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Our study examines the horizontal variation of the nocturnal surface air temperature by analyzing measurements from four contrasting networks of stations with generally modest topography. The horizontal extent of the networks ranges from 1 to 23 km. For each network, we investigate the general relationship of the horizontal variation of temperature to the wind speed, wind direction, near-surface stratification, and turbulence. As an example, the horizontal variation of temperature generally increases with increasing stratification and decreases with increasing wind speed. However, quantitative details vary significantly between the networks. Needed changes of the observational strategy are discussed. Significance Statement: We analyze nocturnal measurements from four different networks of surface stations over different types of terrain. We concentrate on the spatial variation of the surface air temperature that controls frost damage, fog formation, dispersion of particulates, and the effects of terrain changes on climate. For each network, our study examines the general relationship of the horizontal variation of temperature to the wind speed, wind direction, vertical variation of temperature, and turbulence. We identify important inadequacies of the observations and the need for future more appropriate network design including instrumental needs and choice of sites. [ABSTRACT FROM AUTHOR]

The interactions between a katabatic flow on a plain and a circular basin cut into the plain and surrounded by an elevated rim were examined during a 5-h steady-state period during the Second Meteor Crater Experiment (METCRAX II) to explain observed disturbances to the nocturnal basin atmosphere. The approaching katabatic flow split horizontally around Arizona's Meteor Crater below a dividing streamline while, above the dividing streamline, an ~50-m-deep stable layer on the plain was carried over the 30-50-m rim of the basin. A flow bifurcation occurred over or just upwind of the rim, with the lowest portion of the stable layer having negative buoyancy relative to the air within the crater pouring continuously over the crater's upwind rim and accelerating down the inner sidewall. The cold air intrusion was deepest and coldest over the direct upwind crater rim. Cold air penetration depths varied around the inner sidewall depending on the temperature deficit of the inflow relative to the ambient environment inside the crater. A shallow but extremely stable cold pool on the crater floor could not generally be penetrated by the inflow and a hydraulic jump-like feature formed on the lower sidewall as the flow approached the cold pool. The upper nonnegatively buoyant portion of the stable layer was carried horizontally over the crater, forming a neutrally stratified, low-wind speed cavity or wake in the lee of the upwind rim that extended downward into the crater over the upwind sidewall. [ABSTRACT FROM AUTHOR]

A 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. [ABSTRACT FROM AUTHOR]

Climatological features of the surface wind on diurnal and seasonal time scales over a 17-yr period in an area of complex terrain at Dugway Proving Ground in northwestern Utah are analyzed, and potential synoptic-scale, mesoscale, and microscale forcings on the surface wind are identified. Analysis of the wind climatology at 26 automated weather stations revealed a bimodal wind direction distribution at times when thermally driven circulations were expected to produce a single primary direction. The two modes of this distribution are referred to as the 'northerly' and 'southerly' regimes. The northerly regime is most frequent in May, and the southerly regime is most frequent in August. January, May, and August constitute a 'tripole seasonality' of the wind evolution. Although both regimes occur in all months, the monthly changes in regime frequency are related to changes in synoptic and mesoscale phenomena including the climatological position of the primary synoptic baroclinic zone in the western United States, interaction of the large-scale flow with the Sierra Nevada, and thermal low pressure systems that form in the Intermountain West in summer. Numerous applications require accurate forecasts of surface winds in complex terrain, yet mesoscale models perform relatively poorly in these areas, contributing to poor operational forecast skill. Knowledge of the climatologically persistent wind flows and their potential forcings will enable relevant model deficiencies to be addressed. [ABSTRACT FROM AUTHOR]

This study examines marginal cold pools forming in a shallow valley. 'Marginal' refers to cold pools that are generally weak and intermittent. Nineteen stations were instrumented with sonic anemometers and accurate slow-response temperature measurements. The vertical structure of the cold pool is determined from a 20-m tower on the valley floor that includes eight levels of sonic anemometers and eight additional levels of slow-response temperature. On the basis of the data analysis, the traditional concept of a cold pool must be generalized to include cold-pool intermittency, complex variation of temperature related to some three-dimensional effects of terrain, and a diffuse cold-pool top. Different types of cold pools are classified in terms of the stratification and gradient of potential temperature along the slope. The strength of the cold pool is related to a forcing temperature scale that is proportional to the net radiative cooling divided by the wind speed above the valley. The scatter is large partly because of nonstationarity of the marginal cold pool in this shallow valley. [ABSTRACT FROM AUTHOR]

