Upslope Flow Research Articles

An Assessment of Winter Orographic Precipitation and Cloud-Seeding Potential in Wyoming

AbstractThis paper presents an evaluation of the precipitation patterns and seedability of orographic clouds in Wyoming using SNOTEL precipitation data and a high-resolution multiyear model simulation over an 8-yr period. A key part of assessing the potential for cloud seeding is to understand the natural precipitation patterns and how often atmospheric conditions and clouds meet cloud-seeding criteria. The analysis shows that high-resolution model simulations are useful tools for studying patterns of orographic precipitation and establishing the seedability of clouds by providing information that is either missed by or not available from current observational networks. This study indicates that the ground-based seeding potential in some mountain ranges in Wyoming is limited by flow blocking and/or prevailing winds that were not normal to the barrier to produce upslope flow. Airborne seeding generally had the most potential for all of the mountain ranges that were studied.

Fire and Rain: The Legacy of Hurricane Lane in Hawaiʻi

AbstractHurricane Lane (2018) was an impactful event for the Hawaiian Islands and provided a textbook example of the compounding hazards that can be produced from a single storm. Over a 4-day period, the island of Hawaiʻi received an island-wide average of 424 mm (17 in.) of rainfall, with a 4-day single-station maximum of 1,444 mm (57 in.), making Hurricane Lane the wettest tropical cyclone ever recorded in Hawaiʻi (based on all available quantitative records). Simultaneously, fires on the islands of nearby Maui and Oʻahu burned 1,043 ha (2,577 ac) and 162 ha (400 ac), respectively. Land-use characteristics and antecedent moisture conditions exacerbated fire hazard, and both fire and rain severity were influenced by the storm environment and local topographical features. Broadscale subsidence around the storm periphery and downslope winds resulted in dry and windy conditions conducive to fire, while in a different region of the same storm, preexisting convection, incredibly moist atmospheric conditions, and upslope flow brought intense, long-duration rainfall. The simultaneous occurrence of rain-driven flooding and landslides, high-intensity winds, and multiple fires complicated emergency response. The compounding nature of the hazards produced during the Hurricane Lane event highlights the need to improve anticipation of complex feedback mechanisms among climate- and weather-related phenomena.

Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley. Part II: Passive tracer tracking

AbstractUnder wintertime quiescent conditions, thermally driven circulations represent one of the only sources of tracer dispersion over mountainous terrain. Those circulations can be unequally developed at a valley scale since they strongly depend on local morphological arrangement. At the same time, very heterogeneous pollutant distribution can be observed, as for instance in a French Alpine basin located in the Arve River valley. This complex basin regularly shows large variations in pollutant concentrations with certain sectors suffering from poor wintertime air quality. On the other hand, the surrounding tributary valleys appear to be less affected, suggesting that the basin local dynamics may participate in pollutant trapping. The present study intends to classify the pollutant transport mechanisms in terms of efficiency and to identify the most dynamically vulnerable atmospheric volumes regarding pollutant accumulation. This is achieved through a set of semi‐idealized high‐resolution numerical simulations reproducing a full diurnal cycle with passive tracers released continuously at a constant rate. The model is used as a laboratory in order to quantify the influence of several processes on transport mechanism efficiency. This approach underlines the high efficiency of vertical transport by anabatic winds while horizontal transport efficiency by up‐valley wind systems remains weak, leaving the surrounding tributary valleys almost unaffected by the basin pollution during daytime. At night, the efficiency of horizontal transport by the down‐valley wind systems depends on the tracer source location within the basin. In addition, the tracers emitted within the tributary valleys do not reach the basin bottom because of thermal stratification and local morphological arrangement but rather degrade the air quality of mid‐altitude villages lying along the basin sidewalls.

Stability of the anabatic Prandtl slope flow in a stably stratified medium

In the Prandtl model for anabatic slope flows, a uniform positive buoyancy flux at the surface drives an upslope flow against a stable background stratification. In the present study, we conduct linear stability analysis of the anabatic slope flow under this model and contrast it against the katabatic case as presented in Xiao & Senocak (J. Fluid Mech., vol. 865, 2019, R2). We show that the buoyancy component normal to the sloped surface is responsible for the emergence of stationary longitudinal rolls, whereas a generalised Kelvin–Helmholtz (KH) type of mechanism consisting of shear instability modulated by buoyancy results in a streamwise-travelling mode. In the anabatic case, for slope angles larger than to the horizontal, the travelling KH mode is dominant whereas, at lower inclination angles, the formation of the stationary vortex instability is favoured. The same dynamics holds qualitatively for the katabatic case, but the mode transition appears at slope angles of approximately . For a fixed slope angle and Prandtl number, we demonstrate through asymptotic analysis of linear growth rates that it is possible to devise a classification scheme that demarcates the stability of Prandtl slope flows into distinct regimes based on the dimensionless stratification perturbation number. We verify the existence of the instability modes with the help of direct numerical simulations, and observe close agreements between simulation results and predictions of linear analysis. For slope angle values in the vicinity of the junction point in the instability map, both longitudinal rolls and travelling waves coexist simultaneously and form complex flow structures.

