Geostrophic Wind Speed Research Articles

Planetary Boundary-Layer Modelling and Tall Building Design.

Characteristics of flow in the planetary boundary layer (PBL) strongly affect the design of tall structures. PBL modelling in building codes, based as it is on empirical data from the 1960s and 1970s, differs significantly from contemporary PBL models, which account for both "neutral" flows, and "conventionally neutral" flows. PBL heights estimated in these relatively sophisticated models are typically approximately half as large as those obtained using the classical asymptotic similarity approach, and are one order of magnitude larger than those specified in North American and Japanese building codes. A simple method is proposed for estimating the friction velocity and PBL height as functions of specified surface roughness and geostrophic wind speed. Based on published results, it is tentatively determined that, even at elevations as high as 800 m above the surface, the contribution to the resultant mean flow velocity of the component V normal to the surface stress is negligible and the veering angle is of the order of only 5°. This note aims to encourage dialogue between boundary-layer meteorologists and structural engineers.

울산 지역 대기질 모의능력 개선을 위한 배출량자료 평가

배출량 자료는 대기질 수치모의에 있어서 큰 영향을 주는 요소로서 정확한 대기질 수치모의를 위해서 배출량 자료의 정확성과 신뢰도 향상은 대기질 모델 연구에 있어 필수적이다. 본 연구에서는 울산지역을 대상으로 2003년과 2010년 CAPSS 배출량 자료인 CAPSS-2003과 CAPSS-2010를 입력자료로 하여 WRF-CMAQ 모델을 수행하여 울산지역 배출량 지료의 타당성을 검토하였다. 우선 오염물질의 장거리 수송 영향을 배제하기 위하여 울산지역 자체 내의 배출량 영향이 우세한 종관 기상조건을 가진 사례일을 선정하고 선정된 사례일에 대해서 WRF-CMAQ모델을 수행한 후 그 결과인 CO, <TEX>$NO_2$</TEX>, <TEX>$SO_2$</TEX>, <TEX>$PM_{10}$</TEX> 농도와 관측 값을 비교하여, 과소 혹은 과대 모의되는 정도를 분석하여 모의결과를 보정 할 수 있는 'scaling factor'를 제시하였다. 그 결과 CAPSS-2003을 이용한 모의결과는 CO와 <TEX>$NO_2$</TEX>에 대해서는 관측과 유사한 수준으로 모의하였으나 <TEX>$SO_2$</TEX>는 약 12배 과대모의, <TEX>$PM_{10}$</TEX>은 약 27배나 과소모의하는 결과를 얻었다. 반면 CAPSS-2010을 이용한 모의결과에서는 CO와 <TEX>$NO_2$</TEX>의 모의결과는 유사하였으며, <TEX>$SO_2$</TEX>는 약 2배 과대모의, <TEX>$PM_{10}$</TEX>은 약 5배 과소모의 하여 <TEX>$SO_2$</TEX>와 <TEX>$PM_{10}$</TEX> 배출량이 상당히 개선됨을 확인하였다. <TEX>$SO_2$</TEX>와 <TEX>$PM_{10}$</TEX> 역시 이전 보다는 관측에 가깝게 모의되었으나 현실적 모델링 결과를 도출하기 위해서 배출량은 향후 개선되어야 할 부분으로 판단되었다. 따라서 보다 현실적인 모델링 결과를 도출하기 위해서는 본 연구에서 제시한 울산지역 배출량의 'scaling factor'를 이용하면 보다 안정된 모델링 결과가 도출 될 것으로 판단된다. Emission source term is one of the strong controlling factors for the air quality simulation capability, particularly over the urban area. Ulsan is an industrial area and frequently required to simulate for environmental assessment. In this study, two CAPSS (Clean Air Policy Support System) emission data; CAPSS-2003 and CAPSS-2010 in Ulsan, were employed as an input data for WRF-CMAQ air quality model for emission assessment. The simulated results were compared with observations for the local emission dominant synoptic conditions which had negative vorticities and lower geostrophic wind speed at 850hPa weather maps. The measurements of CO, <TEX>$NO_2$</TEX>, <TEX>$SO_2$</TEX> and <TEX>$PM_{10}$</TEX> concentrations were compared with simulations and the 'scaling factors' of emissions for CO, <TEX>$NO_2$</TEX>, <TEX>$SO_2$</TEX>, and <TEX>$PM_{10}$</TEX> were suggested in in aggregative and quantitative manner. The results showed that CAPSS-2003 showed no critical discrepancies of CO and <TEX>$NO_2$</TEX> observations with simulations, while <TEX>$SO_2$</TEX> was overestimated by a factor of more than 12, while <TEX>$PM_{10}$</TEX> was underestimated by a factor of more than 20 times. However, CAPSS-2010 case showed that <TEX>$SO_2$</TEX> and <TEX>$PM_{10}$</TEX> emission were much more improved than CAPSS-2003. However, <TEX>$SO_2$</TEX> was still overestimated by a factor of more than 2, and <TEX>$PM_{10}$</TEX> underestimated by a factor of 5, while there was no significant improvement for CO and <TEX>$NO_2$</TEX> emission. The estimated factors identified in this study can be used as'scaling factors'for optimizing the emissions of air pollutants, particularly <TEX>$SO_2$</TEX> and <TEX>$PM_{10}$</TEX> for the realistic air quality simulation in Ulsan.

