Relationship of Synoptic Winds and Complex Terrain Flows during the MISTRAL Field Experiment

The objective of this study is to describe the seasonal differences in SO2 ground-level fumigations from a power plant situated on very complex terrain in the Iberian Peninsula within the Western Mediterranean Basin (WMB). The study area extends more than 80 km around the power plant on very complex semi-arid terrain. Considering different plume-rise schemes, by experimentation and modelling this study attempts to characterise the seasonal differences in both the plume footprint 80 km around the power plant and the turbulent regime (diurnal or nocturnal) driving the main contribution to the accumulated plume footprints at different distances from the power plant within a complex terrain region. Two markedly different SO2 ground-level distributions around the power plant are presented for the typical summer and winter dispersive scenarios in the area. Simulations show that the SO2 footprint of a plume being advected more than 450 m above ground level in complex terrain is highly dependent on the prevailing meteorological conditions and on the mesoscale perturbations of the synoptic flows within the lower layers of the troposphere. The results obtained show how on complex terrain, despite seasonal meteorological differences and under stable dispersive conditions, the simulated mechanical turbulence leeward of the mountain ranges reproduces highly concentrated SO2 fumigations on the ground more than 50 km away from the power plant. Besides, under summer convective activity, plume fumigations have been successfully simulated less than 15 km from the power plant. In conclusion, this study shows how measurements from air quality networks together with information obtained from atmospheric transport and diffusion models are able to characterise different transport scenarios. This is a clear advantage for the end-users and decision-makers who manage and optimise the regional air quality networks.

<strong class="journal-contentHeaderColor">Abstract.</strong> Aerosol particle optical, physical and chemical properties have been previously studied in the United Arab Emirates (UAE), but there is still a gap in the knowledge of particle sources, and in the horizontal and vertical transport of aerosol particles and their precursors in the area. To investigate how aerosol particle and SO₂ concentrations at the surface responded to changes in horizontal and vertical transport, we used data from a one-year measurement campaign at a background site where local sources of SO₂ where expected to be minimal. The measurement campaign provided a combination of in-situ measurements at the surface, and the boundary layer evolution from vertical and horizontal wind profiles measured by a Doppler lidar. The diurnal structure of the boundary layer in the UAE was very similar from day to day, with deep well-mixed boundary layer during the day transitioning to a shallow nocturnal layer, with the maximum boundary layer height usually being reached around 1400 local time. Both SO₂ and nucleation mode aerosol particle concentrations were elevated for surface winds coming from the east or western sectors. We attribute this to oil refineries located on the eastern and western coasts of the UAE. The concentrations of larger cloud condensation nuclei (CCN) sized particles and their activation fraction did not show any clear dependence on wind direction, but the CCN number concentration showed some dependence on wind speed, with higher concentrations coinciding with the weakest surface winds. Peaks in SO₂ concentrations were also observed despite low surface wind speeds and wind directions unfavourable for transport. However, winds aloft were much stronger, with wind speeds of 10 m s^-1 at 1 km common at night, and with wind directions favourable for transport, and surface-measured concentrations increased rapidly once these particular layers started to be entrained into the growing boundary layer, even if the surface wind direction was from a clean sector. These conditions also displayed higher nucleation mode aerosol particle concentrations, i.e. new particle formation events occurring due to the increase in the gaseous precursor.