A convection-permitting model for the Lake Victoria Basin: evaluation and insight into the mesoscale versus synoptic atmospheric dynamics

Aiming for an improved understanding of the different factors that determine the regional climate of the Lake Victoria Basin, the COSMO-CLM regional climate model is set up in a tropical, convection-permitting configuration and is directly nested in a recent reanalysis product (ERA5). The convection-permitting simulation outperforms state-of-the-art climate integrations that rely on convection parametrisations. Yet the domain-averaged model precipitation is larger than in the multi-observational ensemble, but the latter shows large spread. Overestimations of outgoing TOA shortwave and longwave radiation are much reduced compared to the COSMO-CLM CORDEX-Africa integration, but still suggest a general underestimation of the cloud fraction or frequency. Comparing the control with a no-lake simulation, the presence of Lake Victoria implies strong intensification of over-lake rainfall, but it only slightly increases the total domain-averaged precipitation. In addition, the easterly trade winds are shown to largely affect the mesoscale circulation and precipitation patterns. During daytime, fast trades and anabatic slope winds trigger convection at the lee-wind slopes, and subsidence over the basin. Slow trades allow the stationary air to produce spontaneous convective cells and to develop anabatic winds that result in orographic precipitation. During night-time, trade winds curl around the eastern branch of the East African Rift, generating a southerly and northerly evening convergence front entering the Lake Victoria Basin. Thus, our results highlight the key importance of the easterly trade winds and the complex orography in determining the total accumulation and location of precipitation in the Lake Victoria region.

Planar hydraulic jumps in thin film flow

We reformulate shallow water theory to understand viscous shear induced natural hydraulic jumps in channels slightly deviated from the horizontal. One of the interesting contributions of the study is a modified expression for Froude number to predict jumps in inclined channels. The proposed Froude number is different from the conventional expression which incorporates channel inclination as a straight forward component of gravity. This highlights the complexity that a jump can generate even in single phase laminar flow. We also obtain an analytical expression for predicting jump strength and show that the scaling relationship originally proposed for jump location in horizontal channels is applicable for both upslope and downslope flows. As expected, upslope flow aids jump formation and beyond a critical adverse tilt, a submerged jump results in subcritical flow right from the entry. On the other hand, both Reynolds number and channel tilt suppress the tendency to jump in downslope flows and below a critical downslope inclination, the flow remains supercritical throughout the channel length. The film thickness for fully developed flow can be predicted from the exact solution of the Navier–Stokes equations. As the theory encounters a singularity in the jump region, numerical simulations and experimental results have been used to obtain additional insights into the physics of jump formation. They have revealed the existence of submerged jump, wavy jump, smooth jump and no jump conditions as a function of liquid Reynolds number, scaled channel length and channel inclination. Such a variety of jump geometries in planar laminar flow has not been reported earlier. Both theory and simulations also reveal that the linear free surface profile upstream of the jump is a function of Reynolds number only, while the downstream profiles can be tuned by changing both Reynolds number as well as the channel length and tilt over the range of parameters studied. We thus demonstrate that, despite the simplicity and the approximations involved, shallow water equations formulated assuming self-similar velocity profiles can elucidate the physics of planar laminar jumps over slight inclinations, difficult to avoid in practice. The analytical and simulated results have been extensively validated with experimental data obtained from a specially designed test rig which ensures laminar flow before and after the jump. To the authors’ knowledge, almost no experimental study has to date been reported on films ‘thin enough’ to remain laminar even after the planar jump.