Multiple Regimes of Wind, Stratification, and Turbulence in the Stable Boundary Layer

Abstract A long time series of temporally high-resolution wind and potential temperature data from the 213-m tower at Cabauw in the Netherlands demonstrates the existence of two distinct regimes of the stably stratified nocturnal boundary layer at this location. Hidden Markov model (HMM) analysis is used to objectively characterize these regimes and classify individual observed states. The first regime is characterized by strongly stable stratification, large wind speed differences between 10 and 200 m, and relatively weak turbulence. The second is associated with near-neutral stratification, weaker wind speed differences between 10 and 200 m, and relatively strong turbulence. In this second regime, the state of the boundary layer is similar to that during the day. The occupation statistics of these regimes are shown to covary with the large-scale pressure gradient force and cloud cover such that the first regime predominates under clear skies with weak geostrophic wind speed and the second regime predominates under conditions of extensive cloud cover or large geostrophic wind speed. These regimes are not distinguished by standard measures of stability, such as the Obukhov length or the bulk Richardson number. Evidence is presented that the mechanism generating these distinct regimes is associated with a previously documented feedback resulting from the existence of an upper limit on the maximum downward heat flux that can be sustained for a given near-surface wind speed.

The effects of mean atmospheric forcings of the stable atmospheric boundary layer on wind turbine wake

Interactions between the nocturnal atmospheric boundary layer (ABL) and wind turbines (WTs) can be complicated due to the presence of low level jets (LLJ), a region which creates wind speeds higher than geostrophic wind speed. A study has been performed to isolate the effect of mean forcings of the ABL on turbulence energetics and structures in the wake of WT. Large eddy simulation with an actuator line model has been used as a tool to simulate a full-scale 5-MW WT under two different realistic atmospheric states of the stable ABL corresponding to low- and high-stratification. The study clearly demonstrates that the large-scale forcings of thermally stratified atmospheric boundary characterized by shear- and buoyancy-driven turbulence significantly influence the wake structure of a wind turbine. For the WT in low-stratified ABL, the jets occur above the WT resulting in a strong mixed layer behind the WT. High turbulence results in a faster wake recovery. For the WT in high-stratified ABL, the jets occur near the hub-height resulting in an asymmetric wake structure. The jets confine the mixing to hub-height resulting in a slower wake recovery. Vertical shear causes the interaction of the root- and lower-tip vortices resulting in the instability of the root vortex leading to an enhanced shear stress and turbulent kinetic energy. The tip vortices exhibit mutual inductance between adjacent vortex filaments resulting in vortex merging. LLJs are an important metric associated with mean atmospheric forcings that dictate the turbulence generated in WT wake and the wake recovery of a WT in a stable ABL.