The near-surface wind and temperature regime at three points in the Atacama Desert of northern Chile is described using two years of multilevel measurements from 80-m towers located in an altitude range between 2100 and 2700 m MSL. The data reveal the frequent development of strong nocturnal drainage flows at all sites. Down-valley, nose-shaped wind speed profiles are observed, with maximum values occurring at heights between 20 and 60 m AGL. The flow intensity shows considerable interdaily variability and a seasonal modulation of maximum speeds, which in the cold season can attain hourly average values of more than 20 m s−1. Turbulent mixing appears to be important over the full tower layer, affecting the curvature of the nighttime temperature profile and possibly explaining the observed increase of surface temperatures in the down-valley direction. Nocturnal valley winds and temperatures are weakly controlled by upper-air conditions observed at the nearest aerological station. Estimates of terms in the momentum budget for the development and quasi-stationary phases of the down-valley flows suggest that the pressure gradient force due to the near-surface cooling along the sloping valley axes plays an important role in these drainage flows. A scale for the jet nose height of equilibrium turbulent down-slope jets is proposed that is based on surface friction velocity and surface inversion intensity. At one of the sites, this scale explains about 70% of the case-to-case observed variance of jet nose heights. Further modeling and observations are needed, however, to define better the dynamics, extent, and turbulence structure of this flow system, which has significant wind-energy, climatic, and environmental implications. [ABSTRACT FROM AUTHOR]

Episodic nighttime intrusions of warm air, accompanied by strong winds, enter the enclosed near-circular Meteor Crater basin on clear, synoptically undisturbed nights. Data analysis is used to document these events and to determine their spatial and temporal characteristics, their effects on the atmospheric structure inside the crater, and their relationship to larger-scale flows and atmospheric stability. A conceptual model that is based on hydraulic flow theory is offered to explain warm-air-intrusion events at the crater. The intermittent warm-air-intrusion events were closely related to a stable surface layer and a mesoscale (~50 km) drainage flow on the inclined plain outside the crater and to a continuous shallow cold-air inflow that came over the upstream crater rim. Depending on the upstream conditions, the cold-air inflow at the crater rim deepened temporarily and warmer air from above the stable surface layer on the surrounding plain descended into the crater, as part of the flowing layer. The flow descended up to 140 m into the 170-m-deep crater and did not penetrate the approximately 30-m-deep crater-floor inversion. The intruding air, which was up to 5 K warmer than the crater atmosphere, did not extend into the center of the crater, where the nighttime near-isothermal layer in the ambient crater atmosphere remained largely undisturbed. New investigations are suggested to test the hypothesis that the warm-air intrusions are associated with hydraulic jumps. [ABSTRACT FROM AUTHOR]

Observations are analyzed to explain an unusual feature of the nighttime atmospheric structure inside Arizona's idealized, basin-shaped Meteor Crater. The upper 75%-80% of the crater's atmosphere, which overlies an intense surface-based inversion on the crater's floor, maintains a near-isothermal lapse rate during the entire night, even while continuing to cool. Evidence is presented to show that this near-isothermal layer is produced by cold-air intrusions that come over the crater's rim. The intrusions are driven by a regional-scale drainage flow that develops over the surrounding inclined Colorado Plateau. Cold air from the drainage flow builds up on the upwind side of the crater and splits around the crater at low levels. A shallow layer of cold air, however, spills over the 30-60-m-high rim and descends partway down the crater's upwind inner sidewall until reaching its buoyancy equilibrium level. Detrainment of cold air during its katabatic descent and compensatory rising motions in the crater atmosphere destabilize the basin atmosphere, producing the observed near-isothermal lapse rate. A conceptual model of this phenomenon is presented. [ABSTRACT FROM AUTHOR]