelength variation in wind direction (as in pure rolls), or, more likely, superposed long wavelengths (rolls) and short wavelengths (cells), as described for such structures being advected past a tower in LeMone (1973).…”

Section: Discussionmentioning

confidence: 74%

“…While the evidence supports the presence of rolls, we cannot totally eliminate the possibility that the rolls coexisted with lines of cells (rolls+cells), as is commonly observed in clear-air radar patterns (e.g., Weckwerth et al, 1997), or that we are sampling random cells, although the last possibility is unlikely. The present analysis and the common presence of rolls in other cold-air outbreak studies suggests the presence of rolls or rolls+cells.…”

Section: Discussionmentioning

confidence: 76%

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

Belušić

Večenaj²,

LeMone

2015

Front. Earth Sci.

View full text Add to dashboard Cite

Horizontal Convective Rolls: Determining the Environmental Conditions Supporting their Existence and Characteristics

Cited by 185 publications

References 49 publications

Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

Daytime urban breeze circulation and its interaction with convective cells

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

Contact Info

Product

Resources

About