scholarly journals A Simple Method Based on Routine Observations to Nowcast Down-Valley Flows in Shallow, Narrow Valleys

2016 ◽  
Vol 55 (7) ◽  
pp. 1497-1511 ◽  
Author(s):  
Gert-Jan Duine ◽  
Thierry Hedde ◽  
Pierre Roubin ◽  
Pierre Durand
Keyword(s):  
Temperature Difference ◽  
Potential Temperature ◽  
Threshold Value ◽  
Simple Relation ◽  
Forecast Verification ◽  
Valley Wind ◽  
Simple Method ◽  
Thermally Driven ◽  
Scale Down ◽  
Valley Flows

AbstractA simple relation to diagnose the existence of a thermally driven down-valley wind in a shallow (100 m deep) and narrow (1–2 km wide) valley based on routine weather measurements has been determined. The relation is based on a method that has been derived from a forecast verification principle. It consists of optimizing a threshold of permanently measured quantities to nowcast the thermally driven Cadarache (southeastern France) down-valley wind. Three parameters permanently observed at a 110-m-high tower have been examined: the potential temperature difference between the heights of 110 and 2 m, the wind speed at 110 m, and a bulk Richardson number. The thresholds are optimized using the wind observations obtained within the valley during the Katabatic Winds and Stability over Cadarache for the Dispersion of Effluents (KASCADE) field experiment, which was conducted in the winter of 2013. The highest predictability of the down-valley wind at the height of 10 m (correct nowcasting ratio of 0.90) was found for the potential temperature difference at a threshold value of 2.6 K. The applicability of the method to other heights of the down-valley wind (2 and 30 m) and to summer conditions is also demonstrated. This allowed a reconstruction of the climatology of the thermally driven down-valley wind that demonstrates that the wind exists throughout the year and is strongly linked to nighttime duration. This threshold technique will make it possible to forecast the subgrid-scale down-valley wind from operational numerical weather coarse-grid simulations by means of statistical downscaling.

scholarly journals Local-Scale Valley Wind Retrieval Using an Artificial Neural Network Applied to Routine Weather Observations

2019 ◽  
Vol 58 (5) ◽  
pp. 1007-1022
Author(s):  
Florian Dupuy ◽  
Gert-Jan Duine ◽  
Pierre Durand ◽  
Thierry Hedde ◽  
Pierre Roubin ◽  
...  
Keyword(s):  
Neural Network ◽  
Artificial Neural Network ◽  
Wind Speed ◽  
Temperature Difference ◽  
Potential Temperature ◽  
Winter Period ◽  
Valley Wind ◽  
Weather Observations ◽  
Thermally Driven ◽  
Artificial Neural

AbstractWe hereby present a new method with which to nowcast a thermally driven, downvalley wind using an artificial neural network (ANN) based on remote observations. The method allows the retrieval of wind speed and direction. The ANN was trained and evaluated using a 3-month winter-period dataset of routine weather observations made in and above the valley. The targeted valley winds feature two main directions (91% of the total dataset) that are aligned with the valley axis. They result from downward momentum transport, channeling mechanisms, and thermally driven flows. A selection procedure of the most pertinent ANN input variables, among the routine observations, highlighted three key variables: a potential temperature difference between the top and the bottom of the valley and the two wind components above the valley. These variables are directly related to the mechanisms that generate the valley winds. The performance of the ANN method improves on an earlier-proposed nowcasting method, based solely on a vertical temperature difference, as well as a multilinear regression model. The assessment of the wind speed and direction indicates good performance (i.e., wind speed bias of −0.28 m s−1 and 84% of calculated directions stray from observations by less than 45°). Major sources of error are due to the misrepresentation of cross-valley winds and very light winds. The validated method was then successfully applied to a 1-yr period with a similar performance. Potentially, this method could be used to downscale valley wind characteristics for unresolved valleys in mesoscale simulations.

scholarly journals Analysis of the diurnal development of the <i>Ora del Garda</i> wind in the Alps from airborne and surface measurements

2013 ◽  
Vol 13 (7) ◽  
pp. 19121-19171 ◽  
Author(s):  
L. Laiti ◽  
D. Zardi ◽  
M. de Franceschi ◽  
G. Rampanelli
Keyword(s):  
High Resolution ◽  
Potential Temperature ◽  
Lake Breeze ◽  
Pollution Transport ◽  
Valley Wind ◽  
Combined Analysis ◽  
Airborne Measurements ◽  
Early Afternoon ◽  
Thermally Driven ◽  
Surface Observations

