Evening Transition Period Research Articles

An Application of the Maximum Entropy Production Method in the WRF Noah Land Surface Model

AbstractThe calculation of surface sensible heat (SH) and latent heat (LE) fluxes using the bulk transfer models for complex terrains, tall vegetation regions, and morning and evening transition periods remains a challenging problem in numerical weather and climate models. The maximum entropy production (MEP) model, a new method of calculating surface heat fluxes, is coupled with the Noah land surface model (LSM) in the Weather Research and Forecasting (WRF) model. Surface heat fluxes and meteorological variables including air temperature, relative humidity, soil temperature, and precipitation data at nine intensive observation stations and 453 routine meteorological operational observation stations in the Tibetan Plateau (TP) from 1 June to 31 August 2015 are used to evaluate the coupled model. The results show that the MEP method improves the nonclosure of the surface energy balance in the WRF (with an energy residual from 24.3 to 1.9 W m−2), reducing the overestimations of SH and LE and the underestimation of the surface ground heat flux over the complex terrain of TP (with the bias decreases of 50.6% and 117.1% for SH and LE in turn), especially during the daytime. The improved SH and LE reduce the cold and wet biases in the TP by 29.4% and 51.7%, respectively. The MEP model also reduces the overestimated soil temperature by 63%. Moreover, the overestimated daily precipitation is reduced by 3% over the TP. The successful application of the MEP method demonstrates its advantage over the bulk transfer method, providing a new approach for reducing the overestimation of surface heat fluxes and wet and cold biases in numerical forecast models in complex terrains.

A Simulation Study on Risks to Wind Turbine Arrays from Thunderstorm Downbursts in Different Atmospheric Stability Conditions

Thunderstorm downbursts have been reported to cause damage or failure to wind turbine arrays. We extend a large-eddy simulation model used in previous work to generate downburst-related inflow fields with a view toward defining correlated wind fields that all turbines in an array would experience together during a downburst. We are also interested in establishing what role contrasting atmospheric stability conditions can play on the structural demands on the turbines. This interest is because the evening transition period, when thunderstorms are most common, is also when there is generally acknowledged time-varying stability in the atmospheric boundary layer. Our results reveal that the structure of a downburst’s ring vortices and dissipation of its outflow play important roles in the separate inflow fields for turbines located at different parts of the array; these effects vary with stability. Interacting with the ambient winds, the outflow of a downburst is found to have greater impacts in an “average” sense on structural loads for turbines farther from the touchdown center in the stable cases. Worst-case analyses show that the largest extreme loads, although somewhat dependent on the specific structural load variable considered, depend on the location of the turbine and on the prevailing atmospheric stability. The results of our calculations show the highest simulated foreaft tower bending moment to be 85.4 MN-m, which occurs at a unit sited in the array farther from touchdown center of the downburst initiated in a stable boundary layer.

Large Eddy Simulation of Wind Turbine Wakes during the Evening Transition Period

A transient simulation of a two inline wind turbines during the evening transition period is modelled using Large Eddy Simulation with an actuator disc method. Two inline turbines are placed parallel to the stream-wise direction and the distance between the turbines is seven rotor diameters. The wind profiles and turbulence data are produced by a precursor simulation. The simulation was carried out for four physical hours and the surface heat flux varies with time according to the evening period. The wind profile evolves over the transition period and the atmospheric stability gradually changes from unstable in the late afternoon to a stable condition in the evening. The wake recovers more quickly in the afternoon and it recovers more slowly corresponding to the changes in atmospheric stability. This causes a reduction in the power output and the mean thrust load of the downstream turbine over the period. Additionally, the velocity spectrum displays higher fluctuations at the downstream turbine.