Atmosphere–ice forcing in the transpolar drift stream: results from the DAMOCLES ice-buoy campaigns 2007–2009

Abstract. During the EU research project Developing Arctic Modelling and Observing Capabilities for Long-term Environmental Studies (DAMOCLES), 18 ice buoys were deployed in the region of the Arctic transpolar drift (TPD). Sixteen of them formed a quadratic grid with 400 km side length. The measurements lasted from 2007 to 2009. The properties of the TPD and the impact of synoptic weather systems on the ice drift are analysed. Within the TPD, the speed increases by a factor of almost three from the North Pole to the Fram Strait region. The hourly buoy position fixes would show that the speed is underestimated by 10–20% if positions were taken at only 1–3 day intervals as it is usually done for satellite drift estimates. The geostrophic wind factor Ui / Ug (i.e. the ratio of ice speed Ui and geostrophic wind speed Ug), in the TPD amounts to 0.012 on average, but with regional and seasonal differences. The constant Ui / Ug relation breaks down for Ug &lt; 5 m s−1. The impact of synoptic weather systems is studied applying a composite method. Cyclones (anticyclones) cause cyclonic (anticyclonic) vorticity and divergence (convergence) of the ice drift. The amplitudes are twice as large for cyclones as for anticyclones. The divergence caused by cyclones corresponds to a 0.1–0.5% per 6 h open water area increase based on the composite averages, but reached almost 4% within one day during a strong August 2007 storm. This storm also caused a long-lasting (over several weeks) rise of Ui and Ui / Ug and changed the ice conditions in a way which allowed large amplitudes of inertial ice motion. The consequences of an increasing Arctic storm activity for the ice cover are discussed.

On the interaction between marine boundary layer cellular cloudiness and surface heat fluxes

Abstract. The interaction between marine boundary layer cellular cloudiness and surface fluxes of sensible and latent heat is investigated. The investigation focuses on the non-precipitating closed-cell state and the precipitating open-cell state at low geostrophic wind speed. The Advanced Research WRF (Weather Research and Forecasting) model is used to conduct cloud system-resolving simulations with interactive surface fluxes of sensible heat, latent heat, and of sea salt aerosol, and with a detailed representation of the interaction between aerosol particles and clouds. The mechanisms responsible for the temporal evolution and spatial distribution of the surface heat fluxes in the closed- and open-cell state are investigated and explained. It is found that the closed-cell state imposes its horizontal spatial structure on surface air temperature and water vapor, and, to a lesser degree, on the surface sensible and latent heat flux. The responsible mechanism is the entrainment of dry, free tropospheric air into the boundary layer. The open-cell state is associated with oscillations in surface air temperature, water vapor, and in the surface fluxes of sensible heat, latent heat, and of sea salt aerosol. Here, the responsible mechanism is the periodic formation of clouds, rain, and of cold and moist pools with elevated wind speed. Open-cell cloud formation, cloud optical depth and liquid water path, and cloud and rain water path are identified as good predictors of the horizontal spatial structure of surface air temperature and sensible heat flux, but not of surface water vapor and latent heat flux. It is shown that the open-cell state creates conditions conducive to its maintenance by enhancing the surface sensible heat flux. The open-cell state also enhances the sea salt flux relative to the closed-cell state. While the open-cell state under consideration is not depleted in aerosol and is insensitive to variations in sea salt fluxes, in aerosol-depleted conditions, the enhancement of the sea salt flux may replenish the aerosol needed for cloud formation and hence contribute to the maintenance of the open-cell state. Spatial homogenization of the surface fluxes is found to have only a small effect on cloud properties in the investigated cases.

Idealized dry quasi 2‐D mesoscale simulations of cold‐air outbreaks over the marginal sea ice zone with fine and coarse resolution

A nonhydrostatic model (NH3D) is used for idealized dry quasi 2‐D simulations of Arctic cold‐air outbreaks using horizontal grid spacings between 1.25 and 60 km. Despite the idealized setup, the model results agree well with observations over Fram Strait. It is shown that an important characteristic of the flow regime during cold‐air outbreaks is an ice‐breeze jet (IBJ) with a maximum wind speed exceeding often the large‐scale geostrophic wind speed. According to the present simulations, which agree very well with those of another nonhydrostatic mesoscale model (METRAS), the occurrence, strength, and horizontal extent L of this jet depend strongly on the external forcing and especially on the direction of the large‐scale geostrophic wind relative to the orientation of the ice edge. The latter dependency is explained by the effects of the thermally induced geostrophic wind over open water and Coriolis force. It is found that coarse‐resolution runs underestimate the strength of the jet. This underestimation has important consequences to the surface fluxes of heat and momentum, which are also underestimated by about 10–15% on average over the region between the ice edge and 120–180 km downstream. Our results suggest that a grid spacing of about L/7 is required (about 10–30 km) to simulate the IBJ strength with an accuracy of at least 10%. Thus, the results of large‐scale models as well might contain uncertainties with regard to the simulated IBJ strength which would influence the energy budget in a large region along the marginal sea ice zones.