Statistical characterization of frost zones: Case of tea freeze damage in the Kenyan highlands

Frequent frost occurrences in the Kenyan highlands have had devastating effects on agricultural productivity. With inadequate management systems to mitigate the impacts, farmers have often had to bear the burden of losses resulting from frost damage. While agriculture in Kenya remains dependent on weather and climate, the agricultural economy of Kenya continues to suffer, underscoring the need for building local knowledge as basis for development of early warning systems. The current paper attempts to delineate frost zones by statistically characterizing them based on known risk factors related to topography (elevation, convexity, aspect, upslope flow length) and Land Surface Temperature (LST) derived from Moderate Resolution Imaging Spectroradiometer (MODIS). Through binary logistic regression, a logistic regression model was developed utilizing observation data (frost occurrence and non-occurrence) as a binary dependent variable to estimate the probability of frost occurrence. Assuming a 0.5 probability cut-off threshold between frost occurrence and non-occurrence, an overall accuracy of 81% with area under Receiver Operating Characteristics (ROC) Curve of 0.88 was obtained. No evidence of lack of model fit was detected. This model outperforms the currently operational model that utilizes MODIS LST alone to detect frost zones in the Kenyan tea plantations. It provides an improved method for effective delineation of frost zones by incorporating local topographic characteristics.

On gravity currents of fixed volume that encounter a down-slope or up-slope bottom

We consider a gravity current released from a lock into an ambient fluid of smaller density, that, from the beginning or after some horizontal propagation X1, propagates along an inclined (up- or down-) bottom. The flow (assumed in the inertial-buoyancy regime) is modeled by the shallow-water (SW) equations with a jump condition applied at the nose (front). The behavior of the current is dominated by the slope angle, θ, but is also affected by additional dimensionless parameters: the aspect ratio of the lock x0/h0, the height ratio of the ambient to lock, H/h0, and the distance of the backwall from the beginning of the slope, X1/x0. We show that the stability of the interface, reflected by the value of the bulk Richardson number, Ri, is essential in the interpretation and modeling. In the upslope flow, Ri increases and hence entrainment/mixing effects are unimportant. In the downslope flow, the current first accelerates and Ri decreases; this enhances entrainment and drag, which then decelerate the current. We show that the accelerating-decelerating downstream current is reproduced well by a SW model combined with a simple closure for the entrainment and drag. A comparison of the theoretical results with previously published experimental data for both upslope flow and downslope flow show fair agreement.

Characteristic of anabatic wind in Bandung basin observed by AWS

A field campaign has been done for the wet season in 2015 to identify the pattern of local circulation such as anabatic wind at Bandung basin. In this field campaign, the Automatic Weather Stations (AWS) were installed at seven points from the north until the south region of Bandung basin. The result of this research defines the pattern of anabatic wind in Bandung basin into three categories: the pattern on weak convection, strong convection, and strong convection with rain event. On weak convection condition, after 12.00 LT, the anabatic wind tended to move to the north region of Bandung basin. The wind still moved to the north and south region, on strong convection and strong convection with rain event condition after 12.00 LT. However, after 15.00 LT the wind on strong convection with rain event changed to become northward. On strong convection and strong convection with rain event, the pattern of anabatic wind was influenced by the convective activity in the north and south region of Bandung basin. Whereas on weak convection condition, the pattern of anabatic wind was not influenced by the convective activity.

Numerical Simulations of Seasonal Variations of Rainfall over the Island of Hawaii

AbstractThe seasonal variations of rainfall over the island of Hawaii are studied using the archives of the daily model run from the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) from June 2004 to February 2010. Local effects mainly drive the rainfall on the Kona coast in the early morning and the lower slopes in the afternoon. During the summer, the incoming trade winds are more persistent and moister than in winter. The moisture content in the wake zone is higher than open-ocean values because of the convergent airflow associated with dual counterrotating vortices. As the westerly reversed flow moves toward the Kona coast, it decelerates with increasing moisture and a moisture maximum over the coastal area, especially in the afternoon hours in summer months. The higher afternoon rainfall on the Kona lower slopes in summer than in winter is caused by a moister (>6 mm) westerly reversed flow bringing moisture inland and merging with a stronger upslope flow resulting from solar heating. Higher nocturnal rainfall off the Kona coast in summer than in winter is caused by the low-level convergence between a moister westerly reversed flow and offshore flow. On the windward slopes, the simulated rainfall accumulation in winter is higher because of frequently occurring synoptic disturbances during the winter storm season. Nevertheless, early morning rainfall along the windward coast and afternoon rainfall over the windward slopes of the Kohala Mountains is lower in winter because the incoming trades are drier.