Abstract. A lake-breeze and valley-wind coupled circulation system, known as Ora del Garda, typically arises in the late morning from the northern shorelines of Lake Garda (southeastern Italian Alps), and then channels into the Sarca and Lakes valleys to the north. After flowing over an elevated saddle, in the early afternoon this wind breaks out from the west into the nearby Adige Valley, hindering the regular development of the local up-valley wind by producing a strong and gusty anomalous flow in the area. Two targeted flights of an equipped motorglider were performed in the morning and afternoon of 23 August 2001 in the above valleys, exploring selected vertical slices of the atmosphere, from the lake's shore to the area where the two local airflows interact. At the same time, surface observations were collected during an intensive field measurement campaign held in the interaction area, as well as from routinely-operated weather stations disseminated along the whole study area, allowing the analysis of the different stages of the Ora del Garda development. From airborne measurements, an atmospheric boundary-layer (ABL) vertical structure, typical of deep Alpine valleys, was detected in connection with the wind flow, with rather shallow (∼500 m) convective mixed layers surmounted by deeper, weakly stable layers. On the other hand, close to the lake's shoreline the ABL was found to be stabilized down to very low heights, as an effect of the onshore advection of cold air by the lake breeze. Airborne potential temperature observations were mapped over high-resolution 3-D grids for each valley section explored by the flights, using a geostatistical technique called residual kriging (RK). RK-regridded fields revealed fine-scale features and inhomogeneities of ABL thermal structures associated with the complex thermally-driven wind field developing in the valleys. The combined analysis of surface observations and RK-interpolated fields revealed an irregular propagation of the lake-breeze front in the lower part of the valley, and cross-valley thermal asymmetries amenable both to the differential solar heating of the valley slopes and to the valley curvature in its upper part. The overflowing of the potentially cooler Ora del Garda air from the Lakes Valley in the afternoon produces a strong katabatic wind at the bottom of the underlying Adige Valley, which blows in cross-valley (i.e. westerly) direction and impinges on the opposite eastern valley sidewall. RK-regridded potential temperature field highlighted that this phenomenon gives origin to a "hydraulic jump" flow structure in the urban area north of the city of Trento, leading to the down-stream formation of a ∼1300 m deep well-mixed layer. The improved knowledge of the typical Ora del Garda flow patterns and associated ABL structures, deriving from the combined analysis of surface and airborne observations, has practical application in air quality forecasting for the study area, for it helps in the understanding of pollution transport and dispersion processes by thermally-driven winds in the region. Moreover, 3-D meteorological fields produced by RK are likely to be an excellent basis for comparison with results from high-resolution numerical simulations, as they provide a degree of spatial detail that is fully comparable to the spatial scales resolved by large-eddy simulations.

scholarly journals Evolution of Late Wintertime Thermally Driven Local Flows and Temperature Fields over Far-Western Nepal

2016 ◽  
Vol 21 (1) ◽  
pp. 35-47
Author(s):  
Ram P. Regmi ◽  
Sangeeta Maharjan
Keyword(s):  
Thermal Environment ◽  
Flow Characteristics ◽  
Temperature Fields ◽  
Valley Wind ◽  
Plain Area ◽  
Slope Winds ◽  
Thermally Driven ◽  
Slope Wind ◽  
Wrf Modeling ◽  
Southern Plain

