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Uncrewed aircraft system (UAS) observations collected during the 2018 Lower Atmospheric Process Studies at Elevation—a Remotely Piloted Aircraft Team Experiment (LAPSE-RATE) field campaign were assimilated into a high-resolution configuration of the Weather Research and Forecasting Model using an ensemble Kalman filter. The benefit of UAS observations was assessed for a terrain-driven (drainage and upvalley) flow event that occurred within Colorado’s San Luis Valley (SLV) using independent observations. The analysis and prediction of the strength, depth, and horizontal extent of drainage flow from the Saguache Canyon and the subsequent transition to upvalley and up-canyon flow were improved relative to that obtained both without data assimilation (benchmark) and when only surface observations were assimilated. Assimilation of UAS observations greatly improved the analyses of vertical variations in temperature, relative humidity, and winds at multiple locations in the northern portion of the SLV, with reductions in both bias and the root-mean-square error of roughly 40% for each variable relative to the benchmark run. Despite these noted improvements, some biases remain that were tied to measurement error and/or the impact of the boundary layer parameterization on vertically spreading the observations, both of which require further exploration. The results presented here highlight how observations obtained with a fleet of profiling UAS improve limited-area, high-resolution analyses and short-term forecasts in complex terrain.

A field study was conducted of flows in the Birch Creek Valley in eastern Idaho. There is a distinct topographic constriction in the Birch Creek Valley that creates two subbasins: an upper and lower valley. The data were classified into one of three groups based on synoptic influence (weak/absent, high wind speeds, and other evidence of synoptic influence). Gap flows commonly developed downwind of the constriction in association with the weak/absent group but also occurred in association with the two synoptic groups suggesting the potential for more diverse origins. In general, the frequency and strength of gap flows appeared to be linked to the development of the requisite thermal regime and minimization of any synoptically driven southerly winds that would suppress outflows. Gap flows were characterized by high wind speeds with jetlike vertical profiles along the axis of the lower valley. For all three groups the morning transition in the upper valley and western sidewall usually proceeded slightly ahead of the lower valley, consistent with the principles of the topographic amplification factor. The persistence of southerly winds in the lower valley past evening transition inhibited the development of gap flows, promoted strong nighttime inversions, and delayed the onset of morning transition relative to the upper valley. Nocturnal temperature inversions in the lower valley were largely eliminated with the onset of strong gap flows resulting in earlier morning transitions there. The form for a method of predicting gap flow wind speeds is proposed.

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.

Lothon, Marie et al., © 2014 Author(s). Due to the major role of the sun in heating the earth's surface, the atmospheric planetary boundary layer over land is inherently marked by a diurnal cycle. The afternoon transition, the period of the day that connects the daytime dry convective boundary layer to the night-time stable boundary layer, still has a number of unanswered scientific questions. This phase of the diurnal cycle is challenging from both modelling and observational perspectives: it is transitory, most of the forcings are small or null and the turbulence regime changes from fully convective, close to homogeneous and isotropic, toward a more heterogeneous and intermittent state. These issues motivated the BLLAST (Boundary-Layer Late Afternoon and Sunset Turbulence) field campaign that was conducted from 14 June to 8 July 2011 in southern France, in an area of complex and heterogeneous terrain. A wide range of instrumented platforms including full-size aircraft, remotely piloted aircraft systems, remote-sensing instruments, radiosoundings, tethered balloons, surface flux stations and various meteorological towers were deployed over different surface types. The boundary layer, from the earth's surface to the free troposphere, was probed during the entire day, with a focus and intense observation periods that were conducted from midday until sunset. The BLLAST field campaign also provided an opportunity to test innovative measurement systems, such as new miniaturized sensors, and a new technique for frequent radiosoundings of the low troposphere. Twelve fair weather days displaying various meteorological conditions were extensively documented during the field experiment. The boundary-layer growth varied from one day to another depending on many contributions including stability, advection, subsidence, the state of the previous day's residual layer, as well as local, meso-or synoptic scale conditions. Ground-based measurements combined with tethered-balloon and airborne observations captured the turbulence decay from the surface throughout the whole boundary layer and documented the evolution of the turbulence characteristic length scales during the transition period. Closely integrated with the field experiment, numerical studies are now underway with a complete hierarchy of models to support the data interpretation and improve the model representations., The BLLAST field experiment was made possible thanks to the contribution of several institutions and supports: INSU-CNRS (Institut National des Sciences de l’Univers, Centre national de la Recherche Scientifique, LEFE-IMAGO program), Météo-France, Observatoire Midi-Pyrénées (University of Toulouse), EUFAR (EUropean Facility for Airborne Research) BLLATE-1&2, COST ES0802 (European Cooperation in the field of Scientific and Technical) and the Spanish MINECO projects CGL2009–08609, CGL2012–37416–C04–03, CGL2012–37416– C04–02 and CGL2011-13477-E