Composite relationships among outgoing longwave radiation, sea level pressure, surface winds, and upper tropospheric circulation are examined for northern winter during subseasonal episodes of eastward progression of convection from the Indian Ocean to the western Pacific. This evolution often culminates with westerly wind burst events and strong air‐sea interaction associated with regional‐scale convective blowups in the western equatorial Pacific. We first document some of these interactions in the composites for two timescales, the submonthly (6–30 days) and that of the Madden‐Julian Oscillation (MJO) timescale (30–70 days). We then analyze the December 1992 period during the Tropical Ocean Global Atmosphere Coupled Ocean‐Atmosphere Response Experiment (TOGA COARE) to illustrate how these composite relationships are manifested in a case study. Convection in the Indian Ocean for the composites is shown to be associated with a northern hemisphere wave train at 200 mbar, arcing through the midlatitudes, that can contribute to convective blowups farther east on the submonthly (6–30 days) timescale in the Intertropical Convergence Zone (ITCZ) in the eastern Pacific. The eastern Asian trough that is part of this wave train is associated with pressure surges from the northern hemisphere and subsequent convection over Southeast Asia. As the MJO convective envelope moves east to Australasia, midlatitude wave trains in either hemisphere include upper level troughs east of Asia and Australia and pressure surges from either hemisphere that contribute to pressure rises over the Indonesian region and a subsequent shift of the convective envelope to the western Pacific. The vertical wind structure for the December 1992 case study is consistent with the composite surface and upper level winds and also shows strong vertical wind shear in the boundary layer, a sharply defined westerly maximum near 700 mbar and an intensification of the upper level easterlies near 100 mbar. Very deep westerlies (to 200 mbar) are confined to shorter timescales. The case study illustrates the various time and space scale interactions noted in the composites. Reciprocal interactions between the tropics and the midlatitudes on the submonthly and MJO timescales in both the composites and the case study involve pressure surges and wave interaction that influence subsequent convection as the convective envelope migrates eastward from the tropical Indian to Pacific Ocean region.

Titan&amp;#8217;s climate includes dynamic processes at multiple scales, ranging from local turbulence to global super-rotation. Several mesoscale models are currently being developed which will permit simulations of surface winds at kilometric resolutions. However, mesoscale modeling requires high-resolution topography as an input, as it can be one of the primary controls on surface wind directions and amplitudes. The Cassini-Huygens mission, which spent 13 years in orbit around Saturn, mapped Titan&amp;#8217;s surface at resolutions up to about 300 m/pixels with its RADAR instrument, which operated at a wavelength of 2.17 cm (13.78 GHz frequency, Ku-band). Titan&amp;#8217;s best topography maps (Corlies et al., 2017) consist in a combination of radar altimetry, the SARTopo dataset (Stiles et al., 2009), and SAR stereoradargrammetry, interpolated where data is unavailable. The available topographic information thus has low spatial resolution, unequal coverage, and very variable accuracy (e.g., Fig. 1), rendering it unusable as an input for mesoscale modeling. In the absence of better topography data for the foreseeable future, we create fully parametrized synthetic topography, based on the surface geomorphology and the available topography. A similar method is used to create synthetic maps of the surface albedo, aerodynamic roughness length, and thermal inertia, whose influence on surface winds can thus be tested.In order to study (1) the surface atmospheric and sedimentary environment at the Dragonfly landing site and (2) the role of surface liquids in mesoscale atmospheric circulation, we focus on two regions: a 1000-km-wide region centered on the Dragonfly landing site near Selk crater, and a 250-km-wide area in the northern small lakes region. We first create a geomorphological map based on Cassini Synthetic Aperture Radar (SAR) images (Fig. 1), simpler than previous geomorphological maps of these regions (Malaska et al., 2016; Birch et al., 2017) in order to minimize the number of parameters. Each geomorphological unit (dune fields, mountains, plains, craters, lakes, valleys&amp;#8230;) is given a fully parametrized shape, and a random surface topographic roughness is added everywhere (except on the smooth lakes). Reasonable values of the parameters are chosen from existing topography data, resulting in maps such as the one in Fig. 2. The same method is applied to create synthetic maps of the thermal inertia, surface albedo, and aerodynamic roughness z0: each unit is assigned a value chosen from the literature.The resulting topography, albedo, thermal inertia, and roughness maps are simplified and assume that all features of a unit are identical: these maps therefore look reasonable, but are non-unique and cannot match well with the observations. However, they have the great advantage of being parametrized, which allows us to vary each parameter and test its influence on mesoscale model outputs such as surface winds. Some key questions that we are seeking to answer include: At what height do crater rims and mountains on Titan start to significantly influence surface winds? How do the winds in a realistic lakes region compare to those over an idealized circular lake? Can topography cause wind patterns responsible for the dune morphologies observed on Titan, including cross-hatched dunes at the Dragonfly landing site?In the equatorial region, which is a dry, desert environment, we use the mesoscale model developed by Lef&amp;#232;vre et al. (abstract in this session), which couples the Weather-Research Forecast (WRF) nonhydrostatic dynamical core (Skamarock et al., 2008) with the LMD Titan PCM physics package (Lebonnois et al., 2012). Meanwhile, in the North polar small lakes region, we use the mesoscale Titan WRF, which reproduces the methane cycle, and can therefore model the evaporation and transport of methane (Chatain et al., abstract in this session).While other presentations at EPSC (Chatain et al., Lef&amp;#232;vre et al.) will focus on the mesoscale modeling efforts, including the effect of topography on the model, we will detail the synthetic topography model and present preliminary results on the relationship between surface winds and dune morphology for a &amp;#8220;best guess&amp;#8221; topography. In particular, we use the surface winds output by the mesoscale model in the equatorial regions as inputs to the IPGP dune morphodynamics model (Narteau et al., 2009) to attempt to reproduce the reticulated dunes observed around Selk crater (Malaska et al., 2016).&amp;#160;Figure 1: Mapping the Selk crater region from Cassini SAR and topographyFigure 2: A synthetic topography map of the Selk crater region. Altitude is given in meters.&amp;#160;