Atmospheric processes over the Himalayan complex terrain are yet to be studied extensively. Only a few significant researches are reported from this region and the Far-Western Region (FWR) of Nepal still remains untouched. Thus, the present study was conceived to understand the meteorological flow characteristics and thermal environment over the region and associated areas during the late wintertime with the application of the state-of-the-art-of Weather Research and Forecasting (WRF) Modeling System. The study revealed that the northern mountainous region developed strong down slope wind during the night and morning times, which sweeps out the southern plain area of Nepal and may reach just beyond the border. The wind over the plain was very shallow whose depth was just about 100 m. The down slope winds over the southern slope of the Daijee and Nandhaur mountain ranges were significantly enhanced by the subsidence of the southerly wind that prevails above 1 km height above the mean sea level. Close to the noon time a very gentle southerly valley wind from the southern plain replaced the nighttime down slope. Very shallow but strong surface inversion builds up over the plain that breaks up in the late morning. The depth of the mixed layer and the valley wind may reach up to 1km in the afternoon. The thermal environment over the FWR of Nepal was fairly hot that may remain around 35°C in the afternoon around the Mahendranagar area whereas the temperature during the nighttime may go as low as 23°C. The study revealed that, contrary to the general perception, temperature over plain areas of Nepal was significantly higher than further southern areas belonging to India. The meteorological flow fields over the FWR of Nepal executed diurnal periodicity with little day-to-day variation during the late wintertime.Journal of Institute of Science and TechnologyVolume 21, Issue 1, August 2016, page: 35-47

scholarly journals Great Salt Lake–Effect Precipitation: Observed Frequency, Characteristics, and Associated Environmental Factors

2012 ◽  
Vol 27 (4) ◽  
pp. 954-971 ◽  
Author(s):  
Trevor I. Alcott ◽  
W. James Steenburgh ◽  
Neil F. Laird
Keyword(s):  
Environmental Factors ◽  
Temperature Difference ◽  
Large Scale ◽  
Salt Lake ◽  
Great Salt Lake ◽  
Threshold Value ◽  
Substantial Improvement ◽  
Lake Area ◽  
Lake Effect ◽  
The Difference

Abstract This climatology examines the environmental factors controlling the frequency, occurrence, and morphology of Great Salt Lake–effect (GSLE) precipitation events using cool season (16 September–15 May) Weather Surveillance Radar-1988 Doppler (WSR-88D) imagery, radiosonde soundings, and MesoWest surface observations from 1997/98 to 2009/10. During this period, the frequency of GSLE events features considerable interannual variability that is more strongly correlated to large-scale circulation changes than lake-area variations. Events are most frequent in fall and spring, with a minimum in January when the climatological lake surface temperature is lowest. Although forecasters commonly use a 16°C lake–700-hPa temperature difference (ΔT) as a threshold for GSLE occurrence, GSLE was found to occur in winter when ΔT was only 12.4°C. Conversely, GSLE is associated with much higher values of ΔT in the fall and spring. Therefore, a seasonally varying threshold based on a quadratic fit to the monthly minimum ΔT values during GSLE events is more appropriate than a single threshold value. A probabilistic forecast method based on the difference between ΔT and this seasonally varying threshold, 850–700-hPa relative humidity, and 700-hPa wind direction offers substantial improvement over existing methods, although forecast skill is diminished by temperature and moisture errors in operational models. An important consideration for forecasting because of their higher precipitation rates, banded features—with a horizontal aspect ratio of 6:1 or greater—dominate only 20% of the time that GSLE is occurring, while widespread, nonbanded precipitation is much more common. Banded periods are associated with stronger low-level winds and a larger lake–land temperature difference.

scholarly journals Mechanisms of Up-Valley Winds

2004 ◽  
Vol 61 (24) ◽  
pp. 3097-3111 ◽  
Author(s):  
Gabriele Rampanelli ◽  
Dino Zardi ◽  
Richard Rotunno
Keyword(s):  
Three Dimensional ◽  
The Other ◽  
Two Dimensional ◽  
Free Atmosphere ◽  
Valley Wind ◽  
Main Contributor ◽  
Physical Mechanisms ◽  
Thermally Driven ◽  
Hydrostatic Approximation ◽  
Temperature Contrast

Abstract The basic physical mechanisms governing the daytime evolution of up-valley winds in mountain valleys are investigated using a series of numerical simulations of thermally driven flow over idealized three-dimensional topography. The three-dimensional topography used in this study is composed of two, two-dimensional topographies: one a slope connecting a plain with a plateau and the other a valley with a horizontal floor. The present two-dimensional simulations of the valley flow agree with results of previous investigations in that the heated sidewalls produce upslope flows that require a compensating subsidence in the valley core bringing down potentially warmer air from the stable free atmosphere. In the context of the three-dimensional valley–plain simulations, the authors find that this subsidence heating in the valley core is the main contributor to the valley– plain temperature contrast, which, under the hydrostatic approximation, is the main contributor to the valley– plain pressure difference that drives the up-valley wind.