The Met Office Unified Model has been adapted to run with a 100 m horizontal grid length for a region of complex terrain whose valleys are too narrow to be represented in the operational forecast (1.5 km grid length) model. Forecasts of temperatures are compared with observations from the COLd air Pooling EXperiment. The model provides a realistic representation of valley cold pools which form during stable nights, although the predictions are shown to be sensitive to vertical resolution. Reducing the vertical grid spacing near the surface results in lower night-time temperatures and larger, more realistic, valley–hill temperature contrasts.

Dense observations and numerical experiments were carried out to estimate the modification of mesoscale circulation, particularly cold drainage wind. It was confirmed that nocturnal drainage flow can develop on clear calm summer day and change due to orographical forcing and the heterogeneity of heat flux induced by the discontinuity of land-use. The temperature of nocturnal drainage flow at Sungji Valley, Busan Korea, tended to increase as it passed over the urban surface due to anthropogenic heat. The increase in temperature reached 2.9 K at night. The roughness associated with the exchange of momentum flux alone and the pass of nocturnal drainage flow is important for modifying the characteristics of flow Numerical simulations carried out under various surface conditions showed good agreement with the observations. Urban heat fluxes from the surface during the day are fundamental causes of the changes in the urban mesoscale circulation. In addition, the impact of a discontinuity of surface heat flux on mesoscale flow modification tends to be greater at night than during the day because the direction of urban surface heat fluxes at night is different from that in rural areas. In addition, the criterion to estimate the increase in temperature nocturnal drainage flow was also proposed, and provided results that generally agreed with the numerical results.

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.

SUMMARY This study documents the aire ow within major Alpine river valleys in the presence of precipitation. The analysis seeks a conceptual understanding of the effect of rain on the valley e ow. Ground-based and airborne Doppler radar and surface data from the Mesoscale Alpine Programme show that during persistent rainy periods the subsidence caused by melting and evaporation of precipitation particles contributes to the formation of a down-valley e ow. This down-valley eow opposes moist southerly e ow aloft, which is lifted over the topographic barrier and in which precipitation particles are formed.

The Penn State–NCAR mesoscale model MM5 is used to simulate and better understand the wind observations in the Kali Gandaki Valley reported in the first part of this paper. The Kali Gandaki River originates in Nepal near Tibet, flows southward through the Mustang Basin, crosses the Himalayas in a gorge, and descends to the lowlands of Nepal. Extremely strong diurnal upvalley flow in the gorge and the basin alternates with rather weak drainage flow in the night. As proposed in Part I, the Mustang Basin and the Tibetan Plateau can be considered as an elevated heat source driving the upvalley flow during the day. However, the extreme strength of the diurnal upvalley winds and the order-of-magnitude asymmetry between day and night cannot be explained with a simple plateau circulation theory. The model is successful in simulating almost all aspects of the observations. The simulations strongly suggest that the observed acceleration of the upvalley winds near the entrance to the Mustang Basin is linked...