Diurnal variations of land surface wind speed probability distributions under clear‐sky and low‐cloud conditions

Long‐term 10 min wind tower data and ceilometer backscatter data at Cabauw in the Netherlands provide quantitative information on the influence of low clouds on the diurnal evolution of the near‐surface wind speed (NSWS) probability distribution (probability density function), the wind power density (WPD), and their vertical structure in the bottom 200 m of the atmosphere. Under clear‐sky conditions, pronounced diurnal cycles are identified in the leading three moments of NSWS as well as WPD and the boundary layer thermal structure in all seasons. When low clouds are present, weaker diurnal cycles with a different vertical structure are observed. Under clear skies, skewness at night is positive within the stable air near the surface but negative above 100 m. In the presence of low clouds, wind speeds are positively skewed and the probability of strong winds is higher associated with a larger geostrophic wind speed.

Is the storminess in the Twentieth Century Reanalysis really inconsistent with observations? A reply to the comment by Krueger et al. (2013b)

In a recent study of trends and low frequency variability of extra-tropical cyclone activity in the ensemble of Twentieth Century Reanalyses, we concluded that “For the North Atlantic-European region and southeast Australia, the 20CR cyclone trends are in agreement with trends in geostrophic wind extremes derived from in-situ surface pressure observations”. This conclusion has been challenged by Krueger et al. (Clim Dyn, submitted, 2013b), because a recent study (doi: 10.1175/JCLI-D-12-00309.1 , by the same lead author) comparing annual 95th percentiles (P95) of geostrophic wind speed (geo-wind) derived from surface pressure observations and from the 20CR found that “20CR-geostrophic storminess deviates to a large extent from the observation-based curve” in the period prior to 1950. In this reply, we show that our conclusion is valid; and we clarify that several factors contribute to the reported inconsistencies between the 20CR and observation-based geo-wind extremes. These include the choice of index that is used to represent the temporal variation of extremes (e.g., annual vs. seasonal percentiles), the use of different sampling intervals (6-hourly vs. 3-hourly), and the presence of very large errors in the observations that were not identified, corrected, or excluded in any of the previous studies of observation-based geo-wind extremes. We show that the time series of consecutive seasonal P95 geo-winds derived from the observations and from 20CR are in good agreement back to about 1893, with some deviation earlier when the observations (especially digitized data) remain limited and are more uncertain. We find that the correlation between the 20CR and observation-based geo-wind extremes (P95) time series for the full 134-year record is highly significant statistically, with and without the correction or exclusion of the newly identified erroneous SLP values. The agreement between 20CR and observations is further improved after the correction or exclusion of these erroneous values.

Identification of Long-Range Transported Haze Phenomena and Their Meteorological Features over Northeast Asia

AbstractThis paper presents a technique used to empirically classify operationally defined “haze” days in South Korea from 2000 to 2007 into long-range transported (LRT) and stagnant (STG) haze cases. A total of 547 haze days were classified into these two cases by tracking consecutive 6-day synoptic weather charts and air trajectories. The meteorological features associated with long-range transport of haze were identified by contrasting the values of 35 candidate meteorological parameters corresponding to the two types of haze cases. A suitable subset of synoptic variables was then chosen to diagnose the unique meteorological features of each case. The statistical test showed that geostrophic wind speed, vorticity, vorticity advection at a geopotential height of 850 hPa, and the vertical stability index of the lower atmosphere were indicated as highly effective parameters for distinguishing between the LRT and STG cases. The classification accuracies showed 93.2%, 87.8%, 85.4%, and 84.4% for these four variables, respectively. The STG case was well characterized by negative vorticity, with stable atmospheric stability conditions and weak geostrophic wind speed, that is, ~2.8 m s−1at a geopotential height of 850 hPa, whereas the LRT case had relatively strong geostrophic wind speed, &gt;6 m s−1. For both cases, the location of the anticyclone played an important role in haze occurrence, directly and indirectly. A high pressure system led to stable STG haze with weak ventilation, resulting from upper-atmospheric subsidence. The LRT case was associated with a strong anticyclone that prevailed over southwestern China, maintaining the pressure gradient force that generated the westerly wind that was persistently conducive to downwind long-range transport of haze.