On wind turbine loads during the evening transition period

AbstractThe late afternoon hours in the diurnal cycle precede the development of the nocturnal stable boundary layer. This “evening transition” (ET) period is often when energy demand peaks. This period also corresponds to the time of day that is a precursor to late‐afternoon downbursts, a subject of separate interest. To capture physical characteristics of wind fields in the atmospheric boundary layer (ABL) during this ET period, particularly the interplay of shear and turbulence, stochastic simulation approaches, although more tractable, are not suitable. Large‐eddy simulation (LES), on the other hand, may be used to generate high‐resolution ABL turbulent flow fields. We present a suite of idealized LES four‐dimensional flow fields that define a database representing different combinations of large‐scale atmospheric conditions (characterized by associated geostrophic winds) and surface boundary conditions (characterized by surface heat fluxes). Our objective is to evaluate the performance of wind turbines during the ET period. Accordingly, we conduct a statistical analysis of turbine‐scale wind field variables. We then employ the database of these LES‐based inflow wind fields in aeroelastic simulations of a 5‐MW wind turbine. We discuss how turbine loads change as the ET period evolves. We also discuss maximum and fatigue loads on the rotor and tower resulting from different ABL conditions. Results of this study suggest that, during the ET period, the prevailing geostrophic wind speed affects the mean and variance of longitudinal winds greatly and thus has significant influence on all loads except the yaw moment which is less sensitive to uniform and symmetric incoming flow. On the other hand, surface heat flux levels affect vertical turbulence and wind shear more and, as a result, only affect maximum blade flapwise bending and tower fore‐aft bending loads.

Investigating the impact of atmospheric stability on thunderstorm outflow winds and turbulence

Abstract. Downburst events initialized at various hours during the evening transition (ET) period are simulated to determine the effects of ambient stability on the outflow of downburst winds. The simulations are performed using a pseudo-spectral large eddy simulation model at high resolution to capture both the large-scale flow and turbulence characteristics of downburst winds. First, a simulation of the ET is performed to generate realistic initial and boundary conditions for the subsequent downburst simulations. At each hour in the ET, an ensemble of downburst simulations is initialized separately from the ET simulation in which an elevated cooling source within the model domain generates negatively buoyant air to mimic downburst formation. The simulations show that while the stability regime changes, the ensemble mean of the peak wind speed remains fairly constant (between 35 and 38 m s−1) and occurs at the lowest model level for each simulation. However, there is a slight increase in intensity and decrease in the spread of the maximum outflow winds as stability increases as well as an increase in the duration over which these strongest winds persist. This appears to be due to the enhanced maintenance of the ring vortex that results from the low-level temperature inversion, increased ambient shear, and a lack of turbulence within the stable cases. Coherent turbulent kinetic energy and wavelet spectral analysis generally show increased energy in the convective cases and that energy increases across all scales as the downburst passes.

Investigation of Near-Storm Environments for Tornado Events and Warnings

Abstract In this study, a 13-yr climatology of tornado event and warning environments, including metrics of tornado intensity and storm morphology, is investigated with particular focus on the environments of tornadoes associated with quasi-linear convective systems and right-moving supercells. The regions of the environmental parameter space having poor warning performance in various geographical locations, as well as during different times of the day and year, are highlighted. Kernel density estimations of the tornado report and warning environments are produced for two parameter spaces: mixed-layer convective available potential energy (MLCAPE) versus 0–6-km vector shear magnitude (SHR6), and mixed-layer lifting condensation level (MLLCL) versus 0–1-km storm-relative helicity (SRH1). The warning performance is best in environments characteristic of severe convection (i.e., environments featuring large values of MLCAPE and SHR6). For tornadoes occurring during the early evening transition period, MLCAPE is maximized, MLLCL heights decrease, SHR6 and SRH1 increase, tornadoes rated as 2 or greater on the enhanced Fujita scale (EF2+) are most common, the probability of detection is relatively high, and false alarm ratios are relatively low. Overall, the parameter-space distributions of warnings and events are similar; at least in a broad sense, there is no systematic problem with forecasting that explains the high overall false alarm ratio, which instead seems to stem from the inability to know which storms in a given environment will be tornadic.

Atmospheric stability and diurnal patterns of aeolian saltation on the Llano Estacado

Abstract Aeolian transport is driven by aerodynamic surface stress imposed by turbulent winds in the Earth’s atmospheric boundary layer (ABL). ABL regime is influenced by stratification, which can either enhance or suppress production of turbulence by shear associated with the vertical gradient of streamwise velocity. During the day and night, surface heat fluxes induce a negative (unstable) and positive (stable) vertical gradient of potential temperature, respectively, which modifies the role of buoyancy in turbulence production. During the brief morning and evening transition periods, the vertical gradient of potential temperature vanishes (neutral stratification). The Monin–Obukhov similarity theory describes how the vertical gradient of streamwise velocity varies with stratification. Simultaneous field measurement of wind speed and aeolian activity were obtained over a 218-day period on a bare, sandy surface on the high plains of the Llano Estacado region of west Texas. Wind speed was measured at a height of 2 m with a propeller-type anemometer and aeolian activity was measured at the surface with a piezoelectric saltation sensor. We have used the wind speed measurements within the framework of the Monin–Obukhov similarity theory to estimate “typical” shear velocity, u ∗ , of the ABL as stratification is varied (characterized with the stability parameter). This approach results in a color flood contour of u ∗ against time of day and stability parameter: the procedure demonstrates that aeolian activity is most likely to occur during the day, when buoyancy acts in conjunction with mechanical shear to increase u ∗ .

