pinecone/core-2020-05-10-deduplication huggingface dataset:
Idealized large-eddy simulations of nocturnal low-level jets over subtropical desert regions and implications for dust-generating winds

Nocturnal low-level jets (LLJs) are maxima in the wind profile, which often form above the stable nocturnal boundary layer. Over the Sahara, the world's largest source of mineral dust, this phenomenon is of particular importance to the emission and transport of desert aerosol. We present the first ever detailed large-eddy simulations of dust-generating LLJs. Using sensitivity studies with the UK Met Office large-eddy model (LEM), two key controls of the nocturnal LLJ are investigated: surface roughness and the Coriolis force. Functional relationships derived from the LEM results help to identify optimal latitude-roughness configurations for a maximum LLJ enhancement. Ideal conditions are found in regions between 20 and 27°N with roughness lengths >0.0001 m providing long oscillation periods and large jet amplitudes. Typical LLJ enhancements reach up to 3.5 m s<sup>-1</sup> for geostrophic winds of 10 m s<sup>-1</sup>. The findings are largely consistent with results from a theoretical LLJ model applied for comparison. The results demonstrate the importance of latitude and roughness in creating regional patterns of LLJ influence. Combining the functional relationships with high-resolution roughness data over northern Africa gives good agreement with the location of morning dust uplift in satellite observations. It is shown that shear-induced mixing plays an important role for the LLJ evolution and surface gustiness. With decreasing latitude the LLJ oscillation period is longer and, thus, shear-induced mixing is weaker, allowing a more stable nocturnal stratification to develop. This causes a later and more abrupt LLJ breakdown in the morning with stronger gusts, which can compensate for the slower LLJ evolution that leads to a weaker jet maximum. The findings presented here can serve as the first step towards a parametrization to improve the representation of the effects of nocturnal LLJs on dust emission in coarser-resolution models.European Research Counci
Mountain torques and synoptic systems in the Mediterranean

The mountains surrounding the Mediterranean exert torques T during the passage of North Atlantic systems which affect the angular momentum of the airflow passing over and around the massifs. The Alps, the Atlas range and the\ud
orographic block of Asia Minor are selected to investigate the typical flow conditions during torque events. These mountain ranges are small enough to justify a local angular momentum analysis. Both the zonal and the meridional components of a mountain’s torque (Tλ and Tϕ) are used as stratification parameters in a statistical investigation of the interaction of large-scale perturbations with this mountain. How are these flows affected by the obstacle? A simple scheme is tested which attempts to interpret results.\ud
The torque analysis singles out eastward-moving large-scale systems. Their isobars are oriented from southwest (northwest) to northeast (southeast) near the mountain in zonal torque Tλ (Tϕ) cases. The massifs tend to generate a low-level distortion of the pressure field such that the angular momentum of the flow over the mountain is reduced. These results can be explained within the framework of the scheme. The influence of the mountains on the pressure field is seen only at heights ≤4000 m. The low-level distortions of the pressure field contribute positively to the total torque for lags τ ≤ 0 in the Alps and for all lags −2 ≤ τ ≤ 2 days in Asia Minor, where only Tλ is evaluated. The impact of the Atlas mountains is seen only at τ = 0
Modelling sea breeze climatologies and interactions on coasts in the southern North Sea: Implications for offshore wind energy

