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Predicting winter flooding is critical to protecting people and securing water resources in California's Sierra Nevada. Rain-on-snow (ROS) events are a common cause of widespread flooding and are expected to increase in both frequency and magnitude with anthropogenic climate change in this region. ROS flood severity depends on terrestrial water input (TWI), the sum of rain and snowmelt that reaches the land surface. However, an incomplete understanding of the processes that control the flow and refreezing of liquid water in the snowpack limits flood prediction by operational and research models. We examine how antecedent snowpack conditions alter TWI during 71 ROS events between water years 1981 and 2019. Observations across a 500-m elevation gradient from the Independence Creek catchment were input into SNOWPACK, a one-dimensional, physically based snow model, initiated with the Richards equation and calibrated with collocated snow pillow observations. We compare observed "historical" and "scenario" ROS events, where we hold meteorologic conditions constant but vary snowpack conditions. Snowpack variables include cold content, snow density, liquid water content, and snow water equivalent. Results indicate that historical events with TWI > rain are associated with the largest observed streamflows. A multiple linear regression analysis of scenario events suggests that TWI is sensitive to interactions between snow density and cold content, with denser (>0.30 g cm−3) and colder (<−0.3 MJ of cold content) snowpacks retaining >50 mm of TWI. These results highlight the importance of hydraulic limitations in dense snowpacks and energy limitations in warm snowpacks for retaining liquid water that would otherwise be available as TWI for flooding. Significance Statement: The purpose of this study is to understand how the snowpack modulates quantities of water that reach the land surface during rain-on-snow (ROS) events. While the amount of near-term storm rainfall is reasonably predicted by meteorologists, major floods associated with ROS are more difficult to predict and are expected to increase in frequency. Our key findings are that liquid water inputs to the land surface vary with snowpack characteristics, and although many hydrologic models incorporate snowpack cold content and density to some degree, the complexity of ROS events justifies the need for additional observations to improve operational forecasting model results. Our findings suggest additional comparisons between existing forecasting models and those that physically represent the snowpack, as well as field-based observations of cold content and density and liquid water content, would be useful follow-up investigations. [ABSTRACT FROM AUTHOR]

Stable boundary layers are still a relatively problematic component of atmospheric modeling, despite their frequent occurrence. While general agreement exists that Monin–Obukhov similarity is not applicable in the stable boundary layer (SBL) due to the nonhomogeneous, nonstationary flow, no universal organizing theory for the surface SBL has been presented. The Stable Atmospheric Variability and Transport (SAVANT) field campaign took place in the fall of 2018 to explore under what conditions shallow drainage flow is generated. The campaign took place in an agricultural setting and covered the period of both pre- and postharvest, allowing for not only a basic exploration of the boundary layer but also a robust dataset for applied agricultural understanding of aerosol dispersion and impacts of changes in surface cover on drainage flows. This article provides a description of the field campaign. Examples of publicly available data products are presented, as well as examples of shallow drainage flow and corresponding lidar measurements of dispersion. Additionally, the field campaign was used to provide educational opportunities for students from several disciplines, and the outcomes of these joint educational ventures are discussed as models for future collaborations. [ABSTRACT FROM AUTHOR]

Uncrewed aircraft system (UAS) observations from the Lower Atmospheric Profiling 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 (WRF) Model. The impact of assimilating targeted UAS observations in addition to surface observations was compared to that obtained when assimilating surface observations alone using observing system experiments (OSEs) for a terrain-driven flow case and a convection initiation (CI) case observed within Colorado's San Luis Valley (SLV). The assimilation of UAS observations in addition to surface observations results in a clear increase in skill for both flow regimes over that obtained when assimilating surface observations alone. For the terrain-driven flow case, the UAS observations improved the representation of thermal stratification across the northern SLV, which produced stronger upvalley flow over the eastern half of the SLV that better matched the observations. For the CI case, the UAS observations improved the representation of the pre-convective environment by reducing dry biases across the SLV and over the surrounding terrain. This led to earlier CI and more organized convection over the foothills that spilled outflows into the SLV, ultimately helping to increase low-level convergence and CI there. In addition, the importance of UAS capturing an outflow that originated over the Sangre de Cristo Mountains and triggered CI is discussed. These outflows and subsequent CI were not well captured in the simulation that assimilated surface observations alone. Observations obtained with a fleet of UAS are shown to notably improve high-resolution analyses and short-term predictions of two very different mesogamma-scale weather events. [ABSTRACT FROM AUTHOR]

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]