Effect of the surface wind flow and topography on precipitating cloud systems over the Andes and associated Amazon basin: GPM observations

The characteristics of the precipitation under the influence of topography and surface wind flow are investigated over South America. Here the precipitating cloud systems (PCSs) are identified using the Global Precipitation Measurement Precipitation Radar (GPM-PR) data, which provides the three dimensional radar reflectivity factor (Ze), rain rate, drop size and droplet concentration. For each PCSs the surface wind properties are estimated using European Center for Medium-Range Weather Forecast Interim data. Based on the direction of surface flow the PCSs are classified into five categories. Over the South America the near surface wind flow transports the moisture from Amazon basin to east flank of Andes and validated here. The directional surface flow decides the occurrences of the PCSs, as upslope consists of the higher and larger PCSs at the peak of Andes compared to downslope flow. The directional flow suggests that northern Andes consists of pronounced bright band characteristics compared to southern Andes, and upslope and easterly flow have higher probability of rain at the eastern slope of Andes compared to westerly and downslope flow in northern Andes. The results show that orography also modulates the precipitation characteristics under different directional flow over and near the Andes. Eastern slope of Andes has higher rain rate compared to western slope of Andes in most of the directional flow. Orographically forced moisture loaded flow, over the eastern slope of Andes causing the higher rain rate, drop radius and droplet concentration in northern Andes. At the low lands, effective drop radius and droplet concentration show the opposite characteristics, and effective drop radius (concentration) is least (highest) except for the downslope flow over the northern Andes. The results of DSD parameters along with rainfall intensity show the microphysical evolution of the precipitation under the complex orography over the Andes mountain. The present study suggests that in future, surface flow must be considered for studying the orographic precipitation in numerical modeling.

What Is the Main Cause of Diurnal Variation and Nocturnal Peak of Summer Precipitation in Sichuan Basin, China? The Key Role of Boundary Layer Low‐Level Jet Inertial Oscillations

AbstractThe precipitation in Sichuan Basin (SB), China, exhibits pronounced diurnal variation, including minimum rainfall in daytime and a prominent peak near midnight. This study investigates the primary mechanism of precipitation diurnal variation in SB using forecasts from three summer months of 2013 produced at a 4‐km grid spacing. The model forecasts reproduce the observed spatial distributions and diurnal cycles well, including the peak precipitation in SB at around 02 local solar time (LST). Contrary to the common belief that emphasizes the solenoidal effects associated with the Tibetan and Yunnan‐Guizhou Plateaus, prominent diurnal inertial oscillations of boundary layer south‐southwesterly low‐level jet into SB are shown to play more important roles in modulating the diurnal cycles of precipitation in SB. A basinwide moisture budget analysis is performed to reveal that the moisture flux from the southeast side of the basin dominates within the diurnal oscillations of the net moisture flux into the basin, and the much enhanced nocturnal low‐level jet plays a crucial role in the formation of nocturnal precipitation within the basin. The net moisture flux into SB reaches maximum at around 22 LST, the time boundary layer perturbation winds from the daily mean in the direction normal to the southeastern boundary of SB reach maximum, which is about 4 hr before precipitation peak at around 02 LST. Shallow thermally forced nighttime downslope flows and daytime upslope flows on the Tibetan Plateau and Yunnan‐Guizhou Plateau slopes contribute only a small portion of moisture fluxes through the basin boundaries.

Characteristics of roll and cellular convection in a deep and wide semiarid valley: A large-eddy simulation study

The nature of organized motions within the convective boundary layer (CBL) depends on the surface heating and vertical wind shear. Moderate surface heating and strong shear favor horizontal convective rolls, while strong surface heating and weak shear favor open cellular convection. The transition between rolls and cells over land, particularly complex terrain, is still poorly understood. Due to lack of adequate measurements, variability of structural properties of rolls and cells (such as aspect ratio, wavelength and orientation relative to the mean surface winds) over complex terrain remains unclear. At present resolutions used in operational numerical weather prediction, rolls and cells are now becoming partly resolved, but the reliability of their forecasting in complex terrain remains elusive. The main objective of this research is the investigation of the characteristics of rolls and cells in a mountain valley compared to flat terrain.To address this objective, we use the Weather Research and Forecasting Model (WRF) in LES mode. The domains cover the Owens Valley, California, the site of the Terrain-Induced Rotor Experiment (T-REX). We show that LES adequately simulated the complex valley-slope flow observed from flux towers and a wind profiler. Additionally, the vertical profiles of higher-order moments agree with those of textbook CBL behaviour. We found the transitional regime between rolls and cells to be broader compared to flat terrain when considering the bulk CBL stability parameter −zi/L. The width of the transitional regime depends on the temporal trend of −zi/L and the along-valley flow direction. Structural parameters of rolls and cells were different relative to their flat terrain counterparts, and a valley environment tends to introduce additional linearity into convection organization. Our results do not indicate a relationship between different convection modes and upslope flow strength.