Current understanding of the behaviour of sea breezes in the offshore environment is limited but rapidly requires improvement due, not least, to the expansion of the offshore wind energy industry. Here we report on contrasting characteristics of three sea-breeze types on five coastlines around the southern North Sea from an 11 year model-simulated climatology. We present and test an identification method which distinguishes sea-breeze types which can, in principle, be adapted for other coastlines around the world. The coherence of the composite results for each type demonstrates that the method is very effective in resolving and distinguishing characteristics and features. Some features, such as jets and calm zones, are shown to influence offshore wind farm development areas, including the sites of the proposed wind farms up to 200 km offshore. A large variability in sea-breeze frequency between neighbouring coastlines of up to a factor of 3 is revealed. Additionally, there is a strong association between sea-breeze type on one coastline and that which may form coincidentally on another nearby. This association can be as high as 86% between, for example, the North Norfolk and East Norfolk coasts. We show, through associations between sea-breeze events on coastlines with contrasting orientations, that each coastline can be important for influencing the wind climate of another. Furthermore, we highlight that each sea-breeze type needs separate consideration in wind power resource assessment and that future larger turbines will be more sensitive to sea-breeze impacts
Similarity theory based on the Dougherty-Ozmidov length scale

The article of record as published may be located at http://dx.doi.org/10.1002/qj.2488This article describes a local similarity theory for developed turbulence in the stably
stratified boundary layer that is based on the Brunt–V¨ais¨al¨a frequency and the dissipation
rate of turbulent kinetic energy instead of the turbulent fluxes used in the traditional
Monin–Obukhov similarity theory. Based on dimensional analysis (Pi theorem), it is
shown that any properly scaled statistics of the small-scale turbulence are universal
functions of a stability parameter defined as the ratio of a reference height z and the
Dougherty–Ozmidov length scale, which in the limit of z-less stratification is linearly
proportional to the Obukhov length scale.Measurements of atmospheric turbulence made
at five levels on a 20 m tower over the Arctic pack ice during the Surface Heat Budget
of the Arctic Ocean experiment (SHEBA) are used to examine the behaviour of different
similarity functions in the stable boundary layer. In the framework of this approach the
non-dimensional turbulent viscosity is equal to the gradient Richardson number, whereas
the non-dimensional turbulent thermal diffusivity is equal to the flux Richardson number.
These results are a consequence of the approximate local balance between production of
turbulence by shear in the mean flow and viscous dissipation. The turbulence framework
based on the Brunt–V¨ais¨al¨a frequency and the dissipation rate of turbulent kinetic energy
may have practical advantages for estimating turbulence when the fluxes are not directly
available
A dynamical framework for the origin of the diagonal South Pacific and South Atlantic convergence zones

The South Pacific Convergence Zone (SPCZ) and South Atlantic Convergence Zone (SACZ) are diagonal bands of precipitation that extend from the equator southeastward into the Southern Hemisphere over the western Pacific and Atlantic Oceans, respectively. With mean precipitation rates over 5 mm day−1, they are a major component of the tropical and global climate in austral summer. However, their basic formation mechanism is not fully understood. Here, a conceptual framework for the diagonal convergence zones is developed, based on calculations of the vorticity budget from reanalysis and Rossby wave theory. Wave trains propagate eastward along the Southern Hemisphere subtropical jet, with initially quasi-circular vorticity centres. In the zonally sheared environment on the equatorward flank of the jet, these vorticity centres become elongated and develop a northwest-southeast tilt. Ray tracing diagnostics in a non-divergent, barotropic Rossby wave framework then explain the observed equatorward propagation of these diagonal vorticity structures toward the westerly ducts over the equatorial Pacific and Atlantic. The baroclinic component of these circulations leads to destabilisation and ascent ahead of the cyclonic vorticity anomaly in the wave, triggering deep convection because of the high sea surface temperatures in this region. Latent heat release then forces additional ascent and strong upper-tropospheric divergence, with an associated anticyclonic vorticity tendency. A vorticity budget shows that this cancels out the advective cyclonic vorticity tendency in the wave train over the SPCZ, and dissipates the wave within a day. The mean SPCZ is consequently comprised of the sum of these pulses of diagonal bands of precipitation. Similar mechanisms also operate in the SACZ. However, the vorticity anomalies in the wave trains are stronger, and the precipitation and negative feedback from the divergence and anticyclonic vorticity tendency are weaker, resulting in continued propagation of the wave and a more diffuse diagonal convergence zone
Log-normal distribution based EMOS models for probabilistic wind speed
  forecasting