Cannulas with multi‐staged side holes are the method of choice for femoral cannulation in extracorporeal therapies today. A variety of differently designed products is available on the market. While the preferred tool for the performance assessment of such cannulas are pressure‐flow curves, little is known about the flow and velocity distribution. Within this work flow and velocity patterns of a femoral venous cannula with multi‐staged side holes were investigated. A mock circulation loop for cannula performance evaluation was built and reproduced using a computer‐aided design system. With computational fluid dynamics, volume flows and fluid velocities were determined quantitatively and visually with hole‐based precision. In order to ensure the correctness of the flow simulation, the results were subsequently validated by determining the same parameters with four‐dimensional flow‐sensitive magnetic resonance imaging. Measurement data and numerical solution differed 7% on average throughout the data set for the examined parameters. The highest inflow and velocity were detected at the most proximal holes, where half of the total volume flow enters the cannula. At every hole stage a Y‐shaped inflow profile was detected, forming a centered stream in the middle of the cannula. Simultaneously, flow separation creates zones with significant lower flow velocities. Numerical simulation, validated with four‐dimensional flow‐sensitive magnetic resonance imaging, is a valuable tool to examine flow and velocity distributions of femoral venous cannulas with hole‐based accuracy. Flow and velocity distribution in such cannulas are not ideal. Based on this work future cannulas can be effectively optimized. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A 100 m resolution simulation of radiation fog observed during the Local And Non‐local Fog EXperiment (LANFEX) was performed over the Shropshire hills (UK) in order to understand the impact of local circulation on valley fog formation. The model correctly resolves all valleys and their different geometries, their associated dynamical features, and the different fog conditions between the measurement sites. Passage of stratocumulus during the night led to fog dissipation and gave the opportunity to study two fog formation stages. In the narrow valleys, fog formed at the valley floor and non‐local drainage‐flow processes acted to dissipate it. This equilibrium determined the fog optical thickness, which varied within and between valleys. Wider basins were more subject to dense fog conditions (due to local formation) than narrower valleys, where advecting fog events are locally observed through basins overflowing. The largest and most open valley of Jay Barns was impacted by numerous circulations from narrower tributary valleys, and their complex interactions affected fog formation differently between events. The impact of cloud microphysics on the simulated fog is studied by comparing simulations with one‐moment and two‐moment schemes. The use of a two‐moment scheme brings improvements when the prognostic number concentration is used to compute cloud optical properties, meaning that the radiative impact of droplet concentration is greater than its gravitational settling effect. [ABSTRACT FROM AUTHOR]

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. [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]

Lightning stroke density measured by the Global Lightning Dataset (GLD360) has shown several strong maxima around the globe. Several of these extremes are located over large tropical water bodies surrounded by terrain features. Four prominent maxima are examined and compared in this study: Lake Maracaibo in South America, the Strait of Malacca in equatorial Asia, Lake Victoria in East Africa, and Lake Titicaca in South America. Specifically, the authors observe that all four water bodies exhibit sustained maxima in lightning occurrence all night, the peak lightning frequency occurs very late at night or the following morning at three of the four sites, and the nocturnal maxima are out of phase at the four locations even though the afternoon maxima over the surrounding terrain all occur between 1500 and 1700 local solar time. The meteorological factors affecting the diurnal cycle of lightning occurrence over these four water bodies, which are all adjacent to mountains, are explored in this study. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In the region of Corsica located in the western Mediterranean Sea, the mean daily lightning activity as an indicator for deep convection in late summer and autumn shows a distinct maximum in mid-afternoon and a secondary maximum at night. During the night, most of the lightning activity is located offshore and near the island's coastline. Currently there are no observational data which could be used to explain this nocturnal offshore convection, but understanding its formation mechanism is important for accurately forecasting the regional weather. In this article, we explore two possible mechanisms initiating nocturnal offshore convection: (i) convergence with subsequent lifting due to the interaction between drainage winds and the synoptic flow over the sea and (ii) dynamically induced lee-side convergence due to the island barrier effect. To this end, we perform numerical simulations with the Consortium for Small-scale Modeling (COSMO) model at a convection-permitting horizontal grid spacing of 2.8 km for two cases with different synoptic conditions and low-level wind directions. The simulation results show that the island's drainage flow can either favour or hinder the development of deep convection. Furthermore, convective initiation is very sensitive to terrain elevation and model initialisation time and small changes of these parameters can determine whether deep convection can be successfully simulated. [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]

