Controls on boundary layer ventilation: Boundary layer processes and large-scale dynamics

Midlatitude cyclones are important contributors to boundary layer ventilation. However, it is uncertain how efficient such systems are at transporting pollutants out of the boundary layer, and variations between cyclones are unexplained. In this study 15 idealized baroclinic life cycles, with a passive tracer included, are simulated to identify the relative importance of two transport processes: horizontal divergence and convergence within the boundary layer and large-scale advection by the warm conveyor belt. Results show that the amount of ventilation is insensitive to surface drag over a realistic range of values. This indicates that although boundary layer processes are necessary for ventilation they do not control the magnitude of ventilation. A diagnostic for the mass flux out of the boundary layer has been developed to identify the synoptic-scale variables controlling the strength of ascent in the warm conveyor belt. A very high level of correlation (R-2 values exceeding 0.98) is found between the diagnostic and the actual mass flux computed from the simulations. This demonstrates that the large-scale dynamics control the amount of ventilation, and the efficiency of midlatitude cyclones to ventilate the boundary layer can be estimated using the new mass flux diagnostic. We conclude that meteorological analyses, such as ERA-40, are sufficient to quantify boundary layer ventilation by the large-scale dynamics

The Extratropical Transition of Hurricane Debby (1982) and the Subsequent Development of an Intense Windstorm over Finland

On 22 September 1982, an intense windstorm caused considerable damage in northern Finland. Local forecasters noted that this windstorm potentially was related to Hurricane Debby, a category 4 hurricane that occurred just 5 days earlier. Due to the unique nature of the event and lack of prior research, our aim is to document the synoptic sequence of events related to this storm using ERA-Interim reanalysis data, best track data, and output from OpenIFS simulations. During extratropical transition, the outflow from Debby resulted in a ridge building and an acceleration of the jet. Debby did not reintensify immediately in the midlatitudes despite the presence of an upper-level trough. Instead, ex-Debby propagated rapidly across the Atlantic as a diabatic Rossby wave-like feature. Simultaneously, an upper-level trough approached from the northeast and once ex-Debby moved ahead of this feature near the United Kingdom, rapid reintensification began. All OpenIFS forecasts diverged from reanalysis after only 2 days indicating intrinsic low predictability and strong sensitivities. Phasing between Hurricane Debby and the weak trough, and phasing of the upper- and lower-level potential vorticity anomalies near the United Kingdom was important in the evolution of ex-Debby. In the only OpenIFS simulation to correctly capture the phasing over the United Kingdom, stronger wind gusts were simulated over northern Finland than in any other simulation. Turbulent mixing behind the cold front, and convectively driven downdrafts in the warm sector, enhanced the wind gusts over Finland. To further improve understanding of this case, we suggest conducting research using an ensemble approach.Peer reviewe

Temporal evolution of features that control 10-m wind gusts in moist baroclinic wave simulations identified using non-linear regression

EMS Annual Meeting Abstracts, Vol. 19A majority of insured losses over Europe are related to Extra-Tropical Cyclones (ETC) which are characterised by strong winds, heavy precipitation and powerful ocean waves. Baroclinic wave simulations (BWS) are used to study ETC by varying their background state and measuring their different intensities. However, two main issues limit an exhaustive exploration of ETC intensity and background state relationship: 1) the dimensionality of the feature space, 2) a large number of intensity measures. To alleviate this issue, this study proposes to use a wrapper Feature Selection Algorithm (wFSA) combined with a non-linear regressor applied to an intensity measure. The selected subsets are analysed through time. BWS was performed in the moist case using OpenIFS version Cy43r3v2 configured as an aqua planet with full physics and the radiation scheme deactivated. The atmospheric state proposed by Jablonowski and Williamson was used. The spatial resolution of the simulation was set to TL319/L137 and the time resolution to 20 minutes for 15 days. The initial perturbation was located in 40°N 20°E. A number of 55 measures -called features- were extracted from the BWS and the 10-m wind gust was selected as the intensity measure. A stable wFSA was performed using weighted Random Forest Regressor in the framework proposed by Meinshausen and Bühlmann. The regression was run 10 times on 60% of randomly selected points in the northern hemisphere to infer the 10-m wind gust. Finally, the average feature importance and its variance were computed for each feature every 12 hours. The forecast surface roughness and the specific humidity were the most important features for the first 2 days. Afterwards, mean sea level pressure became predominant for 5 days. For the remaining days, forecast surface roughness, specific humidity and large scale precipitation were the most important features to infer 10-m wind gust. Further work will aim at increasing the number of BWS by modifying the average temperature of the background state. All results will be compared to propose an efficient dimension reduction to study BWS and their evolution.Peer reviewe

