Reduced Sea-Surface Roughness Length at a Coastal Site

Sea-surface roughness length is a key parameter for characterizing marine atmospheric boundary layer. Although aerodynamic roughness lengths for homogeneous land and open water surfaces have been examined extensively, the extension of relevant knowledge to the highly inhomogeneous coastal area is problematic due to the complex mechanisms controlling coastal meteorology. This study presented a lidar-based observational analysis of sea-surface roughness length at a coastal site in Hong Kong, in which the wind data recorded from March 2012 to November 2015 were considered and analyzed. The results indicated the turning of wind near the land-sea boundary, leading to a dominative wind direction parallel to the coastline and an acceleration in wind. Moreover, the roughness lengths corresponding to two representative azimuthal sectors were compared, in which the roughness lengths for the onshore wind sector (i.e., 120°–240°) appear to be larger than the constant value (z0 = 0.2 mm) recommended in much existing literature, whereas the values for the alongshore wind sector (i.e., 60°–90°) are significantly smaller, i.e., about two orders of magnitude less than that of a typical sea surface. However, it is to be noted that the effect of atmospheric stability, which is of crucial importance in governing the marine atmospheric boundary layer, is not taken into account in this study.

Investigating the Aerodynamic Surface Roughness Length over Baghdad City Utilizing Remote Sensing and GIS Techniques

This study calculated the surface roughness length (Zo), zero-displacement length (Zd) and height of the roughness elements (ZH) using GIS applications. The practical benefit of this study is to classify the development of Baghdad, choose the appropriate places for installing wind turbines, improve urban planning, find rates of turbulence, pollution and others. The surface roughness length (Zo) of Baghdad city was estimated based on the data of the wind speed obtained from an automatic weather station installed at Al-Mustansiriyah University, the data of the satellite images digital elevation model (DEM), and the digital surface model (DSM), utilizing Remote Sensing Techniques. The study area was divided into 15 municipalities (Rasheed, Mansour, Shulaa, Karrada, Shaab, Adhamiyah, Sadre 2, Sadre 1, Rusafa, Alghadeer, Baghdad Aljadeedah, Karkh, Kadhumiya, Green zone, and Dora). The results indicated that the variations in Zo depend strongly on zero-displacement length (Zd) and the roughness element height (ZH) and wind speed. The research results demonstrated that Baghdad Aljadeedah has the largest (Zo) with 0.43 m and Rasheed has the lowest value of (Zo) with 0.19 m.; the average (Zo) of Baghdad city was 0.32 m.

The Role of Sublimation on the Surface Mass Balance of the Interior Greenland Ice Sheet

<p>Precipitation along with sublimation and deposition are the main contributors to the surface mass balance (SMB) in the accumulation area of the Greenland Ice Sheet (GrIS). However, precipitation events are rare and intermittent. In between precipitation events the surface snow continuously undergoes sublimation and deposition. Recent studies imply that these surface exchange processes influence the isotopic composition of the surface snow which is later archived as a climate record in ice cores. In order to understand the possible implications on the recorded climate signal, sublimation needs to be quantified on a local scale.</p><p>Here we present simulated SMB components for eight ice core drilling sites on the GrIS using the regional climate model MAR (Modèle Atmosphérique Régional). We validated MAR against in-situ flux observations at the East Greenland Ice Core Project site and found a high sensitivity of sublimation to the downward long wave flux and to the parameterization of the surface roughness length. We propose a surface roughness length optimized for the interior of the GrIS which is supported by our observations.</p><p>Our results show that in the GrIS accumulation area the mass turnover via sublimation and deposition can reach the same order of magnitude as precipitation. This highlights the importance of a better understanding of how the climate signal is imprinted in the surface snow isotopic composition.</p>

Development of surface roughness length and drag parameterizations over deep seas

