Abstract
Despite impressive advances in rain forecasts over the past decades, our understanding of rain formation on a microphysical scale is still poor. Droplet growth initially occurs through ...diffusion and, for sufficiently large radii, through the collision of droplets. However, there is no consensus on the mechanism to bridge the condensation coalescence bottleneck. We extend the analysis of prior methods by including radiatively enhanced diffusional growth (RAD) to a Markovian turbulence parameterization. This addition increases the diffusional growth efficiency by allowing for emission and absorption of thermal radiation. Specifically, we quantify an upper estimate for the radiative effect by focusing on droplets close to the cloud boundary. The strength of this simple model is that it determines growth-rate dependencies on a number of parameters, like updraft speed and the radiative effect, in a deterministic way. Realistic calculations with a cloud-resolving model are sensitive to parameter changes, which may cause completely different cloud realizations and thus it requires considerable computational power to obtain statistically significant results. The simulations suggest that the addition of radiative cooling can lead to a doubling of the droplet size standard deviation. However, the magnitude of the increase depends strongly on the broadening established by turbulence, due to an increase in the maximum droplet size, which accelerates the production of drizzle. Furthermore, the broadening caused by the combination of turbulence and thermal radiation is largest for small updrafts and the impact of radiation increases with time until it becomes dominant for slow synoptic updrafts.
The transition from land to sea affects the wind field in coastal regions. From the perspective of near-coastal offshore wind farms, the coastal transition complicates the task of energy resource ...assessment by, for example, introducing non-homogeneity into the free wind field. To help elucidate the matter, we quantify the average horizontal wind speed gradients at progressively increasing distances from the German coast using two years of hourly ERA5 reanalysis data, and further describe the dependence of wind speed gradients on the measurement height, atmospheric stability, and season. A vertical wind lidar located on Norderney Island near the German mainland acts as our observational reference for the ERA5 data, where a good agreement (R2=0.93$R^2 =\nobreak 0.93$) is found despite the relatively coarse ERA5 data resolution. Interestingly, the comparison of lidar data with the higher-resolution Weather Research and Forecasting (WRF) mesoscale model yields good but relatively weaker agreement (R2=0.85$R^2 =\nobreak 0.85$). The ERA5 data reveal that, for flow over the North Sea originating from the German mainland from the south, the wind speed at 10 m (110 m) above sea level increases by 30 % (20 %) some 80 km from the coast on average, and by 5 % at larger heights. An increased stratification increases the horizontal wind speed gradient at 10 m above sea level but decreases it at 110 m. Case studies using satellite and flight measurements are first analyzed to help reveal some of the underlying mechanisms governing horizontal wind speed gradients, including cases of decreasing wind speed with increasing distance from the coast, in which stable flow of warm air over the colder sea leads to an overall deceleration of the flow. The accuracy of offshore resource assessment appears to profit from utilising the horizontal wind speed gradient information contained in ERA5 reanalysis data.
The atmospheric boundary layer experiences multiple changes in coastal regions, especially with wind directions from land towards the sea, where the wind speed usually increases due to the smaller ...roughness of the ocean surface. These effects are of particular relevance for offshore wind energy utilization; they are summarized under the term coastal effects. This paper provides an overview of coastal effects and their potential impact on the operating conditions of offshore wind farms with a focus on the German Bight. Common numerical and experimental tools to study coastal effects and developing internal boundary layers (IBL) are introduced, and a review on the current state of research is given. The German Bight is an interesting example to illustrate impacts of coastal effects on offshore wind energy, because of the large number of wind turbines with a coastal distance of 100 km or less. Phenomena related to the stability of the boundary layer, like low level jets, are discussed. Spatial variations of vertical heat fluxes in the coastal zone related to variable water depths or Wadden Sea areas are analysed. The study illustrates that due to the increasing size of offshore wind farms, horizontal wind speed gradients caused by coastal effects can lead to significant wind variations within a single farm.Research topics which still need further attention are discussed in the framework of the rapidly developing wind energy sector with increasing wind turbine hub heights and rotor diameters as well as growing wind farm sizes. One example is the interaction of coastal effects with offshore wind farm wakes. The necessity to consider a large spectrum of spatial and temporal scales to understand and describe coastal effects is highlighted. We summarize modelling and observation tools, which are suitable for the investigation and prediction of the boundary layer dynamics in coastal areas. Existing applications and results are described based on several examples with collocated observation and model results obtained in the X‑Wakes project. The study puts particular focus on the large potential provided by the combination of different measurements and modelling techniques and gives recommendations for future developments of integrated approaches including the formulation of priorities.