Mountain Volume Control on Deep-Convective Rain Amount during Episodes of Weak Synoptic Forcing

AbstractThermally driven upslope flows in mountainous areas provide favorable conditions for diurnal deep moist convection especially during episodes of weak synoptic forcing. The present study investigates the response of deep convection to axisymmetric orography as a function of orographic width and height by running ensembles of idealized convection-resolving simulations with a horizontal grid spacing of Δx = 1 km, full-physics parameterizations, and an interactive land surface. Deep convection is explicitly resolved and not parameterized. To cover a wide range of orographic scales, simulations are conducted with heights between 250 and 4000 m and widths between 5 and 30 km. The mountain slope strongly affects upslope wind speed characteristics, the timing and intensity of local updrafts, and local rain intensity. Although the day-to-day variability is substantial, the statistical-mean rain amount extracted by the mountain scales almost linearly with the mountain volume. Simulations with alternative mountain geometries, multiple peaks, and large-scale flow suggest that the linear scaling is valid for a surprisingly large portion of the parameter space. The scaling breaks down in the limit of relatively strong large-scale flows, sufficiently tall mountains, or elongated mountains. The existence of the simple linear scaling over such a wide range of configurations suggests that the response of thermally driven orographic deep convection over many mountainous areas is strongly affected by mountain volume. As a consequence, the rain amount is disproportionally dominated by the large horizontal scales of orography, as they contribute mostly to the mountain volume.

Multiscale characteristics of an extreme precipitation event over Nepal

This study focuses on the analysis of the extreme precipitation event in Central Nepal on 19 July 2007 which was part of a sequence of rain events leading to the devastating South Asia flood of 2007. We investigate synoptic‐scale conditions using reanalyses and attribute moisture sources with a Lagrangian moisture source diagnostic. Further, we characterize the mesoscale precipitation event with a high‐resolution numerical simulation. The simulation reveals an intense wide convective event with a simulated 40 dBZ echo core of considerable horizontal extent (1,550 km 2) exceeding a height of 12 km. Initially small convective cells were invigorated by high CAPE and a potentially unstable layer at mid‐tropospheric levels. This layer reached conditional instability adding latent energy to the system. Isolated convective cells organized upscale into a wide intense convective system, fuelled with moist low‐level inflow. The result was torrential rain with over 250 mm within 24 hr. Several synoptic‐scale conditions contributed to the intense development: (a) supply of moist air with the help of a typical monsoon break condition flow pattern, (b) anomalously significant moisture sources along this path due to prior precipitation events, (c) an upper‐tropospheric trough orienting the atmospheric flow against the Himalayas with associated quasi‐geostrophic forcing creating a favourable environment for convection, and (d) destabilized stratification due to an upslope flow. This analysis encompasses multiple scales and shows how a wide intense convective system, not unusual for this region, can be intensified by distinct synoptic constituents.

Obtaining turbulence statistics of thermally driven anabatic flow by sonic-hot-film combo anemometer

Achieving a better understanding of atmospheric boundary layer flows, and of slope flows in particular, is of paramount importance for the research of climate processes and production of accurate weather forecasts. We addressed the need for high resolution statistics that describe the turbulence of thermally driven anabatic (upslope) flows by the implementation of a novel sonic-hot-film anemometer, the combo probe. A field experiment was staged to obtain continuous 8-day measurements of a thermally driven anabatic flow diurnal cycle on a moderate slope by a single combo probe mounted atop a 2 m high mast. Variations of the mean and fluctuating upslope velocity field components and temperature exhibit a strong correlation of the developing flow with the diurnal solar heating cycle. The provided detailed analysis of turbulence statistics includes characteristic length scales and spectra of velocity fluctuations almost up to the Kolmogorov scale. Identification of spectral shape similarity led to the introduction of a suitable normalization and comparison of the results to a theoretical model. Additionally, empirical fits of several parameters are produced and discussed with respect to variations of thermal forcing that were derived in terms of bulk temperature differences and buoyancy fluxes up the slope. The main outcomes are spectra resolved down to small scales and turbulence statistics made available for numerical simulations and future studies with slow instruments.