Ensembles of forecasts are obtained from multiple runs of numerical weather
forecasting models with different initial conditions and typically employed to
account for forecast uncertainties. However, biases and dispersion errors often
occur in forecast ensembles, they are usually under-dispersive and uncalibrated
and require statistical post-processing. We present an Ensemble Model Output
Statistics (EMOS) method for calibration of wind speed forecasts based on the
log-normal (LN) distribution, and we also show a regime-switching extension of
the model which combines the previously studied truncated normal (TN)
distribution with the LN.
  Both presented models are applied to wind speed forecasts of the eight-member
University of Washington mesoscale ensemble, of the fifty-member ECMWF ensemble
and of the eleven-member ALADIN-HUNEPS ensemble of the Hungarian Meteorological
Service, and their predictive performances are compared to those of the TN and
general extreme value (GEV) distribution based EMOS methods and to the TN-GEV
mixture model. The results indicate improved calibration of probabilistic and
accuracy of point forecasts in comparison to the raw ensemble and to
climatological forecasts. Further, the TN-LN mixture model outperforms the
traditional TN method and its predictive performance is able to keep up with
the models utilizing the GEV distribution without assigning mass to negative
values.Comment: 24 pages, 10 figure
A climatological view of HNO3 partitioning in cirrus clouds

A new in situ climatology of cirrus ice water content (IWC) is used, together with observed molar ratios of HNO3/H2O in cirrus ice particles, to estimate the range of HNO3 content in cirrus ice in the temperature interval 185-240 K. We find that nearly over the complete temperature range HNO3 percentages in ice between 0.01 and 100% are possible in cirrus clouds and that IWC is a major parameter determining the content of HNO3 in ice at given temperatures. Considering average conditions, the HNO3 content increases with decreasing temperature from 1% to about 10% in the range 240-200 K. For colder ice clouds, the average HNO3 content again decreases down to 6%. At higher temperatures, less efficient HNO3 uptake limits the HNO3 content in cirrus ice, while at low temperatures small IWCs permit only little HNO3 in ice, thus causing the convex-shaped average HNO3 content curve. The highest HNO3 content is expected in tropical ice clouds with very large IWCs, especially at temperatures between 190 and 210 K. Thus, tropical cirrus clouds show the highest potential to vertically redistribute HNO
Solar Signals in CMIP-5 Simulations: The Stratospheric Pathway

The 11-year solar cycle component of climate variability is assessed in the historical simulations of models taken from the Coupled Model Inter-comparison Project, phase 5 (CMIP-5). Multiple linear regression is applied to estimate the zonal-temperature, −wind and annular mode responses to a typical solar cycle, with a focus on both the stratosphere and the stratospheric influence on the surface over the period ∼1850-2005. The analysis is performed on all CMIP-5 models, but focuses on the 13 CMIP-5 models that resolve the stratosphere (high-top models), and compares the simulated solar cycle signature with reanalyses data. The 11-year solar cycle component of climate variability is found to be weaker in terms of magnitude and latitudinal gradient around the stratopause in the models than in reanalysis. The peak in temperature in the lower equatorial stratosphere (∼70 hPa) reported in some studies is found in the models to depend on the length of the analysis period, with the last 30 years yielding the strongest response.\ud
\ud
A modification of the Polar Jet Oscillation (PJO) in response to the 11-year solar cycle is not robust across all models, but is more apparent in models with high spectral resolution in the shortwave. The PJO evolution is slower in the models, leading to a stronger response during February, whereas observations indicate it to be weaker. In early winter, the magnitudes of the modelled response is more consistent with observations when only data from 1979–2005 is considered. The observed North Pacific high-pressure surface response during solar maximum is only simulated in some models, of which there are no distinguishing model characteristics. The lagged North Atlantic surface response is reproduced in both high- and low-top models, but is more prevalent in the former. In both cases, the magnitude of the response is generally lower than in observations
Solar Signals in CMIP-5 Simulations: The Ozone Response