The authors investigate atmospheric internal gravity waves (IGWs): their generation and induction of global intermittent turbulence in the nocturnal stable atmospheric boundary layer based on the new concept of turbulence generation discussed in a prior paper by Sun et al. The IGWs are generated by air lifted by convergence forced by the colliding background flow and cold currents near the ground. The buoyancy-forced IGWs enhance wind speed at the wind speed wave crests such that the bulk shear instability generates large coherent eddies, which augment local turbulent mixing and vertically redistribute momentum and heat. The periodically enhanced turbulent mixing, in turn, modifies the air temperature and flow oscillations of the original IGWs. These turbulence-forced oscillations (TFOs) resemble waves and coherently transport momentum and sensible heat. The observed momentum and sensible heat fluxes at the IGW frequency, which are due to either the buoyancy-forced IGWs themselves or the TFOs, are larger than turbulent fluxes near the surface. The IGWs enhance not only the bulk shear at the wave crests, but also local shear over the wind speed troughs of the surface IGWs. Temporal and spatial variations of turbulent mixing as a result of this wave-induced turbulent mixing change the mean airflow and the shape of the IGWs. [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]

We present an intercomparison of three subgrid-scale ( SGS) models for large-eddy simulation ( LES) of katabatic flows. The SGS closures we study include the Smagorinsky formulation, a scale-invariant dynamic model, and a scale-dependent dynamic model. Downslope fluxes of mass and buoyancy deficit are highly sensitive to SGS closure choice. Due to strong shear and strong stratification, the dynamic models show a reduction of the Smagorinsky coefficient near the surface relative to the Smagorinsky model. Furthermore, results from both scale-dependent dynamic models suggest that the assumptions of scale invariance in the dynamic model are violated. We present a second set of experiments which focus on grid resolution requirements of eddy-resolving simulations of drainage flows. Downslope buoyancy and mass flux predictions produced by the scale-dependent dynamic model are more robust with respect to grid resolution and grid anisotropy than the scale-invariant dynamic and Smagorinsky SGS models. Predictions of vertically integrated downvalley mass and buoyancy fluxes are more sensitive to vertical grid resolution than horizontal grid resolution. [ABSTRACT FROM AUTHOR]

The nighttime stable atmospheric boundary layer over real terrain is modeled with nested high-resolution large-eddy simulations (LESs). The field site is located near Leon, Kansas, where the 1999 Cooperative Atmosphere-Surface Exchange Study took place. The terrain is mostly flat with an average slope of 0.5°. The main topographic feature is a shallow valley oriented in the east-west direction. The night of 5 October is selected to study intermittent turbulence under prevailing quiescent conditions. Brief turbulent periods triggered by shear-instability waves are modeled with good magnitude and temporal precision with a dynamic reconstruction turbulence closure. In comparison, conventional closures fail to excite turbulent motions and predict a false laminar flow. A plausible new intermittency mechanism, previously unknown owing to limited spatial coverage of field instruments at this site, is unveiled with the LESs. Turbulence can be generated through gravity wave breaking over a stagnant cold-air bubble in the valley upwind of the main tower. The bubble is preceded by the formation of a valley cold-air pool due to down-valley drainage flows during the evening transition. The bubble grows in depth by entraining cold down-valley and downslope flows from below and is eroded by shear-induced wave breaking on the top. The cyclic process of formation and erosion is repeated during the night, leading to sporadic turbulent bursting. [ABSTRACT FROM AUTHOR]

A numerical model simulation of the formation of a cold pool within a narrow valley during a clear calm night is presented. The results are compared with measurements made during the COLd air Pooling EXperiment (COLPEX). It is demonstrated that the model provides a realistic simulation of the observed cold pool. The cold pool begins to form approximately 1.5 hours before sunset, when the near-surface air within the valley starts to cool more rapidly than the air on the surrounding hills. The model potential temperature budget is used to determine which physical processes are most important for the evolution of the cold pool. The results show that sheltering provided by the valley causes a turbulent heat flux divergence over the lowest 5 m above the valley floor, which is large compared with the other terms in the budget. This causes relatively rapid cooling of the air adjacent to the ground. On the surrounding hills the turbulent heat flux divergence is larger than in the valley, but the enhanced cooling this provides is approximately balanced by an almost equally large warming tendency from advection. The net effect of these two terms is small compared with that in the valley and therefore the cooling rate is smaller. Above 5 m, the cooling in the valley is dominated by local transport of cold air away from the surface into the interior valley atmosphere. [ABSTRACT FROM AUTHOR]