The role of snow‐surface coupling, radiation, and turbulent mixing in modeling a stable boundary layer over Arctic sea ice

To enhance the understanding of the impact of small‐scale processes in the polar climate, this study focuses on the relative role of snow‐surface coupling, radiation and turbulent mixing in an Arctic stable boundary layer. We extend the GABLS1 (GEWEX Atmospheric Boundary‐Layer Study 1) model intercomparison for turbulent mixing with the other relevant physical processes in the stable boundary layer over sea ice. We use the Single Column Model (SCM) version of the Weather Research and Forecasting (WRF) mesoscale meteorological model and run different combinations of boundary layer and radiation schemes, using a state‐of‐the art land surface scheme. With this intercomparison of schemes, we confirm a wide variety in the state of the atmosphere and the surface variables for the selected parameterization schemes. To understand this variety, a sensitivity analysis for one particular combination of parameterization schemes is performed, using a novel analysis method of process diagrams. The variation between the sensitivity runs indicates a relative orientation of model sensitivities to variations in each of the governing processes and these can explain the variety of model results obtained in the intercomparison of different parameterization schemes. Moreover, we apply the same method for several geostrophic wind speeds to represent a large range of synoptic conditions. Results indicate a shift in process significance for different wind regimes. For low wind regimes, the model sensitivity is larger for surface coupling and radiation, while for high wind speeds, the largest sensitivity is found for the turbulent mixing process. An interesting non‐linear feature was found for turbulent mixing for frequently occurring wind speeds and low wind speed cases, where the 2 m temperature increases for decreased amounts of mixing.

Statistical-dynamical downscaling of wind roses over the Czech Republic

The present study describes a new method for statistical-dynamical downscaling that combines two different approaches, namely, a set of patterns simulated with a numerical flow model and a transformation function used to process both calculated data and measurements at a reference station. The combined method produces wind roses and wind speed histograms at an arbitrary location in the model domain. The inflow wind direction represented the key parameter to define a set of wind field simulations. The other two inflow parameters, namely, thermal stratification and geostrophic wind speed, were derived from corresponding averaged soundings. The results showed that in the Czech Republic, there are areas where wind roses are deformed by the surrounding terrain. The deformations occur in relatively shallow and wide valleys, and they are more sensitive to the inflow wind direction. Calculated wind roses are compared to corresponding observations at 22 synoptic stations. The most frequent wind direction sector in simulations agreed with measurements at 17 stations. The resulting error in frequency in that sector was under 5 % at 10 stations. In general, the main features of the wind roses are modelled well, even at a relatively large distance from the reference station. However, better performance was achieved for smaller distances between reference station and the site. In further studies, a more extensive set of flow patterns with reduced intervals of thermal stratification and wind speed will likely improve calculated wind roses.

Decomposition of the windiness index in the Netherlands for the assessment of future long‐term wind supply

ABSTRACTThe main windiness index in the Netherlands (Windex‐CBS) has been subject to a decreasing trend since the systematic recording started in 1988. This seriously complicates the estimation of future long‐term wind supply. Therefore, it is necessary to know the main causes of the reported decline.For this purpose, Windex‐CBS, derived from actual wind turbine yields, is compared with two alternative windiness indices. The first index is based on wind speed observations Wland and the second on geostrophic wind speed Gland. Similar indices are formulated for offshore wind supply in the North Sea, Woff and Goff. High mutual correlations between the indices indicate that they do efficiently account for natural variability. Yet, the trends of the different indices do substantially differ from each other.The decrease of Windex‐CBS is twice as large as (200%) the decrease of Wland. The trend of the difference between both indices is highly significant. This suggests that the Windex‐CBS is contaminated by operational or methodological factors. Wland does not suffer from such factors and is therefore the preferred measure of wind supply. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Estimation of persistence and trends in geostrophic wind speed for the assessment of wind energy yields in Northwest Europe

