We present novel measurements from a field campaign that aims to characterize multi-scale flow patterns, ranging from 0.1 to 10 km in a time-resolved manner, in a mountainous region in northwestern ...Spain with a mountain–valley–ridge configuration. We select two flow cases where topographic-flow interactions were measured by five synchronized scanning Doppler wind lidars along a 10 km transect line that includes a cross section of the valley. We observed a hydraulic jump in the lee side of the mountain. For this case, the Froude number transition from supercritical (>1) at the mountain to subcritical (<1) at the valley is in agreement with previous experiments at a smaller scale. For a 1-year period, the measurements show such a transition about 10 % of the time, indicating a possible high occurrence of hydraulic jumps. The second flow case presents valley winds that are decoupled from the northerly flow aloft and show a stratified layered pattern, which is well captured by the lidar scans and complementary ground-based observations. These measurements can aid the evaluation of multi-scale numerical models as well as improve our knowledge with regards to mountain meteorology.
In numerical models of the climate system and in other applications, the surface energy budget is usually considered closed, allowing for estimation of missing terms as the residual of the others. ...Real measurements of this budget show significant uncertainties in the values of each flux and imbalances that range between 5% and more than 50%, as shown in recent literature. In this article, a derivation of the surface energy budget equation from the prognostic temperature equation is presented and the hypotheses are discussed. Minor terms, which are usually neglected, such as tendency or advection, are estimated. Then, the 2 year statistics for a station in the Ebro Valley are analyzed, focusing on the imbalance, which is found to increase as the other terms in the equation increase, with values on the order of 30% of the net radiation. The same location seen by the model of the European Center for Medium‐Range Weather Forecasts (ECMWF) is analyzed. Large differences between observations and model simulation results occur at a daily scale although the average terms are comparable, with a systematic overestimation of the ground and sensible heat fluxes by the model.
Key Points
Analysis of the imbalance of the observed surface energy budget
Daily and nightly averaged annual cycles of the observed SEB
Comparison between ECMWF model values and observations
The late afternoon upslope–downslope flow transition on the west inner sidewall of Arizona’s Meteor Crater, visualized by photographs of smoke dispersion, is investigated for 20 October 2006 using ...surface radiative and energy budget data and mean and turbulent flow profiles from three towers, two at different distances up the slope and one on the basin floor. The bowl-shaped crater allows the development of the upslope–downslope flow transition with minimal influence from larger-scale motions from outside and avoiding the upvalley–downvalley flow interactions typical of valleys. The slow downslope propagation of the shadow from the west rim causes a change in the surface radiation budget and the consequent loss of heat from the shallow atmospheric layer above the western slope at a time when the sun still heats the crater floor and the inner east sidewall. The onset of the katabatic flow is visualized by the dispersion of the smoke, and the onset occurs at the same time at the two slope towers. The katabatic flow arrives later at the crater floor, cooling the air and contributing to the stabilization of a shallow but strong inversion layer there. A wavelet analysis indicates that the initial upslope current is driven by crater-size scales, whereas the later downslope flow is influenced by the thermal gradient between opposing sidewalls generated by their different cooling rates. A comparison with other days suggests that the timing of the transition is also influenced by the presence of convective eddies in addition to the local energy balance.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
TEAMx (www.teamx-programme.org) is an international research programme that aims at improving the understanding of exchange processes in the atmosphere over mountains at multiple scales and at ...advancing the parameterizations of these processes in numerical models for weather and climate prediction. This document, compiled by the TEAMx Programme Coordination Office, provides a concise overview of the scientific scope of TEAMx.; TEAMx (www.teamx-programme.org) ist ein internationales Forschungsprogramm, das darauf abzielt, das Verständnis von Austauschprozessen in der Atmosphäre über Bergen auf mehreren Ebenen zu verbessern und die Parametrisierung dieser Prozesse in numerischen Modellen für die Wetter- und Klimavorhersage voranzutreiben. Dieses vom TEAMx-Programmkoordinierungsbüro zusammengestellte Dokument bietet einen kurzen Überblick über den wissenschaftlichen Umfang von TEAMx.
The values of the atmospheric variables follow from mass, momentum, and energy conservation equations. These equations involve gradients which, in heterogeneous terrain, lead to a connection of the ...values at one point to the values in their nearby surroundings, due to turbulence mixing or advective local transports. In this work the observed variability of the air and soil variables is analyzed for a 1‐km2 surface in a semirural area for a number of weather stations separated typically 150 m. Data show a large variability at the hectometer scale, with several degrees of difference in temperature sustained in time between neighboring points. While in the daytime turbulence contributes to keep the differences moderate, in weak wind nights the spatial variability increases significantly. The variability of soil variables can be large depending on the soil moisture and the vegetation cover and is less sensitive to the diurnal cycle. The inspection of the vertical gradients of temperature and humidity reveals that their sign and intensity, and consequently the corresponding sensible and latent heat fluxes, differ depending on the measuring point. The thermal advection is obtained from the hectometer‐scale network, which is comparable in magnitude to the turbulent fluxes and to the imbalance of the surface energy budget for clear and calm nights. During the day, the advection term may explain part of the energy budget imbalance, particularly when it is computed on a 30‐min time scale. A similar method is applied to the water vapor, finding that in the very dry conditions of the experiment, moisture advection is significant and with values comparable to the evapotranspiration.
Key Points
The observed atmospheric variability over semiarid terrain at the hectometer scale is significant
Thermal advection is quantitatively close to the turbulent heat fluxes in clear nights with weak winds
In dry conditions, local advection of moisture can be of the same order of evapotranspiration
Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October ...1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence.PUBLICATION ABSTRACT
The simplified hydraulic two-layer model for a katabatic flow is analysed using the outputs from a high-resolution mesoscale simulation. A stably stratified night is simulated for the Duero basin, a ...complex terrain area located in the northern Spanish plateau, with large vertical and horizontal spatial resolution. Well-defined katabatic flows on the basin slopes are generated by the simulation, that are relatively stationary and quasi-bidimensional for some areas in the central part of the night.
The bulk quantities used in the two-layer approach as well as the different terms in the equations are computed from the three-dimensional information provided by the mesoscale simulation. This method allows to inspect how well the simplified approach represents the katabatic flow generated by the mesoscale model. The study shows that the hydraulic model allows for a comprehensive analysis of the basic mechanisms of the slope flows but is not able to close the budget equations, since the residuals are large.
A mesoscale simulation for the island of Majorca, Spain, in the western Mediterranean Sea is used to study the nocturnal system of winds under weak synoptic pressure gradients. A very high vertical ...resolution is used in the first 500 m above ground level to characterize with large detail the thin circulations close to ground, namely, basin and katabatic flows. It is found that the island, the basin, and the slope scales interact strongly, especially when a quasi-steady state is reached in the second part of the night. A high mountain range creates a high pressure area upwind where local winds can develop. Katabatic flows converge to the valleys where they interact with a cold pool, which is advected slowly to sea by the land-sea night-breeze effect combined with a topographic forcing. The katabatic flows experience small entrainment at the top and have well-defined characteristics. The results are partially confirmed by the observations and some satellite images. PUBLICATION ABSTRACT
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
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