The analysis of agricultural drainage requires a computer tool to help a user describe the changes in the shallow water table and the drainage flow, taking into account soil properties, physical characteristics and spatial disposition of drains and the vertical rates of recharge or discharge of the aquifer (infiltration or evapotranspiration). In this study, the computer program with graphic interface called DRENAS was developed, which simulates the hydraulic functioning of subsurface drainage systems; its main calculation module has a numerical solution to the one-dimensional Boussinesq equation for unconfined aquifers, considering the storage coefficient of the aquifer as a function of the hydraulic head and representing the vertical recharge as a function of time. In this module, non-linear relationships between the drainage flow and the hydraulic head on the drain can be used, known as fractal radiation and convex radiation boundary conditions. The program was complemented by including a database with information about soil properties and a calculation module with simplified analytical solutions for agricultural drainage. The module of the numerical solution was validated, first, by comparing its results with those obtained when applying the analytical solution for the transitory regime that is derived by assuming linear radiation boundary conditions and considering the transmissivity and storage coefficient of the unconfined aquifer as constants; then, it was validated by considering data from drained water depth measured in an experimental system. Both validations showed the ability of the model to describe the change of the water table and the volume drained for different conditions of drain design (maximum differences 0.025 % and 0.48 cm, respectively). Therefore, the user can assume that the results described by DRENAS are reliable, if the scenarios analyzed fulfill the hypothesis for which it was developed. [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]

Abstract: Although the use of liquid waste from piggeries as agricultural fertilizer is common practice, problems arise from contamination of water by chemical compounds. This paper reports on the presence of three hormones and six veterinary antibiotics in water of the surface runoff and drainage flow from volumetric lysimeters to which pig slurry was applied, and which had been planted with cereals, horticultural crops and permanent pasture. High-efficiency liquid chromatography was used for chemical analysis of soil water following two applications of liquid pig slurry. Concentrations of the hormones and antibiotics found were between 4.6 and 1350.8ngL−1, and were detectable up to the ninth day after the second application of manure. The presence of different crops did not influence the occurrence of the analytes. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

In this study, we investigate the validity of similarity scaling over a steep mountain slope (30-41 $$^\circ $$). The results are based on eddy-covariance data collected during the Slope Experiment near La Fouly (SELF-2010); a field campaign conducted in a narrow valley of the Swiss Alps during summer 2010. The turbulent fluxes of heat and momentum are found to vary significantly with height in the first few metres above the inclined surface. These variations exceed by an order of magnitude the well-accepted maximum 10 % required for the applicability of Monin-Obukhov similarity theory in the surface layer. This could be due to a surface layer that is too thin to be detected or to the presence of advective fluxes. It is shown that local scaling can be a useful tool in these cases when surface-layer theory breaks down. Under convective conditions and after removing the effects of self-correlation, the normalized standard deviations of slope-normal wind velocity, temperature and humidity scale relatively well with $$z/\varLambda $$, where $$z$$ is the measurement height and $$\varLambda (z)$$ the local Obukhov length. However, the horizontal velocity fluctuations are not correlated with $$z/\varLambda $$ under all stability regimes. The non-dimensional gradients of wind velocity and temperature are also investigated. For those, the local scaling appears inappropriate, particularly at night when shallow drainage flows prevail and lead to negative wind-speed gradients close to the surface. [ABSTRACT FROM AUTHOR]

The overflow of the dense water mass across the Greenland-Scotland Ridge (GSR) from the Nordic Seas drives the Atlantic meridional overturning circulation (AMOC). The Nordic Seas is a large basin with an enormous reservoir capacity. The volume of the dense water above the GSR sill depth in the Nordic Seas, according to previous estimates, is sufficient to supply decades of overflow transport. This large capacity buffers overflow's responses to atmospheric variations and prevents an abrupt shutdown of the AMOC. In this study, the authors use a numerical and an analytical model to show that the effective reservoir capacity of the Nordic Seas is actually much smaller than what was estimated previously. Basin-scale oceanic circulation is nearly geostrophic and its streamlines are basically the same as the isobaths. The vast majority of the dense water is stored inside closed geostrophic contours in the deep basin and thus is not freely available to the overflow. The positive wind stress curl in the Nordic Seas forces a convergence of the dense water toward the deep basin and makes the interior water even more removed from the overflow-feeding boundary current. Eddies generated by the baroclinic instability help transport the interior water mass to the boundary current. But in absence of a robust renewal of deep water, the boundary current weakens rapidly and the eddy-generating mechanism becomes less effective. This study indicates that the Nordic Seas has a relatively small capacity as a dense water reservoir and thus the overflow transport is sensitive to climate changes. [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]