Convective boundary layer evolution from lidar backscatter and its relationship with surface aerosol concentration at a location of a central China megacity

AbstractBased on the 1 min backscatter ratio R profiles from the all‐day lidar measurements in Wuhan, China (30.5°N, 114.4°E), hourly convective boundary layer (CBL) height was calculated with the variance method. The calculated CBL height sequence displays the regular diurnal cycle of the CBL top. The prevalent mixing process within the CBL is also revealed. During the CBL growth period, the backscatter ratio R falls visibly with increasing altitude and has large variance within the CBL, suggesting that more abundant aerosol particles from lower altitudes are being transported upward and being mixed with the local background or advected aerosol layers. During the CBL quasistationary period, R tends to be vertically uniform, and its variance reaches a daytime minimum within the CBL, indicating that the vertical homogenization of aerosol particles produced by the convectively driven mixing reaches its maximum. During the afternoon and early evening transition period, the vertical uniformity of R weakens and the variance enlarges again, implying that the reduced convectively driven mixing fails to maintain a high vertical homogeneity. When the 1 min R profiles were plotted together in terms of each 1 h interval, the fluctuating R curves at heights around the CBL top looked like a “node”, representing the structure of the entrainment zone between the CBL and the free troposphere. The moving node depicts the evolution of the entrainment zone. The diurnal variation of the CBL height shows an obvious seasonal dependence which coincides with the annual variation of the local surface temperature. The surface fine particle concentration generally has a more complex diurnal cycle than that expected from the CBL‐dilution/CBL‐accumulation effect. But, it shows a strong annual variation which is out of phase with respect to that of the monthly mean maximum CBL height. This tends to suggest that the seasonal behavior of the surface fine particle concentration mainly depends on the seasonal variation in available volume (determined by the CBL height) for aerosol dispersion.

Countergradient heat flux observations during the evening transition period

Abstract. Gradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several intensive observational periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower instrumented with high-speed temperature and velocity sensors. The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are ~ 30–80 min. The existence of the delay and its duration can be explained by considering the convective timescale and the competition of forces associated with the classical Rayleigh–Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turnover timescale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover timescale.

Exploring a geophysical process‐based attribution technique for the determination of the atmospheric boundary layer depth using aerosol lidar and near‐surface meteorological measurements

A new objective method for the determination of the atmospheric boundary layer (ABL) depth using routine vertically pointing aerosol lidar measurements is presented. A geophysical process‐based analysis is introduced to improve the attribution of the lidar‐derived aerosol gradients, which is so far the most challenging part in any gradient‐based technique. Using micrometeorological measurements of Obukhov length scale, both early morning and evening transition periods are determined which help separate the turbulence regimes during well‐mixed convective ABL and nocturnal/stable ABL. The lidar‐derived aerosol backscatter signal intensity is used to determine the hourly‐averaged vertical profiles of variance of the fluctuations of particle backscatter signal providing the location of maximum turbulent mixing within the ABL; thus, obtained mean ABL depth guides the attribution by searching for the appropriate minimum of the gradients. An empirical classification of the ABL stratification patterns into three different types is proposed by determining the changes in the near‐surface stability scenarios. First results using the lidar observations obtained between March and July in 2011 at SIRTA atmospheric observatory near Palaiseau (Paris suburb) in France demonstrate that the new attribution technique makes the lidar estimations of ABL depth more physically reliable under a wide spectrum of meteorological conditions. While comparing lidar and nearby radiosonde measurements of ABL depths, an excellent concordance was found with a correlation coefficient of 0.968 and 0.927 for daytime and nighttime measurements, respectively. A brief climatology of the characteristics of the ABL depth, its diurnal cycle, a detailed discussion of the morning and evening transitions are presented.