scholarly journals Thermally Driven Flows at an Asymmetric Valley Exit: Observations and Model Studies at the Lech Valley Exit

2009 ◽  
Vol 137 (10) ◽  
pp. 3437-3455 ◽  
Author(s):  
Thomas Spengler ◽  
Jan H. Schween ◽  
Markus Ablinger ◽  
Günther Zängl ◽  
Joseph Egger
Keyword(s):  
Shear Zone ◽  
Weather Conditions ◽  
Western Slope ◽  
Eastern Slope ◽  
Valley Wind ◽  
Model Studies ◽  
Synoptic Conditions ◽  
Thermally Driven ◽  
The Alps ◽  
Alpine Foreland

Abstract The summertime thermal circulation in the region of an asymmetric valley exit is investigated by means of observations and high-resolution model simulations. The northeastward-oriented Alpine Lech Valley opening into the Bavarian Alpine foreland has an eastern slope exceeding the western slope by about 15 km. Northerly winds along the eastern slope are frequently observed, reaching substantial strength during fair weather conditions. A field experiment has been conducted to explore this phenomenon and to pinpoint the connection of the northeasterly flow to the Lech Valley wind circulation. Numerical simulations have also been carried out to support the interpretation of the observations. It is found that the northerlies owe their existence to the dominantly easterly flow along the foothills of the Alps, which is partly induced by the Alpine heat low but may be strengthened by favorable synoptic conditions. Examples for both situations will be discussed. The diurnal flow in the Lech Valley has little obvious impact on these northeasterlies. On days with moderate synoptic easterly flow, a wake is present on the lee of the eastern slope of the exit region, accompanied by a shear zone along the edge of the wake. This shear zone is forced southward during the daytime because of thermally initiated pressure gradients between the Alpine foreland and the Alps, leading to sudden wind changes in the exit area at the time of its passage.

Simulation Study of the Wind Dynamics over Mont Tai during the Transition Periods

2021 ◽  
Author(s):  
Iheng Tsai ◽  
Meigen Zhang
Keyword(s):  
Performance Metrics ◽  
Surface Ozone ◽  
A Priori ◽  
Atmospheric Modeling ◽  
Shortwave Radiation ◽  
Temperature Structure ◽  
Valley Wind ◽  
Thermally Driven ◽  
Shadowing Effect ◽  
Rigorous Model

&lt;p&gt;Tai-An city located near the southern foothill of Mont Tai (117.105 &amp;#176;E, 36.256 &amp;#176;N, 1526 m a.s.l.) is known for severe ozone air pollution, frequent nocturnal surface ozone enhancement events, and especially the non-negligible contribution of ozone region transport, owing to diurnal thermally driven circulations induced by steep conical isolated topography. Therefore, In this study, mesoscale wind and temperature structure around Mont Tai region in summer 2018 is predicted by the Regional Atmospheric Modeling System (RAMS). After rigorous model validation, &lt;em&gt;viz.&lt;/em&gt; the De Ridder's interpolation technique within the roughness sublayer and the statistical performance metrics, objectively ensuring the credibility of the simulation results, the &lt;em&gt;a priori&lt;/em&gt; selection of Valley-wind days identifies are expected to be dominated by the thermally driven flow. We focus on the wind dynamic in the morning and evening transition periods on the valley-wind days. RAMS model not only reproduced the temporal sequence of the flow reversal between different above-ground heights, various local aspects and upstream/downstream positions but also captured the majority of energy transfer mechanisms during transition periods. Besides, we developed the code simulation of direct shortwave radiation included the topographic shadowing effect to repair RAMS missing module.&lt;/p&gt;

Long-ranged order in a thermally driven system with temperature dependent interactions

Soft Matter ◽  
2022 ◽  
Author(s):  
Rahul Karmakar ◽  
Jaydeb Chakrabarti
Keyword(s):  
Temperature Difference ◽  
Metallic Nanoparticles ◽  
Particle Interaction ◽  
Temperature Dependent ◽  
Driving Situation ◽  
Driven System ◽  
Thermally Driven

Aggregation of macro-molecules under external drive is far from understood. An important driving situation is achieved by temperature difference. The inter-particle interaction in metallic nanoparticles with ligand capping is reported...

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