Relationships between extra-tropical cyclone intensity measures

EMS Annual Meeting Abstracts, Vol. 19Extra-Tropical Cyclones (ETC) cause the most variability in weather and a significant portion of total insured losses in Europe. Their impacts are caused by high wind speeds, heavy precipitation and large ocean waves. The intensity of ETCs can be quantified with multiple different measures such as Mean Sea Level Pressure (MSLP), relative vorticity or storm severity indices. Currently, it is not known how the various measures of ETC intensity relate to each other. The aim of this study is to determine relationships between different intensity measures, their dependence on geographical region, and on the structure and evolution of the ETCs. ERA5 reanalysis data from 1979 to 2021 was used to study the relationships. The analysis was restricted to the cold season (from October to March) which is when the strongest ETCs most often occur. ETCs were tracked using feature tracking software TRACK with values of 850-hPa relative vorticity every three hours as input. To focus on the most relevant ETCs affecting Europe, only tracks in the North Atlantic were chosen and stationary and short-lived systems were excluded. The intensity measures were calculated by combining the ETC tracks with parameters from ERA5 reanalysis. The intensity measures analysed include the maximum 850-hPa relative vorticity, minimum MSLP, maximum wind gusts, and a storm severity index which is based on extreme 10-metre winds and their occurrence probability. Relationships between different intensity measures were analysed for land and sea areas separately using mutual information and density heatmaps.    The initial results shows that there is a correlation between maximum 850-hPa vorticity and minimum MSLP, and that this correlation is stronger over sea than land areas. However, this relationship is non-linear, with considerable spread associated with it. Additional results concerning the other measures of intensity will also be presented.Extra-Tropical Cyclones (ETC) cause the most variability in weather and a significant portion of total insured losses in Europe. Their impacts are caused by high wind speeds, heavy precipitation and large ocean waves. The intensity of ETCs can be quantified with multiple different measures such as Mean Sea Level Pressure (MSLP), relative vorticity or storm severity indices. Currently, it is not known how the various measures of ETC intensity relate to each other. The aim of this study is to determine relationships between different intensity measures, their dependence on geographical region, and on the structure and evolution of the ETCs. ERA5 reanalysis data from 1979 to 2021 was used to study the relationships. The analysis was restricted to the cold season (from October to March) which is when the strongest ETCs most often occur. ETCs were tracked using feature tracking software TRACK with values of 850-hPa relative vorticity every three hours as input. To focus on the most relevant ETCs affecting Europe, only tracks in the North Atlantic were chosen and stationary and short-lived systems were excluded. The intensity measures were calculated by combining the ETC tracks with parameters from ERA5 reanalysis. The intensity measures analysed include the maximum 850-hPa relative vorticity, minimum MSLP, maximum wind gusts, and a storm severity index which is based on extreme 10-metre winds and their occurrence probability. Relationships between different intensity measures were analysed for land and sea areas separately using mutual information and density heatmaps.    The initial results shows that there is a correlation between maximum 850-hPa vorticity and minimum MSLP, and that this correlation is stronger over sea than land areas. However, this relationship is non-linear, with considerable spread associated with it. Additional results concerning the other measures of intensity will also be presented.Peer reviewe

Climatology, variability, and trends in near-surface wind speeds over the North Atlantic and Europe during 1979-2018 based on ERA5