A new wake model and comparison of eight algorithms for layout optimization of wind farms in complex terrain

The variability and predictability of the surface wind field at the regional scale is explored over a complex terrain region in the northeastern Iberian Peninsula by means of a downscaling technique based on Canonical Correlation Analysis. More than a decade of observations (1992–2005) allows for calibrating and validating a statistical method that elicits the main associations between the large scale atmospheric circulation over the North Atlantic and Mediterranean areas and the regional wind field. In an initial step the downscaling model is designed by selecting parameter values from practise. To a large extent, the variability of the wind at monthly timescales is found to be governed by the large scale circulation modulated by the particular orographic features of the area. The sensitivity of the downscaling methodology to the selection of the model parameter values is explored, in a second step, by performing a systematic sampling of the parameters space, avoiding a heuristic selection. This provides a metric for the uncertainty associated with the various possible model configurations. The uncertainties associated with the model configuration are considerably dependent on the spatial variability of the wind. While the sampling of the parameters space in the model set up moderately impact estimations during the calibration period, the regional wind variability is very sensitive to the parameters selection at longer timescales. This fact illustrates that downscaling exercises based on a single configuration of parameters should be interpreted with extreme caution. The downscaling model is used to extend the estimations several centuries to the past using long datasets of sea level pressure, thereby illustrating the large temporal variability of the regional wind field from interannual to multicentennial timescales. The analysis does not evidence long term trends throughout the twentieth century, however anomalous episodes of high/low wind speeds are identified.

To examine spatial and temporal scales of katabatic flow, a distributed temperature sensing (DTS) optical fiber was deployed 2 km down a mild slope irregularly interrupted by small-scale drainage features as part of the Mountain Terrain Atmospheric Modeling and Observation (MATERHORN) experiment conducted at the U.S. Army Dugway Proving Ground, Utah. The fiber was suspended at two heights near the surface, enabling measurement of variations in lapse rate near the surface at meter-scale spatial resolution with 1-min temporal resolution. Experimental results derived from the DTS and tower-mounted instrumentation indicate that airflow through small-scale drainage features regulated the local cooling rate whereas topographic slope and distance along the drainage strongly influenced the larger-scale cooling rate. Empirical results indicate that local cooling rate decays exponentially after local sunset and basin-wide cooling rate decreases linearly with time. The difference in the functional form for cooling rate between local and basin-wide scales suggests that small-scale features have faster timescales that manifests most strongly shortly after local sunset. More generally, partitioning drainage flow by scale provides insight and a methodology for improved understanding of drainage flow in complex terrain.