<p>Many formulations to determine the sea surface roughness length (z<sub>0</sub>) have been proposed in the past. The well-known Charnock’s equation is applied in most of the previous research. In this study, a different point of view is adopted to develop a new formulation. The starting point is an alternative method for surface roughness length calculation, i.e., the Lettau’s method. This method has already been validated onshore in the presence of obstacles over a domain; for obstacles with a defined cross-section perpendicular to the wind direction plane. Over deep waters, it is expected to find only one type of obstacle, i.e., consecutive waves forming straight lines. Different wave systems and the presence of swell add complexity to determine the sea surface profile. Hence, the adaptation of Lettau’s method seems reasonable, but the demonstrated dependency of z<sub>0</sub> to wave age cannot be neglected.</p><p>Wind-generated waves result from a kinetic energy transfer between the atmosphere and the sea surface. However this physical process is not represented in the well-known logarithmic law. While this effect can be neglected onshore, in offshore environments it can be significant, as 20% of the time z<sub>0</sub> is found to be over the expected range. Therefore, a kinetic energy transfer correction is included into an offshore logarithmic law. With an aerodynamic z<sub>0</sub>, achieved by the adaptation of the Lettau’s equation, and the new offshore logarithmic law, an empirical method for the kinetic energy transfer correction is proposed.</p>

Wind climate of the Melbourne metropolitan area

This paper describes a probabilistic analysis of data recorded by the Bureau of Meteorology (BoM) for the wind climate of the Melbourne metropolitan area. It is based on 10-minute average wind data from four automatic weather stations (AWS) ‒ at Melbourne and Essendon airports, Fawkner Beacon in Port Phillip Bay, and Moorabbin Airport. Corrections to the data were made to adjust to standard terrain conditions and height. For the land stations, these were based on estimates of the surface roughness length at each site as a function of wind direction, making use of recorded gust factors. For the Fawkner Beacon, which is completely surrounded by open water, the surface roughness length is a function of mean wind speed, and the Charnock relationship was used in determining the corrections. For each station the terrain-corrected wind data were fitted with Weibull probability distributions, as an all-direction group and for sixteen direction sectors. Directional probabilities were also determined. The parameters of the all-direction Weibull distributions are very similar for all four stations, but there are differences in directional probabilities for some directions, with a geographic trend from north to south in the region being apparent. Some possible explanations based on the general topography are given.

The making of the New European Wind Atlas – Part 1: Model sensitivity

This is the first of two papers that document the creation of the New European Wind Atlas (NEWA). It describes the sensitivity analysis and evaluation procedures that formed the basis for choosing the final setup of the mesoscale model simulations of the wind atlas. The suitable combination of model setup and parameterizations, bound by practical constraints, was found for simulating the climatology of the wind field at turbine-relevant heights with the Weather Research and Forecasting (WRF) model. Initial WRF model sensitivity experiments compared the wind climate generated by using two commonly used planetary boundary layer schemes and were carried out over several regions in Europe. They confirmed that the most significant differences in annual mean wind speed at 100 m a.g.l. (above ground level) mostly coincide with areas of high surface roughness length and not with the location of the domains or maximum wind speed. Then an ensemble of more than 50 simulations with different setups for a single year was carried out for one domain covering northern Europe for which tall mast observations were available. We varied many different parameters across the simulations, e.g. model version, forcing data, various physical parameterizations, and the size of the model domain. These simulations showed that although virtually every parameter change affects the results in some way, significant changes in the wind climate in the boundary layer are mostly due to using different physical parameterizations, especially the planetary boundary layer scheme, the representation of the land surface, and the prescribed surface roughness length. Also, the setup of the simulations, such as the integration length and the domain size, can considerably influence the results. We assessed the degree of similarity between winds simulated by the WRF ensemble members and the observations using a suite of metrics, including the Earth Mover's Distance (EMD), a statistic that measures the distance between two probability distributions. The EMD was used to diagnose the performance of each ensemble member using the full wind speed and direction distribution, which is essential for wind resource assessment. We identified the most realistic ensemble members to determine the most suitable configuration to be used in the final production run, which is fully described and evaluated in the second part of this study (Dörenkämper et al., 2020).

