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Peatlands have often been neglected in Earth system models (ESMs). Where they are included, they are usually represented via a separate, prescribed grid cell fraction that is given the physical characteristics of a peat (highly organic) soil. However, in reality soils vary on a spectrum between purely mineral soil (no organic material) and purely organic soil, typically with an organic layer of variable thickness overlying mineral soil below. They are also dynamic, with organic layer thickness and its properties changing over time. Neither the spectrum of soil types nor their dynamic nature can be captured by current ESMs. Here we present a new version of an ESM land surface scheme (Joint UK Land Environment Simulator, JULES) where soil organic matter accumulation – and thus peatland formation, degradation and stability – is integrated in the vertically resolved soil carbon scheme. We also introduce the capacity to track soil carbon age as a function of depth in JULES and compare this to measured peat age–depth profiles. The new scheme is tested and evaluated at northern and temperate sites. This scheme simulates dynamic feedbacks between the soil organic material and its thermal and hydraulic characteristics. We show that draining the peatlands can lead to significant carbon loss, soil compaction and changes in peat properties. However, negative feedbacks can lead to the potential for peatlands to rewet themselves following drainage. These ecohydrological feedbacks can also lead to peatlands maintaining themselves in climates where peat formation would not otherwise initiate in the model, i.e. displaying some degree of resilience. The new model produces similar results to the original model for mineral soils and realistic profiles of soil organic carbon for peatlands. We evaluate the model against typical peat profiles based on 216 northern and temperate sites from a global dataset of peat cores. The root-mean-squared error (RMSE) in the soil carbon profile is reduced by 35 %–80 % in the best-performing JULES-Peat simulations compared with the standard JULES configuration. The RMSE in these JULES-Peat simulations is 7.7–16.7 kg C m−3 depending on climate zone, which is considerably smaller than the soil carbon itself (around 30–60 kg C m−3). The RMSE at mineral soil sites is also reduced in JULES-Peat compared with the original JULES configuration (reduced by ∼ 30 %–50 %). Thus, JULES-Peat can be used as a complete scheme that simulates both organic and mineral soils. It does not require any additional input data and introduces minimal additional variables to the model. This provides a new approach for improving the simulation of organic and peatland soils and associated carbon-cycle feedbacks in ESMs.

Northern peatland stores a large amount of organic soil carbon and is considered to be one of the most significant CH4 sources among wetlands. The default wetland CH4 emission scheme in JULES (land surface model of the UK Earth System model) only takes into account the CH4 emissions from inundated areas in a simple way. However, it is known that the processes for peatland CH4 emission are complex. In this work, we coupled the process-based peatland CH4 emission model HIMMELI (HelsinkI Model of MEthane buiLd-up and emIssion for peatlands) with JULES (JULES-HIMMELI) by taking the HIMMELI input data from JULES simulations. Firstly, the soil temperature, water table depth (WTD) and soil carbon simulated by JULES, as well as the prescribed maximum leaf area index (LAI) in JULES were evaluated against available datasets at the studied northern wetland sites. Then, the simulated CH4 emissions from JULES and JULES-HIMMELI simulations were compared against the observed CH4 emissions at these sites. Moreover, sensitivities of CH4 emissions to the rate of anoxic soil respiration (anoxic Rs), surface soil temperature and WTD were investigated. Results show that JULES can well represent the magnitude and seasonality of surface (5–10 cm) and relatively deep (34–50 cm) soil temperatures, whereas the simulated WTD and soil carbon density profiles show large deviations from the site observations. The prescribed maximum LAI in JULES was within one standard deviation of the maximum LAIs derived from the Sentinel-2 satellite data for Siikaneva, Kopytkowo and Degerö sites, but lower for the other three sites. The simulated CH4 emissions by JULES have much smaller inter-annual variability than the observations. However, no specific simulation setup of the coupled model can lead to consistent improvements in the simulated CH4 emissions for all the sites. When using observed WTD or modified soil decomposition rate, there were only improvements in simulated CH4 fluxes at certain sites or years. Both simulated and observed CH4 emissions at sites strongly depend on the rate of anoxic Rs, which is the basis of CH4 emission estimates in HIMMELI. By excluding the effect from the rate of anoxic Rs on CH4 emissions, it is found that the Rs-log-normalized CH4 emissions (log normalization of the ratio of CH4 emission to anoxic Rs rate) show similar increasing trends with increased surface soil temperature from both observations and simulations, but different trends with raised WTD which may due to the uncertainty in simulated O2 concentration in HIMMELI. In general, we consider the JULES-HIMMELI model is more appropriate in simulating the wetland CH4 emissions than the default wetland CH4 emission scheme in JULES. Nevertheless, in order to improve the accuracy of simulated wetland CH4 emissions with the JULES-HIMMELI model, it is still necessary to better represent the peat soil carbon and hydrologic processes in JULES and the CH4 production and transportation processes in HIMMELI, such as plant transportation of gases, seasonality of parameters controlling oxidation and production, and adding microbial activities.