Flow and turbulence in an industrial/suburban roughness canopy

A field study conducted to investigate the flow and turbulence structure of the urban boundary layer (UBL) over an industrial/suburban area is described. The emphasis was on morning and evening transition periods, but some measurements covered the entire diurnal cycle. The data analysis incorporated the dependence of wind direction on morphometric parameters of the urban canopy. The measurements of heat and momentum fluxes showed the possibility of a constant flux layer above the height $$z\approx 2{H}$$ , wherein the Monin-Obukhov Similarity Theory (MOST) is valid; here $$H$$ is the averaged building height. For the nocturnal boundary layer, the mean velocity and temperature profiles obeyed classical MOST scaling up to $$\sim 0.5\Lambda \left( {\sim 6{H}}\right) $$ , where $$\Lambda $$ is the Obukhov length scale, beyond which stronger stratification may disrupt the occurrence of constant fluxes. For unstable and neutral cases, MOST scaling described the mean data well up to the maximum measured height $$(\sim 6{H})$$ . Available MOST functions, however, could not describe the measured turbulence structure, indicating the influence of additional governing parameters. Alternative turbulence parameterizations were tested, and some were found to perform well. Calculation of integral length scales for convective and neutral cases allowed a phenomenological description of eddy characteristics within and above the urban canopy layer. The development of a significant nocturnal surface inversion occurred only on certain days, for which a criterion was proposed. The nocturnal UBL exhibited length scale relationships consistent with the evening collapse of the convective boundary layer and maintenance of buoyancy-affected turbulence overnight. The length and velocity scales so identified are useful in parameterizing turbulent dispersion coefficients in different diurnal phases of the UBL.

An observational and numerical study of a regional‐scale downslope flow in northern Arizona

Boundary layer observations taken during the METCRAX field study in October of 2006 near Winslow in Northern Arizona revealed the frequent presence of a near‐surface wind maximum on nights with relatively quiescent synoptic conditions. Data from a sodar, a radar wind profiler, several surface stations, and frequent high‐resolution rawinsonde soundings were used to characterize this boundary layer wind phenomenon and its relation to synoptic conditions and the ambient environment. The data analyses are augmented by high‐resolution mesoscale numerical modeling. It is found that the observed nocturnal low‐level wind maximum is part of a regional‐scale downslope flow converging from high terrain of the Colorado Plateau toward the Little Colorado River Valley. The depth of this downslope flow is between 100 and 250 m with a peak speed of 4–6 m s−1occurring usually within the lowest 50 m above ground. Opposing ambient winds lead to a longer evening transition period, shallower slope flows, and a smaller horizontal extent as compared to supporting synoptic winds. A simple analytical solution based on local equilibrium appears to agree fairly well with the observed layer mean downslope wind speed, but the classic Prandtl solution for maximum downslope wind speed fails to match the observations. The properties of the flow appear to be insensitive to changes in soil moisture, land cover, and surface roughness length. The contribution to the low‐level wind maximum by inertial oscillation at night is found to be insignificant.

Linearly Organized Turbulence Structures Observed Over a Suburban Area by Dual-Doppler Lidar

Dual-Doppler lidar observations are used to investigate the structure and evolution of surface-layer flow over a suburban area. The observations were made during the Joint Urban 2003 (JU2003) field experiment in Oklahoma City, U.S.A. in the summer of 2003. This study focuses specifically on a 10-h sequence of scan data beginning shortly after noon local time on 7 July 2003. During this period two coherent Doppler lidars performed overlapping low elevation angle sector scans upwind and south of Oklahoma City’s central business district. Radial velocity data from the two lidars are processed to reveal the structure and evolution of the horizontal velocity field in the surface layer throughout the afternoon and during the evening transition period. The retrieved velocity fields clearly show a tendency for turbulence structures to be elongated in the direction of the mean flow throughout the entire 10-h study period. In order to quantify the observed anisotropy and its dependence on stability, integral length scales are estimated directly from the spatially resolved velocity retrievals. As the flow became more stably stratified the characteristic cross-stream dimension of the linear structures decreased. The streamwise component was consistently more anisotropic than the cross-stream component, and both velocity components exhibited maximum anisotropy under neutral conditions. The ratio of the streamwise to cross-stream length scale was estimated to be about eight for the streamwise component, and four for the cross-stream component under neutral conditions.