A composite analysis of the morning cyclone in two Asian deserts

In some areas of the world, there are certain patterns of airflow which are created as a result of the interaction between different environmental factors. This study is trying to introduce one of these specific patterns that emerges particularly in closed and semi-closed deserts, especially in the arid and semi-arid areas. This phenomenon is highly influenced by the mountain-plain wind systems and that is defined as “morning cyclone.” In addition to “NOAA/AVHRR” satellite images, the ERA-interim reanalysis data are used as the main data in this study. The morning cyclone has been studied in the Taklamakan Desert in the northwest of China, and the Margo Desert in Afghanistan. The common feature of these two deserts is being surrounded by high mountains. This feature, alongside with some climatic characteristics such as air aridity, influences the formation of morning cyclones. The results of this study demonstrate the formation mechanisms of these cyclones. According to these mechanisms, morning cyclones form when the sun rises in the morning, the land surface temperature increases, and the mixed layer grows in mountain-surrounded deserts, while the mountain-plain winds (Katabatic winds) are blowing. As long as katabatic winds are blowing and warming trend continues across the deserts, the morning cyclones continue to exist. However, approaching noon, anabatic winds dominate over the lands and the morning cyclone disappears quickly. In other words, the morning cyclone has a lifetime spanning the hours between sunrise and pre-noon.

Characterizing the Synoptic Expression of the Angola Low

The Angola low is a key feature of the southern Africa wet season atmosphere that influences precipitation across the continent. This paper uses ERA-Interim to show that the synoptic expression of the Angola low is a combination of dry heat lows and moist tropical low pressure systems. The Angola heat low and Angola tropical low composites are contrasted against similar lows observed in other continental tropical regions and found to be broadly comparable. The implications that the distinction between dry and moist events has for the interannual relationship among the Angola low, precipitation, and ENSO are examined. The tropical lows exhibit unusual semistationary behavior by lingering in the Angola region rather than traveling offshore. This behavior is proposed to be caused by an integrated sea breeze–anabatic wind that enhances (inhibits) cyclonic vorticity stretching and convection inland (near the coast). The combined effect of the heat lows and the anchored tropical lows creates the Angola low in the climatological average. By elucidating the mechanisms of the Angola low, this research improves the foundation of process-based evaluation of southern Africa present and future climate in CMIP and AMIP models.

The Impact of Mount Washington on the Height of the Boundary Layer and the Vertical Structure of Temperature and Moisture

Discrimination of the type of air mass along mountain slopes can be a challenge and is not commonly performed, but is critical for identifying factors responsible for influencing montane weather, climate, and air quality. A field campaign to measure air mass type and transitions on the summit of Mount Washington, New Hampshire, USA was performed on 19 August 2016. Meteorological observations were taken at the summit and at several sites along the east and west slopes. Ozone concentrations were measured at the summit and on the valley floor. Additionally, water vapor stable isotopes were measured from a truck that drove up and down the Mount Washington Auto Road concurrent with radiosonde launches that profiled the free atmosphere. This multivariate perspective revealed thermal, moisture, and air mass height differences among the free atmosphere, leeward, and windward mountain slopes. Both thermally and mechanically forced upslope flows helped shape these differences by altering the height of the boundary layer with respect to the mountain surface. Recommendations for measurement strategies hoping to develop accurate observational climatologies of air mass exposure in complex terrain are discussed and will be important for evaluating elevation-dependent warming and improving forecasting for weather and air quality.

Turbulent mixing on sloping bottom of an energetic tidal channel

Measurements of turbulence dissipation, current, and stratification in an energetic, sloping tidal channel, the Penghu Channel (PHC), in the Taiwan Strait were conducted to investigate temporal variations of turbulence properties in the bottom boundary layer (BBL) under different stratified conditions. It was found that the PHC exhibits a unique feature of semidiurnal cycle of turbulence in the BBL due to the fact that current speeds during the flood are much higher than those during the ebb. Turbulent mixing in the BBL, produced mainly by the tidal current shear, has high values of dissipation (~10−5 W Kg−1) and eddy diffusivity and extends upward to approximately 40 m above the bottom during the flood. During the flood upslope flow, significant temperature drop and destratification of the near-bottom layer occur due to turbulence mixing associated with the shear instabilities, confirmed by the gradient Richardson number less than the critical value of 1/4. By contrast, stratification produced during the ebb is discernible only in the upper part of the BBL above the mixed layer. The stratification is weak (strong) during enhanced (suppressed) turbulence. The observed dissipation rate of turbulent kinetic energy is proportional to the cubic power of current speed, suggesting that the observed turbulence is generated via the boundary layer shear instability.