This study presents the monthly 10‐m wind speed climatology, decadal variability and possible trends in the North Atlantic and Europe from ERA5 reanalysis from 1979 to 2018 and investigates the physical reasons for the decadal variability. Additionally, temporal time series are examined in three locations: the central North Atlantic, Finland and Iberian Peninsula. The 40‐year mean and the 98th percentile wind speeds emphasize a distinct land‐sea contrast and a seasonal variation with the strongest winds over the ocean and during winter. The strongest winds and the highest variability are associated with the storm tracks and local wind phenomena such as the mistral. The extremeness of the winds is examined with an extreme wind factor (the 98th percentile divided by mean wind speeds) which in all months is higher in southern Europe than in northern Europe. Mostly no linear trends in 10‐m wind speeds are identified in the three locations but large annual and decadal variability is evident. The decadal 10‐m wind speeds were stronger than average in the 1990s in northern Europe and in the 1980s and 2010s in southern Europe. These decadal changes were largely explained by the positioning of the jet stream and storm tracks and the strength of the north–south pressure gradient in the North Atlantic. The 10‐m winds have a positive correlation with the North Atlantic Oscillation in the central North Atlantic and Finland on annual scales and during cold season months and a negative correlation in Iberian Peninsula mostly from July to March. The Atlantic Multi‐decadal Oscillation has a moderate negative correlation with the winds in the central North Atlantic but no correlation in Finland and Iberian Peninsula. Overall, our results emphasize that while linear trends in wind speeds may show a general long‐term trend, more information on the changes is obtained by analysing long‐term variability.This study presents the monthly 10-m wind speed climatology, decadal variability and possible trends in the North Atlantic and Europe from ERA5 reanalysis from 1979 to 2018 and investigates the physical reasons for the decadal variability. Additionally, temporal time series are examined in three locations: the central North Atlantic, Finland and Iberian Peninsula. The 40-year mean and the 98th percentile wind speeds emphasize a distinct land-sea contrast and a seasonal variation with the strongest winds over the ocean and during winter. The strongest winds and the highest variability are associated with the storm tracks and local wind phenomena such as the mistral. The extremeness of the winds is examined with an extreme wind factor (the 98th percentile divided by mean wind speeds) which in all months is higher in southern Europe than in northern Europe. Mostly no linear trends in 10-m wind speeds are identified in the three locations but large annual and decadal variability is evident. The decadal 10-m wind speeds were stronger than average in the 1990s in northern Europe and in the 1980s and 2010s in southern Europe. These decadal changes were largely explained by the positioning of the jet stream and storm tracks and the strength of the north-south pressure gradient in the North Atlantic. The 10-m winds have a positive correlation with the North Atlantic Oscillation in the central North Atlantic and Finland on annual scales and during cold season months and a negative correlation in Iberian Peninsula mostly from July to March. The Atlantic Multi-decadal Oscillation has a moderate negative correlation with the winds in the central North Atlantic but no correlation in Finland and Iberian Peninsula. Overall, our results emphasize that while linear trends in wind speeds may show a general long-term trend, more information on the changes is obtained by analysing long-term variability.Peer reviewe

Evaluating solar radiation forecast uncertainty

The presence of clouds and their characteristics have a strong impact on the radiative balance of the Earth and on the amount of solar radiation reaching the Earth's surface. Many applications require accurate forecasts of surface radiation on weather timescales, for example solar energy and UV radiation forecasts. Here we investigate how operational forecasts of low and mid-level clouds affect the accuracy of solar radiation forecasts. A total of 4 years of cloud and solar radiation observations from one site in Helsinki, Finland, are analysed. Cloud observations are obtained from a ceilometer and therefore we first develop algorithms to reliably detect cloud base, precipitation, and fog. These new algorithms are widely applicable for both operational use and research, such as in-cloud icing detection for the wind energy industry and for aviation. The cloud and radiation observations are compared to forecasts from the Integrated Forecast System (IFS) run operationally and developed by the European Centre for Medium-Range Weather Forecasts (ECMWF). We develop methods to evaluate the skill of the cloud and radiation forecasts. These methods can potentially be extended to hundreds of sites globally. Over Helsinki, the measured global horizontal irradiance (GHI) is strongly influenced by its northerly location and the annual variation in cloudiness. Solar radiation forecast error is therefore larger in summer than in winter, but the relative error in the solar radiation forecast is more or less constant throughout the year. The mean overall bias in the GHI forecast is positive (8 W m(-2)). The observed and forecast distributions in cloud cover, at the spatial scales we are considering, are strongly skewed towards clear-sky and overcast situations. Cloud cover forecasts show more skill in winter when the cloud cover is predominantly overcast; in summer there are more clear-sky and broken cloud situations. A negative bias was found in forecast GHI for correctly forecast clear-sky cases and a positive bias in correctly forecast overcast cases. Temporal averaging improved the cloud cover forecast and hence decreased the solar radiation forecast error. The positive bias seen in overcast situations occurs when the model cloud has low values of liquid water path (LWP). We attribute this bias to the model having LWP values that are too low or the model optical properties for clouds with low LWP being incorrect.Peer reviewe