CO 2 transport over complex terrain

CO 2 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 CO 2-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 CO 2 advection dominated the total CO 2 transport in the NEE estimate at night although there are large uncertainties because of partial cancellation of horizontal and vertical advection. The horizontal CO 2 advection captured not only the CO 2 loss at night, but also the CO 2 uptake during daytime. We found that horizontal CO 2 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 CO 2 concentration is not well mixed by turbulence. The long-term net effect of all the CO 2 budget terms on estimates of NEE needs to be investigated.

The surface energy balance and boundary layer over urban street canyons

AbstractA model is developed for the energy balance of an urban area, represented as a sequence of two‐dimensional street canyons. The model incorporates a novel formulation for the sensible‐heat flux, that has previously been validated against wind tunnel models, and a formulation for radiation that includes multiple reflections and shadowing. This energy balance model is coupled to a model for the atmospheric boundary layer. Results are analysed to establish how the physical processes combine to produce the observed features of urban climate, and to establish the roles of building form and fabric on the urban modification to climate.Over a diurnal cycle there are morning and evening transition periods when the net flux of radiation is largely balanced by the flux of heat into the surface. The urban surface has a large surface area in contact with the air, and hence a large active heat capacity, and so the urban area needs to absorb a larger amount of heat than a rural area to change the surface temperature. The morning and evening transitions are therefore prolonged over urban areas, delaying the onset of convective or stable boundary layers after sunrise and sunset. The model shows that the energy balance of the roof behaves very differently from the combined energy balance of the street canyon system of walls and street. The sensible‐heat flux from the street canyon into the boundary layer is increased by the increased surface area, but is decreased by the buildings reducing the local flow speeds. The net result is that, for the two‐dimensional geometry investigated here, the sensible‐heat flux from the canyon is not strongly sensitive to canyon geometry. The sensible‐heat flux from the roof is larger than from the street, and so the total sensible‐heat flux into the boundary layer, and hence also the air temperature, is strongly dependent on the fraction of plan area occupied by roofs. The radiation budget of the street canyon, which largely drives the temperatures of the canyon surfaces, is significantly changed by the limited sky view and multiple reflections caused by the local building form. The canyon surface temperatures thus depend strongly on local building morphology.Finally, two mechanisms are suggested for how urban areas might maintain a positive sensible‐heat flux at night. Firstly, if the roof material has much lower heat capacity than the street canyon surfaces, then the roof can cool the boundary‐layer air faster than the street canyon surfaces cool, leading to a positive heat flux out of the street canyon. Secondly, advection decouples the boundary layer from the local surface energy balance. In this way, cool air, perhaps from a rural area, advected on to an urban surface can lead to a positive sensible‐heat flux which then tends to neutralize any stable stratification in the boundary layer. Copyright © 2006 Royal Meteorological Society

Impact of vertical atmospheric structure on Beijing aerosol distribution

In August 2004, original informations on Beijing aerosol layers were obtained downtown from experiments achieved during clear and polluted days along the IAP 325-m meteorological tower and a micro-pulse LIDAR operating at BMEMC. Beijing urban boundary layer (UBL) is seen to follow the expected diurnal pattern with low height values at night (80±50 m) and much higher values (up to 3000 m) at daytime. An original process of temperature profiles is shown to closely reconstruct the UBL and sub-layer evolution. UBL vertical extension becomes significant after a short transition period at 10:00 during which at all the tower levels, wind speed increases and wind direction changes dramatically from the synoptic northerly flow to almost the opposite direction (SSW). Additional data from background stations show pollution episodes that may be related to horizontal transport at low altitude during the morning or evening transition periods. Vertical profiles of black carbon (BC) and particle number concentration (N) and LIDAR signal evidence the formation of accumulation layers at 60 m and 90 m probably related to the urban canopy. Using the N/BC ratio, an important chemical reactivity may be observed within the accumulation layers and is still distinguishable during cloudy weather. Reactivity in the vertical column appears to increase with altitude, which might point to the significance of solar radiation shielding by particles. Chemical analyses confirm, in the morning, the presence of a very distinct layer at the top of the tower and show, in the afternoon, a significant mixing (17%) between the UBL and the regional flow above.