A multiple linear regression statistical method is applied to model data taken from the Coupled Model Intercomparison Project, phase 5 (CMIP-5) to estimate the 11-yr solar cycle responses of stratospheric ozone, temperature, and zonal wind during the 1979-2005 period. The analysis is limited to the six CMIP-5 models that resolve the stratosphere (high-top models) and that include interactive ozone chemistry. All simulations assumed a conservative 11-yr solar spectral irradiance (SSI) variation based on the NRL model. These model responses are then compared to corresponding observational estimates derived from two independent satellite ozone profile data sets and from ERA Interim Reanalysis meteorological data. The models exhibit a range of 11-yr responses with three models (CESM1-WACCM, MIROC-ESM-CHEM, and MRI-ESM1) yielding substantial solar-induced ozone changes in the upper stratosphere that compare favorably with available observations. The remaining three models do not, apparently because of differences in the details of their radiation and photolysis rate codes. During winter in both hemispheres, the three models with stronger upper stratospheric ozone responses produce relatively strong latitudinal gradients of ozone and temperature in the upper stratosphere that are associated with accelerations of the polar night jet under solar maximum conditions. This behavior is similar to that found in the satellite ozone and ERA Interim data except that the latitudinal gradients tend to occur at somewhat higher latitudes in the models. The sharp ozone gradients are dynamical in origin and assist in radiatively enhancing the temperature gradients, leading to a stronger zonal wind response. These results suggest that simulation of a realistic solar-induced variation of upper stratospheric ozone, temperature and zonal wind in winter is possible for at least some coupled climate models even if a conservative SSI variation is adopted
Increases in moist-convective updraught velocities with warming in radiative-convective equilibrium

The scaling of updraught velocities over a wide range of surface temperatures is investigated in simulations of radiative-convective equilibrium with a cloud-system resolving model. The updraught velocities increase with warming, with the largest fractional increases occurring in the upper troposphere and for the highest percentile updraughts. A plume model approximately reproduces the increases in updraught velocities if the plume environment is prescribed as the mean profile in each simulation while holding the entrainment and microphysical assumptions fixed. Convective available potential energy (CAPE) also increases with warming in the simulations but at a much faster fractional rate when compared with the square of the updraught velocities. This discrepancy is investigated with a two-plume model in which a weakly entraining plume represents the most intense updraughts, and the environment is assumed to adjust so that a more strongly entraining plume has negligible buoyancy. The two-plume model suggests that updraught velocities increase with warming at a lower fractional rate than implied by the CAPE because of the influence of entrainment on both the mean stratification and the updraughts themselves.National Science Foundation (U.S.) (grant AGS-1148594)United States. National Aeronautics and Space Administration. Research Opportunities in Earth and Space Science (grant 09-IDS09-0049
Frontal-wave cyclogenesis in the North Atlantic –a climatological characterisation

The ambient conditions during cyclogenesis on fronts in the North Atlantic (NA) are characterised climatologically in the ERA-Interim dataset. These ‘secondary’ cyclones, which grow from frontal waves, are typically small in scale, shallow and deepen rapidly, and pose a challenge to forecasters and numerical weather prediction models. Frontal-wave development has been studied intensively in the past based on observations (e.g. FASTEX), real-case studies or analytical models. In this study, frontal-wave cyclogenesis is studied from a climatological perspective and is detected by a combination of two automated identification methods for the period between 1979 and 2014. A climatology of frontal-wave cyclogenesis is presented. Horizontal composites provide an estimate of the climatological mean of conditions during frontal-wave cyclogenesis which may prove useful in the assessment of the realism of proposed idealized models of frontogenesis. The findings are also compared with assumptions made in earlier idealized studies of frontal waves. The composites reveal coherent structures of diabatically generated low-level PV in the lower troposphere, neutral moist static stability at cyclogenesis and an ambient lower troposphere close to saturation. The vertical PV structure in the east NA is more vertically aligned than in the west NA and PV anomalies from the upper and lower levels reach deeper into the mid-troposphere, which indicates an enhanced phase-lock between the two anomalies. Previous studies suggest the existence of an upper limit of along-frontal stretching above which frontal-wave development is strongly suppressed. The presented multi-decadal analysis confirms its existence in a range between 0.6 and 0.8 × 10−5 s−1. Finally a climatology of along-frontal stretching for the NA is presented and compared to the fraction of cyclogenesis forming on frontal waves
Serial clustering of extratropical cyclones in a multi-model ensemble of historical and future simulations