Drainage or katabatic flows are common mesoscale circulations established as a result of differential radiative cooling of near-surface air masses in sloping terrain. The initial irruption of these flows, with sudden shifts in wind speed and direction, may result in vertical displacements of air parcels from their equilibrium position, which prove to be a common source of internal gravity waves. This paper illustrates this mechanism and describes the main features of the oscillations following the study of observational data gathered throughout one night during the Stable Atmospheric Boundary Layer Experiment in Spain 2006 (SABLES2006) field campaign. Pressure differences, measured by microbarometers set at different levels of a tower, help to interpret the evolution of other atmospheric variables, provide a detailed picture of the irruption of a drainage current, and reveal the formation of gravity waves at its top. The main parameters of the waves are derived from wavelet cross correlation of pressure time series, recorded by a surface array of microbarometers. The analysis yields, among other parameters, the horizontal component of the phase and group velocities of the gravity waves, which compare well with the velocity of irruption of the drainage current. Wavelet and other multiresolution techniques are also applied to sonic anemometer records to study the interaction between turbulence and larger-scale motions. The analysis shows evidence of heat flux divergence induced by the gravity waves, which may constitute a key factor for the vertical thermal profile in the nocturnal boundary layer (NBL) in situations of weak turbulence and important wave activity. [ABSTRACT FROM AUTHOR]

Hourly PM10 observational data from a network of sites in the western half of the city of Auckland have been analysed in conjunction with concurrent surface wind observations covering the period 2006 - 2008 inclusive. The chosen study area is bounded by elevated land on three sides and a harbour on the fourth, and is characterised by generally sloping topography and a number of ridges forming shallow valleys. The analysis sought to establish the general relationship between surface wind and PM10 concentrations over the study area as a whole, and to investigate whether low wind conditions led to systematic fine-scale spatial variation in PM10 delineated by the valley-ridge terrain in the study area. A PM10 index was established from the average concentration of data from several sites to distinguish area-wide from locally specific air quality. This analysis established that peaks in area-wide PM10 were limited to evenings and mornings with low wind conditions (< 1.2 m s-1) in winter (mid-May to mid-August). Data from two sites in one of the area's shallow valleys indicate that these conditions led to the formation of a local micro-climate consistent with drainage flow decoupled from synoptic conditions. The effect of this micro-climate on PM10 was to lead to locally elevated concentrations, especially between midnight and dawn. The analysis has been used to inform the air quality assessment for a major motorway project planned to pass through the valley (for which the local monitoring was conducted). It is proposed that the analysis leads to a more robust and representative baseline assessment for project assessment receptors distributed over a wide spatial area. [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]

Abstract: Night-time airflow within a deep and dense canopy near the top of a mountain ridge is investigated based on measurements at Bily Kriz, Czech Republic. The site is characterized by a young Norway spruce forest on a 13° slope and the occurrence of almost exclusively upslope or downslope flows. The forest canopy reaches the ground surface. A decoupled two-layer structure of canopy flow typically develops at night. While the above-canopy flow is most frequently an upslope-directed larger-scale flow over the ridge, the lower-canopy flow is downslope (katabatic). However, the lower-canopy flow can be forced upslope when the wind speed above the canopy exceeds a well-defined limit. Less frequently, on the lee slope to the larger-scale flow, both the above-canopy and the lower-canopy flow are usually downslope, although a flow reversal in the lower canopy is also observed, accompanied with a large shear stress (friction velocity) above the canopy. The occurrence of opposing flows is not limited to sunset/sunrise transition periods. In a simplified modelling approach to the dynamics of the nocturnal lower-canopy flow decoupled from above, local equilibrium is assumed of solely two opposing driving forces – one induced by the negative buoyancy (due to radiative cooling of the canopy) and the other by the hydrodynamic pressure gradient (resulting from the larger-scale flow over the ridge) – and the canopy drag as a retarding force. The diagnostic model gives realistic values of the major driving terms for Bily Kriz, and the downslope or upslope direction and speed of the lower-canopy flow that agree well with the measurements. The model contributes to better interpretation of the experimental results, which are in accordance with recent publications on the flow patterns on forested hills. Knowledge of the lower-canopy flow behaviour and of the degree of its decoupling from the flow aloft is necessary for assessing the contribution of advection to the CO2 budget at sloping forest sites, and for analysis of the flux footprint. [Copyright &y& Elsevier]

This study examines the diurnal variations of the warm-season precipitation over northern China using the high-resolution precipitation products obtained from the Climate Prediction Center’s morphing technique (CMORPH) during May–August of 2003–09. The areas of focus are the Yanshan–Taihangshan Mountain ranges along the east peripheries of the Loess and Inner Mongolian Plateaus and the adjacent North China Plains. It is found that the averaged peak in local precipitation begins early in the afternoon near the top of the mountain ranges and propagates downslope and southeastward at a speed of ∼13 m s−1. The peak reaches the central North China Plains around midnight and the early morning hours resulting in a broad area of nocturnal precipitation maxima over the plains. The diurnal precipitation peak (minimum) is closely collocated with the upward (downward) branch of a mountain–plains solenoid (MPS) circulation. Both the MPS and a low-level southwesterly nocturnal jet are likely to be jointly responsible for the nighttime precipitation maxima over the plains. [ABSTRACT FROM AUTHOR]