An Evaluation of Mesoscale Model Predictions of Down-Valley and Canyon Flows and Their Consequences Using Doppler Lidar Measurements during VTMX 2000

Abstract A mesoscale model, a Lagrangian particle dispersion model, and extensive Doppler lidar wind measurements during the Vertical Transport and Mixing (VTMX) 2000 field campaign were used to examine converging flows over the Salt Lake valley in Utah and their effect on vertical mixing at night and during the morning transition period. The simulated wind components were transformed into radial velocities to make a direct comparison with about 1.3 million Doppler lidar data points and to evaluate critically the spatial variations in the simulated wind fields aloft. The mesoscale model captured reasonably well the general features of the observed circulations, including the daytime up-valley flow; the nighttime slope, canyon, and down-valley flows; and the convergence of the flows over the valley. When there were errors in the simulated wind fields, they were usually associated with the timing, structure, or strength of specific flows. The simulated flow reversal during the evening transition period prod...

The influence of thermally driven circulation on PM 10 concentration in the Salt Lake Valley

Ground level and vertical measurements of aerosols (PM 10) and meteorological parameters in the Salt Lake Valley were conducted as a part of the Vertical Transport and MiXing in the atmospheric boundary layer (VTMX) experiment, conducted during the period 1–18 October 2000. This component of the VTMX experiment was directed toward developing a deeper understanding of the factors affecting the distribution and transport of pollutants in urban areas within complex terrain. The present study focused on the temporal (diurnal) and vertical variation of PM 10 concentration during the stable nighttime period as well as the morning and evening transition periods. The temporal variations in the aerosol concentration during these periods was dominated by thermally driven diurnal winds, with only minor influences from background synoptic winds. The increased aerosol concentration between sunrise and the afternoon was associated with the westerly up-slope winds that advected PM 10 from the nearest source, the downtown area of Salt Lake City. In mid-afternoon the PM 10 concentration decreased significantly due to the arrival of a northwesterly lake breeze near the ground. The lowest aerosol concentrations during the period from late afternoon to early morning are the result of the draining of cleaner air from the mountain slope after the wind shifted from westerly to easterly. The temporal variations in the wind direction and aerosol levels during the nighttime are discussed. A hypothesis is advanced, with some supporting evidence, that periodic variations of the nighttime PM 10 concentrations owe to the shedding of vortices from the Wasatch mountain range bordering the lake. The vertical distribution of aerosols is sensitive to the thermal forcing, which exerts the driving influence on thermal circulation within the Salt Lake Valley.

THE VTMX 2000 CAMPAIGN

A month-long meteorological field campaign sponsored by the Department of Energy's Environmental Meteorology Program was conducted during October 2000 in the Salt Lake Valley to study vertical transport and mixing (VTMX) processes. The goals of the program are to increase our understanding of these processes, to improve our ability to measure and characterize them, and to incorporate that improved knowledge into conceptual and numerical models that can be used to describe and predict them. The program is currently concentrating on nocturnal stable periods and morning and evening transition periods, and it is further focused on urban areas located in valleys, basins, or other settings affected by nearby elevated terrain. Approximately 75 people participated in the campaign. The campaign featured a wide range of remote sensing and in situ measurements, including those from six radar wind profilers, six sodars, five radio acoustic sounding systems, a Doppler lidar, two aerosol lidars, and a water vapor lidar, as many as 22 rawinsonde soundings per Intensive Observing Period (IOP), and the simultaneous release of up to seven perfluorocarbon tracers. Preliminary results show the existence of strong cold pools forming over the valley center with significant wind shear aloft and intermittent turbulence close to themore » surface, a heat island over the downtown area at night and areas with substantially cooler temperatures nearby, regions of strong convergence and divergence affected by a narrow jet through a gap in the mountains to the south and flows out of the canyons to the east, and extensive wave activity.« less

Diurnal rhythm of the bioluminescent field in the ocean epipelagic zone

Characteristics of the diurnal rhythm of the bioluminescence field are analyzed on the basis of experimental data obtained in the region around Kamchatka peninsula (50°31′N; 163°50′E). The feasibility of reconstructing the nocturnal structure of the bioluminescence field by measurements made at other times of the day has been demonstrated. Diurnal rhythm parameters for the bioluminescent field of several other studied areas of the ocean have been defined. The temporal parameters feature linear latitudinal dependence. At the equator the boundaries of morning and evening transition periods are 05:06 to 07:30 and 17:48 to 19:54 hrs, respectively. In the Barents Sea in spring these boundaries are 02:42 to 04:06 and 19:48 to 21:12 hrs. Boundary times of diurnal rhythm do not deviate from the linear regression by more than 10%.