Improvements of the coupled WRF-Lake model over Lake Nam Co, Central Tibetan Plateau

A series of model sensitivity simulations are carried out to calibrate and improve the Weather Research and Forecasting Model coupled with a one-dimensional lake model (WRF-Lake) based on observations over Lake Nam Co. Using the default lake model parameters, the solution of WRF-Lake exhibits significant biases in both the lake thermodynamics and regional climatology, i.e., higher lake surface temperature (LST), earlier onset of summer thermal stratification, and overestimated near-surface air temperature and precipitation induced by the lake’s excessive warming and moistening impacts. The performance of WRF-Lake is improved through adjusting the initial lake temperature profile, the temperature of maximum water density (Tdmax), the surface roughness length, and the light extinction coefficient. Results show that initializing the water temperature with spring observation mitigates the LST overestimation and reduces the timing error of the onset of thermal stratification. By further adjusting Tdmax from 4 °C to the observed value of 3.5 °C, the LST increase from June to mid-July is enhanced and the buildup of thermal stratification is more accurately predicted. Through incorporating the parameterized surface roughness length and decreasing the light extinction coefficient, the model better reproduces the observed daily evolution of LST and vertical lake temperature profile. The calibrated WRF-Lake effectively mitigates the overestimation of over-lake air temperature at 2 m height and precipitation over regions downwind the lake. This suggests that an improved lake scheme within the coupled WRF-Lake is essential for realistically simulating the lake–air interactions and the regional climate over the lake-rich Tibetan Plateau.

Idealized Simulations of City-Storm Interactions in a Two-Dimensional Framework

Numerous studies have identified spatial variability in convective parameters such as rainfall totals and lightning flashes in the vicinity of large urban areas, yet many questions remain regarding the storm-scale processes that are altered during interaction with a city as well as which urban features are most responsible for storm modification. This study uses an idealized, two-dimensional cloud model to investigate structural and evolutionary changes in a squall line as it passes over a simplified representation of a large city. A parameter space exploration is done in which the parameters of the city—surface temperature and surface roughness length—are systemically increased relative to the region surrounding the idealized city. The resultant suite of simulations demonstrates that storm parameters such as vertical velocity, hydrometeor mass, upward mass flux, and buoyant accelerations are enhanced when the storm passes over the idealized city. No such enhancement occurs in the control simulation without an idealized city.

The Making of the New European Wind Atlas, Part 1: Model Sensitivity

This is the first of two papers that documents the creation of the New European Wind Atlas (NEWA). It describes the sensitivity analysis and evaluation procedures that formed the basis for choosing the final setup of the mesoscale model simulations of the wind atlas. An optimal combination of model setup and parameterisations was found for simulating the climatology of the wind field at turbine-relevant heights with the Weather Research and Forecasting (WRF) model. Initial WRF model sensitivity experiments compared the wind climate generated by using two commonly used planetary boundary layer schemes and were carried out over several regions in Europe. They confirmed that the largest differences in annual mean wind speed at 100 m above ground level mostly coincide with areas of high surface roughness length and not with the location of the domains or maximum wind speed. Then an ensemble of more than 50 simulations with different setups for a single year was carried out for one domain covering Northern Europe, for which tall mast observations were available. Many different parameters were varied across the simulations, for example, model version, forcing data, various physical parameterisations and the size of the model domain. These simulations showed that although virtually every parameter change affects the results in some way, significant changes on the wind climate in the boundary layer are mostly due to using different physical parameterisations, especially the planetary boundary layer scheme, the representation of the land surface, and the prescribed surface roughness length. Also, the setup of the simulations, such as the integration length and the domain size can considerably influence the results. The degree of similarity between winds simulated by the WRF ensemble members and the observations was assessed using a suite of metrics, including the Earth Mover's Distance (EMD), a statistic that measures the distance between two probability distributions. The EMD was used to diagnose the performance of each ensemble member using the full wind speed distribution, which is important for wind resource assessment. The most realistic ensemble members were identified to determine the most suitable configuration to be used in the final production run, which is fully described and evaluated in the second part of this study.