Improving ozone modeling in regions of complex terrain using observational nudging in a prognostic meteorological model

Abstract. Due to the stability of the boundary-layer stratification, high-latitude winds over complex terrain are strongly affected by blocking and channelling effects. Consequently, at many low-lying communities in the Canadian Archipelago, including Cape Dorset and Iqaluit considered in this study, surface winds for the most part are from two diametrically opposed directions, following the orientation of the elevated terrain. Shifts between the two prevailing wind directions can be sudden and are associated with geostrophic wind directions within a well defined narrow range. To quantitatively investigate the role of large-scale pressure gradients and the quasi-geostrophic overlying flow, an idealised dynamical system for the evolution of channelled surface winds is derived from the basic equations of motion, in which stability of stationary along-channel wind directions is described as a function of the geostrophic wind. In comparison with long-term horizontal wind statistics at the two locations it is shown that the climatologically prevailing wind directions can be identified as stationary states of the idealised wind model, and that shifts between prevailing wind directions can be represented as stability transitions between these stationary states. In that sense, the prevailing local wind conditions can be interpreted as attracting states of the actual flow, with observed surface winds adjusting to a new stable direction as determined by the idealised system within 3–9 h. Over these time-scales and longer it is therefore advantageous to determine the relatively slow evolution of the observationally well-resolved large-scale pressure distribution, instead of modelling highly variable surface winds directly. The simplified model also offers a tool for dynamical downscaling of global climate simulations, and for determining future scenarios for local prevailing wind conditions. In particular, it allows an estimation of the sensitivity of local low-level winds to changes in the large-scale atmospheric circulation.

This chapter describes the application of a variety of remote sensors to complex terrain studies, including sodars, lidars (aerosol mapping and Doppler), clear-air radars and wind profilers, and optical crosswind sensors. It outlines sensor characteristics relevant to operation in complex terrain settings and examines the origin of turbulence microstructure in complex terrain flows, a significant factor in the use of clear-air radars and sodars. It also summarizes the major programs that have made extensive use of remote sensors and promoted their integration with conventional in-situ instruments. Examples and case studies from these programs are then provided including observations of the initiation and destruction of drainage winds. The behavior of quasi-steady drainage winds and their irregularities during marine layer intrusions, seiches, waves, and strong ambient winds are also described. Recent results are presented that extend the use of Doppler lidar in channeled and merging valley flows as well as in the observation of large-scale mountain-plain circulations. The integration of remote and in-situ sensors in a major transport and dispersion study yields further examples of how different measurement methods can complement each other. Future directions in the study of complex terrain processes are suggested that can take advantage of newly developing remote sensors especially in application to larger-scale mountain-valley circulations.

Variability in surface winds at subseasonal time scales can affect the estimates of the mean and seasonal stress over the ocean, owing to the nonlinear dependence of stress on wind speed. A global ocean wind product that merges the European Centre for Medium-Range Weather Forecasts (ECMWF) fields with satellite and in situ data is used to assess the nonlinear effects of wind variability, in particular synoptic signals (taken here to include all periods of <6 days), on estimates of the mean and the seasonal cycle in zonal and meridional stress. Climatologies based on the period March 1988–February 1999 are considered. Synoptic effects are most pronounced at mid- and high latitudes, where they can amount up to 20% of the mean or seasonal stress. Uncertainties in stress values associated with synoptic wind errors are assessed by comparing estimates from merged-data winds to those from original ECMWF winds. Differences in synoptic winds contribute noticeably to the stress differences. Outside the Tro...

The representation of precipitation variability over mountainous regions by ground-based sensors is an open problem in hydrometeorological applications that necessitates the use of satellite-based precipitation products (SPPs). An extended network of ground-based X-band radar (GR) deployments over complex terrain areas, including the northeastern Italian Alps, North Carolina, Olympic Mountain, and the southern tip of Vancouver Island, is used in this study as a benchmark rainfall data set for error characterization and modeling of Level 2 PMW retrievals (Goddard profiling (GPROF) V05 algorithm) for the different sensors: the Microwave Humidity Sounder (MHS), the Special Sensor Microwave Imager/Sounder (SSMIS), the Global Precipitation Measurement Microwave Imager (GMI), and the Advanced Microwave Scanning Radiometer 2 (AMSR2). Matchups of Level 2 PMW/GR rainfall are extracted based on a matching methodology that identifies GR volume scans with PMW overpasses, and scales GR parameters to the satellite products’ nominal spatial resolution. The error model is the nonparametric machine learning tree-based quantile regression forest (QRF), which we developed using matchups of PMW/GR rainfall data from the different study areas. Validation of the error model is conducted using three cross-validation techniques: the k-fold, leave-one region out, and enforced. All validations showed that the error model-based corrections can significantly reduce both the mean relative error and the random component of PMW products. Moreover, the error reduction demonstrated with the leave-one region out cross-validation technique indicated that the error model is transferable among complex terrain regions. Algorithm developers may find this error model useful to integrate in the Level 3 products.