AcceptedArticleThis is the peer reviewed version of the following article: Economou, T., Stephenson, D. B., Pinto, J. G., Shaffrey, L. C. and Zappa, G. (2015), Serial clustering of extratropical cyclones in a multi-model ensemble of historical and future simulations. Q.J.R. Meteorol. Soc. , which has been published in final form at http://dx.doi.org/10.1002/qj.2591. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.This study has investigated serial (temporal) clustering of extra-tropical cyclones simulated by 17 climate models that participated in CMIP5. Clustering was estimated by calculating the dispersion (ratio of variance to mean) of 30 December-February counts of Atlantic storm tracks passing nearby each grid point. Results from single historical simulations of 1975-2005 were compared to those from historical ERA40 reanalyses from 1958-2001 ERA40 and single future model projections of 2069-2099 under the RCP4.5 climate change scenario.
Models were generally able to capture the broad features in reanalyses reported previously: underdispersion/regularity (i.e. variance less than mean) in the western core of the Atlantic storm track surrounded by overdispersion/clustering (i.e. variance greater than mean) to the north and south and over western Europe. Regression of counts onto North Atlantic Oscillation (NAO) indices revealed that much of the overdispersion in the historical reanalyses and model simulations can be accounted for by NAO variability.
Future changes in dispersion were generally found to be small and not consistent across models. The overdispersion statistic, for any 30 year sample, is prone to large amounts of sampling uncertainty that obscures the climate change signal. For example, the projected increase in dispersion for storm counts near London in the CNRMCM5 model is 0.1 compared to a standard deviation of 0.25. Projected changes in the mean and variance of NAO are insufficient to create changes in overdispersion that are discernible above natural sampling variations.Natural Environment Research Council (NERC)Consortium on Risk in the Environment: Diagnostics, Integration, Benchmarking, Learning, and Elicitation (CREDIBLE
Decoding the precision of historical temperature observations

Historical observations of temperature underpin our ability to monitor Earth’s climate. We identify a pervasive issue in archived observations from surface stations, wherein the use of varying conventions for units and precision has led to distorted distributions of the data. Apart from the original precision being generally unknown, the majority of archived temperature data are found to be misaligned with the original measurements because of rounding on a Fahrenheit scale, conversion to Celsius, and re-rounding. Furthermore, we show that commonly used statistical methods including quantile regression are sensitive to the finite precision and to double-rounding of the data after unit conversion. To remedy these issues, we present a Hidden Markov Model that uses the differing frequencies of specific recorded values to recover the most likely original precision and units associated with each observation. This precision-decoding algorithm is used to infer the precision of the 644 million daily surface temperature observations in the Global Historical Climate Network database, providing more accurate values for the 63% of samples found to have been biased by double-rounding. The average absolute bias correction across the dataset is 0.018 ◦C, and the average inferred precision is 0.41 ◦C, even though data are archived at 0.1 ◦C precision. These results permit better inference of when record temperatures occurred, correction of rounding effects, and identification of inhomogeneities in surface temperature time series, amongst other applications. The precision-decoding algorithm is generally applicable to rounded observations–including surface pressure, humidity, precipitation, and other temperature data–thereby offering the potential to improve quality-control procedures for many datasets.Earth and Planetary Science