Wind climate in Northwest Europe is subject to long-term persistence (LTP), also called the Hurst phenomenon. Ignorance of LTP causes underestimation of climatic variability. The quantification of multi-year variability is important for the assessment of the uncertainty of future multi-year wind yields. Estimating LTP requires long homogeneous time series. Such series of wind observations are rare, but annual mean geostrophic wind speed (U) can be used instead. This study demonstrates a method to estimate the 10-year aggregated mean U for the near and the far future and its uncertainty in Northwest Europe. Time series of U were derived from daily sea level pressure from the European Climate Assessment Dataset. Minor inhomogeneities cannot be ruled out, but were shown to hardly affect the estimated Hurst exponent $$(\hat{H})$$ . A maximum likelihood method was adjusted to remove the biases in $$\hat{H}$$ . The geostrophic wind speed over the North Sea, the British Isles and along the Scandinavian coast are characterised by statistically significant H between 0.58 and 0.74, (H = 0.5 implies no LTP). The additional affect of the parameter uncertainty is estimated in a Bayesian way and is highly dependent on the record length. The assessment of structural changes in future wind fields requires general circulation models. An ensemble of seventeen simulations (ESSENCE) with one single climate model (ECHAM5/MPI-OM) was used to evaluate structural trends and LTP. The estimated $$\hat{H}$$ in the ESSENCE simulations are generally close to 0.5 and not significant. Significant trends in U are found over large parts of the investigated domain, but the trends are small compared to the multi-year variability. Large decreasing trends are found in the vicinity of Iceland and increasing trends near the Greenland coast. This is likely related to the sea ice retreat within the ESSENCE simulations and the associated change in surface temperature gradients.

An examination of storm activity in the northeast Atlantic region over the 1851–2003 period using the EMULATE gridded MSLP data series

[1] The gridded mean sea level pressure (MSLP) series developed by the European and North Atlantic Daily to Multidecadal Climate Variability (EMULATE) project is used in this paper to analyze changes in storm activity in the northeast Atlantic region over the period 1851–2003. Zonal and meridional geostrophic wind speed components were calculated for each grid square over the domain 65°W–45°E, 30°N–65°N, and seasonal percentiles from these data were used as a measure of storm activity in the region. Despite the relatively coarse temporal (daily) and spatial (5° × 5°) resolution of the data, the results indicate that the series are able to provide useful information about storm activity across the northeast Atlantic domain back to 1881 and in the data-rich area of the North Sea back to 1851. The variability of winter storm activity observed across the North Sea over the last 153 years, and particularly the increase to high values in the 1990s, is closely associated with the variability of the North Atlantic storm track. In contrast, high summer values in the late nineteenth century appear to have been related to a Greenland-blocking system, leading to high meridional flow in the region. This latter conclusion is limited, however, by poor data coverage in the northwest Atlantic region.

Changes in the mean and extreme geostrophic wind speeds in Northern Europe until 2100 based on nine global climate models

AbstractThis study aims at analyzing the mean and extreme geostrophic wind speeds in Northern Europe. The analyses are based on nine global climate models and the Special Report on Emission Scenarios (SRES) A1B, A2 and B1 scenarios. The time frames studied consist of the baseline 1971–2000 and the future periods 2046–2065 and 2081–2100. The SRES scenarios are considered both separately and combined. The extremes are calculated for the September–April period for various return periods. The analysis is done by applying the program R and the Generalized Extreme Value ‐methodology.All projections indicate that both the mean and extreme geostrophic wind speeds will increase in the southern and eastern parts of Northern Europe and decrease over the Norwegian Sea in September–April. The change over the ocean is pronounced already in 2046–2065, over the continents in 2081–2100. For the model mean, the smallest change (2–6%) was projected under the B1 and the largest (4–10%) under the A1B and A2 scenarios. However, spread among the individual global circulation models (GCMs) was fairly large.The ratios between the return level estimates for various return periods and the annual maximum wind speeds were found nearly homogeneously independent of the time frame studied. For the baseline and future periods, the extreme winds occurring once in 10 or 50 years were 13% ± 2% and 22% ± 5% stronger than the mean annual maxima, respectively. The present findings serve as support for risk assessment such as required when planning the forest management practices. Copyright © 2011 Royal Meteorological Society

Trends and low-frequency variability of storminess over western Europe, 1878–2007

This study analyzes extremes of geostrophic wind speeds derived from sub-daily surface pressure observations at 13 sites in the European region from the Iberian peninsula to Scandinavia for the period from 1878 or later to 2007. It extends previous studies on storminess conditions in the Northeast (NE) Atlantic-European region. It also briefly discusses the relationship between storminess and the North Atlantic Oscillation (NAO). The results show that storminess conditions in the region from the Northeast Atlantic to western Europe have undergone substantial decadal or longer time scale fluctuations, with considerable seasonal and regional differences (especially between winter and summer, and between the British Isles-North Sea area and other parts of the region). In the North Sea and the Alps areas, there has been a notable increase in the occurrence frequency of strong geostrophic winds from the mid to the late twentieth century. The results also show that, in the cold season (December–March), the NAO-storminess relationship is significantly positive in the north-central part of this region, but negative in the south-southeastern part.