The diurnal wind system of the Kali Gandaki Valley in Nepal was explored in September and October 1998 in a field campaign using pilot balloons as the main observational tool. This valley connects the Plateau of Tibet with the Indian plains. The river crosses the Himalayas forming the deepest valley on Earth. Intense upvalley winds blow up this valley during the day. Observations were made along the river at various spots selected between the exit point from the Himalayas and the source close to the Plateau of Tibet. The strongest upvalley winds were found between Marpha and Chuksang with typical speeds of 15–20 m s−1. The upvalley wind sets in first at the ground but an upvalley wind layer of 1000–2000-m depth forms quickly after the onset. This deep inflow layer persists up to Lo Manthang, a town located a few kilometers south of the Plateau of Tibet. Deceleration in the late afternoon and evening also appears to commence near the ground. Weak drainage flow forms late in the night. The causes o...

This paper describes the simulation of the most important characteristics of the flow and temperature fields observed during the tracer release of the 1990 TRANSALP campaign. The goal of the campaign was to follow the tracer from Leventina Valley to the other side of the Alpine range to investigate whether a re-circulation process could occur due to the valley and mountain breezes. Model output is compared with observations of wind and temperature over an entire day (29 September 1990). Meteorological measurements were obtained at a number of ground stations. Vertical wind and temperature profiles were also available. Model initialization was performed using airborne meteorological measurements and profiles from the synoptic station of Payerne. RAMS results correctly describe the generation of the drainage flow during the night and the successive formation of the valley breeze due to the difference in temperature gradients in the various valleys of the domain. Simulated windfields are in general more accurate during the daytime than during the night hours, since vertical variability in stable conditions is more difficult to reproduce.

A month-long field program to study the springtime katabatic wind confluence zone (where katabatic winds converge) has been carried out near Siple Coast, West Antarctica. Based on previous observations and numerical studies, two surface camps, Upstream B (83.5°8, 136.1°W) and South Camp (84.5°S, 134.3°W), were established. Ground-based remote sensing equipment (sodar and RASS), along with conventional observations, were used. Combining the analyses of surface observations and wind and temperature profiles at the above camps, the following picture for the cross-sectional structure of the confluence zone emerges. A relatively cold katabatic airflow, which probably comes from Fast Antarctica, occupies the layer between the surface and roughly 500 m ACL. Low-level jets are present below 200 m AGL and are stronger near the Transantarctic Mountains. Diurnal variation is present in this cold drainage flow and decreases toward the Transantarctic Mountains. Weak-inversion-layer tops are found near 500 m A...

Laboratory experiments were conducted to simulate the diurnal heating-cooling cycle in the vicinity of a ridge of constant cross section. In the model the fluid is a water solution stratified with salt to simulate the background stratification of the atmosphere. The flow is driven by recirculating water of a controlled temperature beneath the model; the model surface temperature is thus varied in a specified way to simulate the surface heating by solar insolation during the daytime hours and surface cooling by radiation during the nighttime. The pertinent similarity parameters are shown to be Gc, for daytime convective flow and Gd for nocturnal flow; here Gc = Hb/Hc, Gd = Hb/Hd, where Hb, is the mountain height, Hc the neutral buoyancy height of free convection. and Hd the characteristic thickness of the nighttime drainage flow. The model demonstrates some of the principal features of thermally driven mountain circulations, including daytime upslope winds and nocturnal downslope drainage flows. T...

The nighttime local wind and accumulation of cold air over the Kanto Plain in Japan were investigated with a three-dimensional hydrostatic numerical model under the condition of no synoptic pressure gradient. Numerical results strongly support the hypothesis that cold air accumulated in the western portion of the plain owing to the convergence of drainage flow from the mountain slopes in the northern and western parts of the plain. The accumulated cold air generated a deep surface inversion layer over the region, as suggested by observations. The cold air drained along three large rivers located in the plain after 1900 LST, when the daytime valley winds ceased. The heat flux between the surface and the atmosphere was exchanged mainly over the relatively steep slopes of forest-covered mountains, although the drainage flow there was not very strong. The low-temperature area generated in the center of the Kanto Plain could be distinguished even after noon the following day.