Prognostic air pollution models such as TAPM are used to study air pollution on multi-day to multi-seasonal scales, driven by daily synoptic forcings. In complex terrain, mesoscale regimes can introduce diurnally reversing wind systems that can be persistent features flow low level climatology -- especially during stagnant anti-cyclonic conditions that result in deterioration of air quality. In this study, TAPM is modified to ignore synoptic forcings yet still produce terrain induced flows that are common features over the Canterbury Plains in New Zealand. In addition, surface-based climatology for two different regimes is assimilated into the model in order to study their effects on temporal evolution of PM10. Two common modes of surface wind conditions are observed in Christchurch during evenings of high pollution days where the national guideline for PM10 is exceeded Low level flow during the evening hours with generally low wind speeds is dominated by either a north-westerly or a south-westerly wind direction. Results from TAPM simulations of dispersion indicate north-easterly flow opposes north-westerly to westerly drainage flow from the Southern Alps and traps pollutants over the city for an increased period of time and there with delays flushing by a few hours. Simulated pollution levels during the evening are close to observed values and indicate that atmospheric control mechanisms are simulated well. This study delivers a viable tool for local environmental monitoring authorities to estimate generalised spatiotemporal pollution behaviour under different low level flow conditions. [ABSTRACT FROM AUTHOR]

Abstract: A method to determine the soil hydraulic conductivity via an internal drainage experiment is presented. Identifying the parameters of the hydraulic conductivity is achieved by solving an inverse global optimization problem that uses the water contents measured at different depths and times as matching flow variables. The optimization procedure is combined with a recently developed analytical model for the water content propagation, which essentially assumes that the flow is gravity-driven. A crucial (from the identification point of view) parameter of such a model is the initial position of the draining front, determining the interface between the wetted and dried zone. By using an evolutionary algorithm specifically developed for this problem, it is shown that if information upon is not a priori available, the identification of the hydraulic conductivity is not possible. However, assuming that is known (i.e. measured), and by dividing the model variables by , the optimization is able to fully identify the soil hydraulic conductivity. Finally, in order to show the robustness of the proposed approach, it is shown that the method leads to very good estimates of the hydraulic conductivity even if data are noise-affected, provided that the optimization procedure is coupled to the (Tikhonov) regularization approach. [Copyright &y& Elsevier]

Abstract: Stable isotopes provide insight into ecosystem carbon cycling, plant physiological processes, atmospheric boundary-layer dynamics, and are useful for the integration of processes over multiple scales. Of particular interest is the carbon isotope content (δ13C) of nocturnal ecosystem-respired CO2 (δ R). Recent advances in technology have made it possible to continuously examine the variation in δ R within a forest canopy over relatively long time-scales (months–years). We used tunable diode laser spectroscopy to examine δ R at within- and below-canopy spatial locations in a Colorado subalpine forest (the Niwot Ridge AmeriFlux site). We found a systematic pattern of increased δ R within the forest canopy (δ R-c) compared to that near the ground (δ R-g). Values of δ R-c were weakly correlated with the previous day''s mean maximum daytime vapor pressure deficit (VPD). Conversely, there was a negative but still weak correlation between δ R-g and time-lagged (0–5 days) daily mean soil moisture. The topography and presence of sustained nightly drainage flows at the Niwot Ridge forest site suggests that, on nights with stable atmospheric conditions, there is little mixing of air near the ground with that in the canopy. Atmospheric stability was assessed using thresholds of friction velocity, stability above the canopy, and bulk Richardson number within the canopy. When we selectively calculated δ R-g and δ R-c by removing time periods when ground and canopy air were well mixed, we found stronger correlations between δ R-c and VPD, and δ R-g and soil moisture. This suggests that there may be fundamental differences in the environmental controls on δ R at sub-canopy spatial scales. These results may help explain the wide variance observed in the correlation of δ R with different environmental parameters in other studies. [Copyright &y& Elsevier]