Evaluation of an Air Pressure–Based Proxy for Storm Activity

Abstract Yearly percentiles of geostrophic wind speeds serve as a widely used proxy for assessing past storm activity. Here, daily geostrophic wind speeds are derived from a geographical triangle of surface air pressure measurements and are used to build yearly frequency distributions. It is commonly believed, however unproven, that the variation of the statistics of strong geostrophic wind speeds describes the variation of statistics of ground-level wind speeds. This study evaluates this approach by examining the correlation between specific annual (seasonal) percentiles of geostrophic and of area-maximum surface wind speeds to determine whether the two distributions are linearly linked in general. The analyses rely on bootstrap and binomial hypothesis testing as well as on analysis of variance. Such investigations require long, homogeneous, and physically consistent data. Because such data are barely existent, regional climate model–generated wind and surface air pressure fields in a fine spatial and temporal resolution are used. The chosen regional climate model is the spectrally nudged and NCEP-driven regional model (REMO) that covers Europe and the North Atlantic. Required distributions are determined from diagnostic 10-m and geostrophic wind speed, which is calculated from model air pressure at sea level. Obtained results show that the variation of strong geostrophic wind speed statistics describes the variation of ground-level wind speed statistics. Annual and seasonal quantiles of geostrophic wind speed and ground-level wind speed are positively linearly related. The influence of low-pass filtering is also considered and found to decrease the quality of the linear link. Moreover, several factors are examined that affect the description of storminess through geostrophic wind speed statistics. Geostrophic wind from sea triangles reflects storm activity better than geostrophic wind from land triangles. Smaller triangles lead to a better description of storminess than bigger triangles.

Storminess in northern Italy and the Adriatic Sea reaching back to 1760

This study investigates storminess in northern Italy and the northern Adriatic Sea through the examination of several storm proxies. These proxies are based on homogenized daily mean pressure series given at a set of stations (Genoa, Milan, Padua, Turin, and Hvar). The application of widely accepted and well-known methods on pressure series allows for a long-term year-to-year analysis of the intra-seasonal storm variability. As storminess is usually more intense throughout the cold season, our analysis is limited to the October–March period of each year. The following proxies are considered in this study: First, we assess the statistics of geostrophic wind speed. These statistics are derived from two adjacent triangles that are located across the Adriatic Sea (Padua–Hvar–Genoa) and in northern Italy (Genoa–Padua–Turin). Second, we evaluate annual statistics of time series of pressure tendency. Last, intra-seasonal low percentiles of pressure are also made use of. These proxies are used to describe the evolution of the storm climate far back in time, covering in some cases a 260-year long period. The proxies show pronounced interannual and interdecadal variability, but no sustained long-term trend.

Entrainment and budgets of heat, water vapor, and carbon dioxide in a convective boundary layer driven by time-varying forcing

[1] A large-eddy simulation (LES) code is coupled with a land surface model to investigate the diurnal variation of the atmospheric boundary layer (ABL). The diurnal evolution of the ABL is driven by a time-varying incoming solar radiation. The results show that the domain average surface fluxes of sensible heat, water vapor, and carbon dioxide are smooth functions of time but the fluxes at any given surface grid point show random variations, especially the sensible heat flux. At the ABL top, the LES-resolved entrainment fluxes of these scalars also evolve with time and are not fixed fractions of their respective surface fluxes. Entrainment efficiency (the ratio of entrainment flux at zi to weδϕ, where zi is the ABL height, we is entrainment velocity, and δϕ is the jump of scalar across the entrainment zone) is highest for CO2 and lowest for sensible heat. The first-order jump condition model is very good approximation to simulated entrainment fluxes which are largely controlled by the vertical gradients of the scalars across the capping inversion. Our results suggest that over the range of geostrophic winds considered (0–5 m s−1), neither the surface nor the entrainment flux reveals sensitivity to the geostrophic wind speed variations.