Characterization of the transport and dispersion of pollutants in a narrow mountain valley region by means of an atmospheric tracer

A preliminary geophysical model function, relating the sea surface brightness temperatures to ocean surface wind speed and direction, was developed using the data acquired at 45/spl deg/, 55/spl deg/, and 65/spl deg/ incidence angles by Jet Propulsion Laboratory's (JPL) aircraft 19- and 37-GHz polarimetric radiometers in 1994 and 1995. Radiometric temperatures from all polarization channels under cloud-free conditions showed clear dependence on surface wind direction. When there were stratus or scattered clouds, T/sub /spl nu// and T/sub h/ were significantly influenced by the radiation from cloud water, but the polarimetric channel U was found to be insensitive to clouds. The Fourier harmonic coefficients of the wind direction signals were derived from experimental data and related to the wind speed and direction, incidence angle and frequency. In general, all harmonic coefficients increase from low to moderate wind speeds, except the sin 2/spl phi/ component of U at 65/spl deg/ incidence, which peaked at low winds with a peak-to-peak amplitude of 0.6 to 1 Kelvin at about 3 m/s winds. At moderate wind speeds, 45/spl deg/ incidence angle exhibits larger second harmonic signals, but smaller first harmonic signals, than higher incidence angles. Wind direction signals were similar in 19 and 37 GHz channels, but the 37 GHz channel showed a slightly stronger wind direction sensitivity than the 19 GHz channel. The results suggest promising applications of passive microwave radiometers to ocean wind vector measurements.

A climatological and numerical study of “Karakkaze,” a type of local wind in Japan, was conducted. First, winter days under a winter-type synoptic pressure pattern with daily minimum relative humidity of less than 40% were classified according to strong wind (wind speed ≥ 9 ms-1, Karakkaze day), medium wind (6 ms-1 ≤ wind speed -1), and weak wind (wind speed -1). Secondly, the spatial patterns of the surface wind in each category are confirmed by the Japan Meteorological Agency (JMA)-Automated Meteorological Data Acquisition System (AMeDAS) observation data. In addition, we compared the boundary-layer wind of the three categories using wind speed data from the observation tower of the Meteorological Research Institute (MRI) in Tsukuba and from the JMA wind profiler in Kumagaya. Finally, we performed one-dimensional numerical experiments using a column Planetary Boundary Layer (PBL) model to evaluate the impact of solar radiation and upper-level wind on the formation of the Karakkaze. The results are summarized as follows. On the strong-wind days, strong northwesterly winds appear in the area along the Arakawa River and the Tonegawa River from Maebashi. The surface wind speed has a clear diurnal variation with a peak in the early afternoon. Such a diurnal variation is observed up to a 200 m level, but this diurnal pattern nearly reverses itself between 200 m and 400 m levels. On weak-wind days, the diurnal variation pattern is similar to that in other two categories, but the reversed pattern appears at a 100 m level, not at a 200 m level. Stronger surface winds appear under a clearer winter-type pressure pattern. The correlation coefficient is 0.632 between the daily maximum surface wind speed and the daily mean wind speed at a height of 2759 m, whereas the correlation is 0.284 between the surface wind speed and daily sunshine duration, which, in a previous study, was suggested to be the most significant factor. Numerical experiments indicate that a large amount of solar radiation is a necessary condition for a strong daytime wind, but this is not a sufficient explanation for the difference in the surface wind speed between weak and strong-wind days.