The characteristics of tributary drainage flow in stable, nocturnal conditions in three closely located tributaries are compared. The orientation of the tributaries with respect to Kimball Creek, into which they drain, appears to be a controlling factor in the tributary flow. In particular, oscillations in the drainage flow are found to be weakest and drainage mass per unit area greatest in the tributary most closely aligned with the main canyon.

[1] This study focuses on observations of the sea and land breeze circulations in the southern Arabian Gulf. During the summer months, the Indian monsoon creates light northwesterly winds over the Arabian Gulf, allowing for the formation of thermally driven circulations. Observations from a network of stations are used to develop a wind climatology for the Arabian Gulf region. Characteristics of the sea and land breeze circulations, such as onset time, duration, and horizontal and vertical extent are described. The dense network of surface stations in the United Arab Emirates (UAE) allows for a fine-scale observational study in this region. It is found that the sea breeze occurs during all seasons of the year in this region. It occurs in the late afternoon and continues through the evening. A land breeze sets in during the night. Surface offshore winds in the land breeze are strong probably due to drainage flow down the inland hills.

Wind and temperature profiles and corresponding acoustic sounder data collected in September 1982 are presented for nocturnal drainage flow and inversion breakup in two Alberta mountain valleys. The results show that the diurnal amplitude of average valley air temperatures for the Canyon Creek Valley is greater than the Elbow River Valley for four case days. The two-dimensional topographic amplification factors for both valleys show that this difference can be explained on the basis of differing valley geometry. The bottom half of the Canyon Creek Valley drainage flow had a weak inversion (1–2 K/100 m) with the top half of much greater strength (7 K/100 m). The Elbow River Valley drainage flow had an inversion strength of 2.9 K/100 m which was invariant with height. In the Canyon Creek Valley when the ridgetop winds had an up-valley component, the maximum down-valley wind speed occurs at 30%–40% of the drainage depth as opposed to 50%–60% when there is no up-valley component. There is also a line...

The variations of Σθ and ΣΦ in the drainage flow in the Brush Creek valley of western Colorado are investigated using data from Doppler acoustic sodars and instrumented towers. The data were obtained on two experimental nights during the 1984 ASCOT field study. There is good agreement between the Σ variations derived from low-level observations of the sodars and those derived from the towers located throughout the valley. The observed hourly average Σθ and ΣΦ in the nocturnal drainage flow are about 20 ° to 25 ° and 5 °, respectively; these values are much larger than those generally observed over flat terrain during nighttime stable conditions. After sunrise (about 0600 MST), as the valley warms and the flow direction changes to up-valley, these parameters increase sharply to their peak values at about 0800 MST and then decrease to their normal daytime values after about two hours.

The monthly frequencies of nocturnal drainage flows in the Brush Creek Valley were estimated over the period August 1982–January 1985 for the purpose of evaluating the representativeness of the drainage flows observed during a few intensive study periods. These estimates were made on the basis of data from three short meteorological towers situated in the valley. The highest frequencies were observed during the July to October timeframes: 30%–40% during 1983 and 5%–25% during 1984. Of the ten experimental nights when intensive investigations were conducted within the Brush Creek Valley, seven were during strong drainage flow periods and three were during weaker drainage flow periods. The variability of a σθ in complex terrain areas was investigated since this parameter is often used to estimate diffusion of pollutants. Measurements made during strong drainage flow periods within two valleys in The Geysers geothermal area in northern California and within the Brush Creek Valley yielded median hour...