Abstract: CO2 transport processes relevant for estimating net ecosystem exchange (NEE) at the Niwot Ridge AmeriFlux site in the front range of the Rocky Mountains, Colorado, USA, were investigated during a pilot experiment. We found that cold, moist, and CO2-rich air was transported downslope at night and upslope in the early morning at this forest site situated on a 5% east-facing slope. We found that CO2 advection dominated the total CO2 transport in the NEE estimate at night although there are large uncertainties because of partial cancellation of horizontal and vertical advection. The horizontal CO2 advection captured not only the CO2 loss at night, but also the CO2 uptake during daytime. We found that horizontal CO2 advection was significant even during daytime especially when turbulent mixing was not significant, such as in early morning and evening transition periods and within the canopy. Similar processes can occur anywhere regardless of whether flow is generated by orography, synoptic pressure gradients, or surface heterogeneity as long as CO2 concentration is not well mixed by turbulence. The long-term net effect of all the CO2 budget terms on estimates of NEE needs to be investigated. [Copyright &y& Elsevier]

Abstract: Local-scale wind regimes over an idealized two-dimensional (2D) mountain having similar horizontal and vertical scales as Vancouver Island were investigated using a high-resolution mesoscale model. The model-generated flow outputs were then used as ersatz data to assess the impact of limiting assumptions in the eddy-covariance (EC) method as well as the effectiveness of tower flux data time-filtering for the main tower site of the Fluxnet-Canada British Columbia flux station. In this paper, we describe the simulated mesoscale and local-scale flow regimes, and in Part II we describe their use in assessing tower-data analyses of CO2 fluxes. The numerical model was enhanced to include parameterizations for: tree drag, radiation effects of forest canopies on the surface energy budget, and soil heat conduction. Simulations were performed both over an idealized bare hill and over a forested hill. The simulated flow involved interactions between the land/sea breeze, convective thermals, and mountain circulations under fair-weather conditions. The resulting simulated winds over the forested slope were much weaker than those over a bare slope. The nocturnal drainage flow over the forested slope was separated into sub-canopy and above-canopy regimes due to the temperature inversion at treetop. The strongest downslope flows occurred above the canopy with a minimum velocity occurring in the upper canopy where canopy drag is the strongest. The overturning of large convective eddies during daytime resulted in intermittent downslope flows under the canopy during the day. Wind regimes had a rapid shift from upslope to downslope flow above the canopy just after sunset. Further testing will be made using 3D simulations with higher resolution. [Copyright &y& Elsevier]

Abstract: Downslope thermotopographic flows (drainage flows) and the resulting horizontal flow divergence and advection have been suggested to cause systematic underestimates to nighttime ecosystem–atmosphere exchange measurements of CO2 and other scalars above tall vegetation in hilly terrain. Flux correction methods have been proposed using simple models which link measured vertical flow convergence above canopy to horizontal divergence. In Part I (Froelich, N.J., Schmid, H.P., Grimmond, C.S.B., Su, H.-B., Oliphant, A.J., 2005. Flow divergence and density flows above and below a deciduous forest. Part 1: non-zero mean vertical wind above canopy. Agric. For. Meteorol. 133, 140–152.), we examined vertical velocities above canopy at a flux tower in Morgan-Monroe State Forest (MMSF), Indiana, USA, and found evidence of vertical flow convergence and horizontal divergence. Here, we examine whether sub-canopy flow through a small gully in the vicinity of the flux tower was thermotopographically driven, and was linked to the flow divergence found above canopy. While flow in the gully was frequently aligned with the mean wind aloft, indicating dynamic coupling, there were periods when the wind in the gully appeared to be decoupled from the flow aloft and was consistent with thermotopographic flow forcings (including geometry, temperature gradient, and net radiation). During the leaf-off season, these episodes exhibited a classic thermotopographic pattern, with down-gully nighttime flow and up-gully daytime flow. However, during the leaf-on season, the pattern was reversed: during the daytime, flow was down-gully consistent with inversion conditions occurring below the dense leaf canopy; at night, flow was up-gully, consistent with below-canopy lapse conditions. The thermotopographic flow during the leaf-on season suggests horizontal flow convergence at night and divergence during the day, and is shown to be decoupled from the flow aloft. While this research focuses only on flow patterns and not explicitly on CO2 gradients or fluxes, these findings suggest that inferences about drainage flow/advection and corrections to flux measurements based on above-canopy conditions alone may be inappropriate. [Copyright &y& Elsevier]