Factors affecting atmospheric vertical motions as analyzed with a generalized omega equation and the OpenIFS model

A statistical analysis of the physical causes of atmospheric vertical motions is conducted using a generalized omega equation and a one-year simulation with the OpenIFS atmospheric model. Using hourly output from the model, the vertical motions associated with vorticity advection, thermal advection, friction, diabatic heating, and an imbalance term are diagnosed. The results show the general dominance of vorticity advection and thermal advection in extratropical latitudes in winter, the increasing importance of diabatic heating towards the tropics, and the significant role of friction in the lowest troposphere. As this study uses notably higher temporal resolution data than previous studies which applied the generalized omega equation, our results reveal that the imbalance term is larger than the earlier results suggested. Moreover, for the first time, we also explicitly demonstrate the seasonal and geographical contrasts in the statistics of vertical motions as calculated with the generalized omega equation. Furthermore, as our analysis covers a full year, significantly longer than any other previous studies, statistically reliable quantitative estimates of the relative importance of the different forcing terms in different locations and seasons can be made. One such important finding is a clear increase in the relative importance of diabatic heating for midtropospheric vertical motions in the Northern Hemisphere midlatitudes from the winter to the summer, particularly over the continents. We also find that, in general, the same processes are important in areas of both rising and sinking motion, although there are some quantitative differences.Peer reviewe

Daytime along-valley winds in the Himalayas as simulated by the Weather Research and Forecasting (WRF) model

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Sensitivity of idealised baroclinic waves to mean atmospheric temperature and meridional temperature gradient changes

The sensitivity of idealised baroclinic waves to different atmospheric temperature changes is studied. The temperature changes are based on those which are expected to occur in the Northern Hemisphere with climate change: (1) uniform temperature increase, (2) decrease of the lower level meridional temperature gradient, and (3) increase of the upper level temperature gradient. Three sets of experiments are performed, first without atmospheric moisture, thus seeking to identify the underlying adiabatic mechanisms which drive the response of extra-tropical storms to changes in the environmental temperature. Then, similar experiments are performed in a more realistic, moist environment, using fixed initial relative humidity distribution. Warming the atmosphere uniformly tends to decrease the kinetic energy of the cyclone, which is linked both to a weaker capability of the storm to exploit the available potential energy of the zonal mean flow, and less efficient production of eddy kinetic energy in the wave. Unsurprisingly, the decrease of the lower level temperature gradient weakens the resulting cyclone regardless of the presence of moisture. The increase of the temperature gradient in the upper troposphere has a more complicated influence on the storm dynamics: in the dry atmosphere the maximum eddy kinetic energy decreases, whereas in the moist case it increases. Our analysis suggests that the slightly unexpected decrease of eddy kinetic energy in the dry case with an increased upper tropospheric temperature gradient originates from the weakening of the meridional heat flux by the eddy. However, in the more realistic moist case, the diabatic heating enhances the interaction between upper- and low-level potential vorticity anomalies and hence helps the surface cyclone to exploit the increased upper level baroclinicity.Peer reviewe

The summer 2018 heatwave in Finland

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