Calculations of radiative transfer within a V-shaped valley show that the radiative cooling of the valley air is greater than the cooling over a flat surface. The measured cooling of a drainage flow in a deep, steep-walled valley is compared to calculations of radiative transfer within the same valley. It is found that cooling throughout the full watershed of the valley, instead of only within the steep walled part of the valley, must be considered to account for the energy loss of air passing through the valley. Some 10% to 20% of this energy loss occurs as a result of direct radiative cooling of the air. DOI: 10.1111/j.1600-0870.1987.tb00303.x

This paper presents an analysis of nocturnal drainage flows in a mountainous coastal environment where the elevation of the terrain is comparable with the elevation of the marine temperature inversion. The analysis traces the initiation, evolution, and breakup of the drainage flow using acoustic sounder facsimile data and tethered sonde measurements of wind and temperature. Conditions addressed include (1) opposing seabreeze flow ranging from 2 to 8m s-1, (2) aiding flow, and (3) large-scale and drainage-induced subsidence. The effect of deep marine temperature inversions pervades the observations, as seen in deeper, more stratified echo layers, weaker drainage, and delayed destruction of the inversion in the morning.

The three-dimensional, diagnostic, particle-in-cell transport and diffusion model MATHEW/ADPIC is used to test its transferability from one site in complex terrain to another with different characteristics, under stable nighttime drainage flow conditions. The two sites were subject to extensive drainage flow tracer experiments under the multilaboratory Atmospheric Studies in Complex Terrain (ASCOT) program: the first being a valley in the Geysers geothermal region of northern California, and the second a canyon in western Colorado. The domain in each case is approximately 10 × 10 km. The 1980 Geysers model evaluation is only quoted. The 1984 Brush Creek model evaluation is described in detail. Results from comparing computed with measured concentrations from a variety of tracer releases indicate that 52% of the 4531 samples from five experiments in Brush Creek and 50% of the 831 samples from four experiments in the Geysers agreed within a factor of 5. When an angular 10° uncertainty, consistent w...

Wind and temperature data collected by an instrumented tethered balloon and a Doppler lidar in a deep valley are used to investigate the mean properties of the nocturnal drainage flow down the valley on four nights when the wind at ridgetop had an up-valley component. We examine the vertical structure of temperature and the vertical and horizontal structure of the drainage wind. An empirical description of the wind field is derived and used to estimate the mass flux resulting from the drainage flow. Mean properties of the flow are presented and relationships among some of the parameters are examined.

Micro-scale cold-air flow along a gentle slope was analyzed using thermal infrared imaging (TIR), focusing exclusively on the lowermost 2 m above ground. Cold-air pulses were analyzed with regard to their vertical temperature stratification as well as flow characteristics, such as flow speed. Analyses on the transition zone between the near-surface very stable inversion layer and the less stable, warmer air above highlight turbulent situations and detrainment effects at the cold-air inversion top. Using thermal imaging in a high spatiotemporal resolution with up to 90 vertical data points and TIR pixels for 1.5 m cold-air depth, a high-precision cold-air flow analysis was realized.

This study on cold-air pool formation in the wide Cerdanya Valley in the Pyrenees mountain range was conducted using available observational information from September 2010 to August 2014. Cold-air pools occur during almost 60% of the nights, mainly during winter. Cold pools develop even under significant synoptic pressure gradients. Additionally, drainage currents transporting air down-valley occur most of the nights. In particular one representative cold-air pool event has been analysed in detail by a high-resolution mesoscale simulation, combined with an analysis of data from both ground-based stations and satellites. Radiative processes dominate the evolution of cold-air pools, together with turbulence in the lowest layers, while drainage flows down from the high mountains mainly through the tributary valleys and from the valley sidewall slopes play a key role in bringing air to the pool. Cold pool formation begins approximately 1 hr after sunset, and it extends across most of the valley bottom, with a very strong thermal inversion close to the surface that has a depth of up to 100¿m in the lowest parts of the valley. Wind veers down-valley along the main axis 2–3 hr after sunset and the wind direction is approximately maintained until after sunrise.