Abstract: Prediction of nitrate leaching from cropland is crucial for preventing surface or ground water degradation. Accurate modeling of nitrate leaching requires simulations of both soil hydrological and biogeochemical processes. This paper reports an attempt to improve an existing biogeochemical model, Denitrification–Decomposition or DNDC, for estimation of nitrate leaching from crop fields with tile drainage system. DNDC was equipped with detailed biogeochemical processes of nitrogen turnover but a simple module for one-dimensional movement of soil water. Observations from nine drainage tiles with three different fertilizer treatments in 4 years (1996–1999) at an experimental field in Iowa were used for model modifications. Preliminary comparisons with observed tile discharge flow indicated that the original DNDC lacked the water leaching recession character. To correct this deviation, new water retention features were added to DNDC by: (1) adopting a recession curve to regulate the gravity drainage flow in the explicitly simulated soil profile (0–50cm) and (2) introducing a virtual water pool for the space between the bottom of the modeled soil profile (50cm) and the tile lines depth of placement (145cm) to control the tile discharge flow. With these modifications, model prediction of water leaching fluxes from the tile drainage lines was improved. An adsorbed N pool was created in DNDC to simulate the buffering effect of soil on the amount of nitrate available for leaching. The Langmuir equation was adopted to simulate adsorption and desorption of ammonium ions on the soil absorbents. This modification enhanced the model capacity for simulating free ammonium dynamics, nitrification, and nitrate leaching. Sensitivity tests of the modified DNDC showed that the modeled impact of differences of precipitation, soil texture, soil organic carbon content, and fertilizer application rates on nitrate leaching rates were consistent with observations reported by other researchers. This study indicated that a biogeochemical model with limited modifications in hydrology could serve nitrate leaching prediction and be useful for sustainable agricultural management. [Copyright &y& Elsevier]

Abstract: Ecosystem–atmosphere exchange flux measurements above tall vegetation in hilly terrain are well known to suffer from systematic underestimates of nighttime fluxes of CO2 and other scalars with significant sources in or below the canopy. This bias is commonly attributed to advection driven by thermotopographic density flows and the resulting horizontal flow divergence. Flux correction methods have been proposed based on this notion. To examine the structure and dynamics of horizontal flow divergence and vertical convergence mean vertical velocities are analyzed. These are derived from sonic anemometers at 1.8 and 1.3 times canopy height for 3 years above a deciduous forest in hilly terrain at the Morgan–Monroe State Forest (Indiana, USA) Fluxnet site. Measured vertical velocities are linked to forcing parameters represented in the equations of motion and heat for sloping terrain. In the leaf-off season, the data suggest that the dynamics and daily patterns of horizontal flow divergence (implied from vertical convergence through continuity) are entirely consistent with the hypothesis that the divergence is driven by thermotopographic density flows. However, in the vegetative season with a full canopy, a more complex picture emerges, suggesting strong dynamic and thermal decoupling of the horizontal divergence below canopy from flow conditions above. Thus we conclude that flux correction methods based on above-canopy conditions alone may significantly misrepresent scalar transport below canopy during the vegetative season and should be avoided. [Copyright &y& Elsevier]

Wind speed profiles above a forest canopy relate to scalar exchange between the forest canopy and the atmosphere. Many studies have reported that vertical wind speed profiles above a relatively flat forest can be classified by a stability index developed assuming wind flow above a flat plane. However, can such a stability index be used to classify vertical wind speed profiles observed above a sloping forest at nighttime, where drainage flow often occurs? This paper examines the use of the bulk Richardson number to classify wind speed profiles observed above a sloping forest at nighttime. Wind speed profiles above a sloping forest were classified by the bulk Richardson numberRi B. Use ofRi B distinguished between drainage flow, shear flow, and transitional flow from drainage flow to shear flow. These results suggest thatRi B is useful to interpret nighttime CO2 and energy fluxes above a sloping forest. Through clear observational evidence, we also show that the reference height should be high enough to avoid drainage-flow effects when calculatingRi B. [ABSTRACT FROM AUTHOR]

• Rate of gas drainage from a semi-sealed channel strongly depends on thermal boundary condition. Draining a viscous gas from a semi-sealed narrow conduit is a pore-scale problem of fundamental interest to primary fluid recovery from a petroleum reservoir as well as other applications. Such a drainage flow is entirely driven by the volumetric expansion of the gas and its mass flow rate is determined by the time-rate of decrease of the gas density within the conduit. It was found previously that when thermal effect is completely neglected, the drainage rate differs significantly from that based on the lubrication theory. The present work parametrically explores the influence of thermal boundary condition on the non-isothermal drainage flow from a narrow channel. It is found that as the wall transitions from adiabatic to isothermal condition, the excess density changes from a plane wave solution to a non-plane wave solution; and the drainage rate increases due to thermal damping on the wall, as a strong damping of the acoustic wave accelerates the process towards the final equilibrium. It is shown that when the exit is also cooled and the wall is non-adiabatic, the total recovered fluid mass exceeds the amount based on the isothermal theory determined by the initial and final density difference alone. The isothermal wall condition with the wall maintained at the initial gas temperature produces the most amount of fluid in the shortest time. [ABSTRACT FROM AUTHOR]