A total of 20 urban neighbourhood-scale eddy covariance flux tower datasets are made openly available after being harmonized to create a 50 site–year collection with broad diversity in climate and urban surface characteristics. Variables needed as inputs for land surface models (incoming radiation, temperature, humidity, air pressure, wind and precipitation) are quality controlled, gap-filled and prepended with 10 years of reanalysis-derived local data, enabling an extended spin up to equilibrate models with local climate conditions. For both gap filling and spin up, ERA5 reanalysis meteorological data are bias corrected using tower-based observations, accounting for diurnal, seasonal and local urban effects not modelled in ERA5. The bias correction methods developed perform well compared to methods used in other datasets (e.g. WFDE5 or FLUXNET2015). Other variables (turbulent and upwelling radiation fluxes) are harmonized and quality controlled without gap filling. Site description metadata include local land cover fractions (buildings, roads, trees, grass etc.), building height and morphology, aerodynamic roughness estimates, population density and satellite imagery. This open collection can help extend our understanding of urban environmental processes through observational synthesis studies or in the evaluation of land surface environmental models in a wide range of urban settings. These data can be accessed from https://doi.org/10.5281/zenodo.7104984 (Lipson et al., 2022).

Wetlands occupy a special place in the mosaic of landscapes around the globe, which, as a moist areas covered with vegetation, intensively release methane. Long-term research on vertical methane exchange between wetlands and the atmosphere has shown that the intensity of this process is a function of the climatic conditions at the observation site and the physico-chemical properties of the soil, such as moisture, temperature, pH and oxidation-reduction potential (redox) which reflects the ability of the soil to develop oxidizing or reducing conditions and thus indicates whether soil conditions are currently aerobic or anaerobic, necessary for the development of methanogenesis. Wetlands with a permanently high level of soil moisture content, located in mid-latitudes, are characterized by a clear annual variation in the vertical flux of methane to the atmosphere with clear relation to soil temperature. In recent years, permanently lowered or strongly fluctuating groundwater levels lead to continuous or episodic drying of the soil, which causes a continuous or temporary reduction in the intensity of methanogenesis. Thus, in dry years, the variability of methane fluxes is disturbed and the annual methane emissions is several times lower than in wet years.In the years 2013-2018, continuous measurements of methane flux (FCH4) were carried out in the marshes of the Biebrza National Park (NE Poland). The results, similar to those from other stations in middle latitudes, showed a clear annual variability of FCH4 in wet years (2013 and 2014) with minimum values in winter and intense methane release to the atmosphere from April to September (up to +0.35 gCH4·m-2·day-1). In dry years (2017 and 2018) in turn, the annual variability was clearly disturbed due to lower groundwater levels.The aim of the study is to perform a comparative analysis of the variability of FCH4 under typical and reduced soil moisture conditions, as well as an analysis of the temporal variability of the redox potential measured at five depths as a parameter supporting the analysis of the variability of methane fluxes in dry years. The variability of FCH4, disturbed in comparison to the wet years, was analyzed based on the variability of the redox potential in the soil, with particular emphasis on the relationship between the intensity of methanogenesis and the depth at which favorable conditions for methanogenesis appear. In such years, only the occurrence of intense but short-term methanogenesis was observed in April-May (up to + 0.1 gCH4·m-2·day-1), then a rapid decrease in the FCH4 value with the groundwater level falling to values close to those of winter and an irregular appearance of elevated FCH4 values in the period from June to November. Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219 and University of Lodz under project 4/IDUB/DOS/2021. The authors thank the authorities of the Biebrza National Park for allowing the continuous measurements in the area of the Park.

Reversing the natural feedbacks that limit the rise in temperature is one of the major threats of climate change. One such mechanism is the exchange of carbon gases between the ecosystem and the atmosphere in wetlands. Wetlands cover only about 3% of the Earth's surface, but in natural conditions act as a CO2 sink and store a significant amount of carbon in the soil. The organic carbon accumulated in the Northern peatland is estimated as one-third of the world’s pool of organic carbon, equivalent to more than half the amount of carbon in the atmosphere. Climate extremes such as droughts and hot spell, can reduce or even invers this role. The water table drawdown and higher temperatures lead to enhanced peat oxidation and releasing a large portion of peat carbon as CO2. It can switch a peatland from sink to source of carbon. However, some studies suggest that other mechanisms may compensate or even turn away this effect in real peatland ecosystems. Consequently, it is vitally important to empirically verify whether the paradigm of peatland transition from carbon sink to source in hot and dry conditions is valid for natural ecosystems. Despite the growing number of observations, it is hard to find datasets clearly showing such effect in the sense that they were collected in an undisturbed environment, represent for the whole ecosystem scale, and span full annual totals.In this study we provide a strong empirical confirmation of switching of the mid-European temperate mire from carbon sink to source under extremely dry and hot climate conditions. The analysis is based on eight-year eddy-covariance measurements at site (53°35′30.8′′ N, 22°53′32.4′′ E, 109 m a.s.l.) located in a one of the largest coherent lowland wetlands in Central Europe – the Biebrza National Park (north-eastern Poland). In the analyzed measurement period (2013-2020) the studied ecosystem was affected by severe droughts in 2015 and 2018-2020. In wet years the peatland was a significant sink of CO2 (down to −990 gCO2∙m−2∙yr−1) whereas in dry years we observed a substantial release of CO2(up to +1020 gCO2∙m−2∙yr−1). At the same time, a CH4 emission dropped from 29 gCH4∙m−2∙yr−1 in the wettest year to about 1−4 gCH4∙m−2∙yr−1 in dry years, which does not compensate for the amount of carbon released in the form of CO2(even taking into account higher global warming potential of CH4). At the same time, relatively small differences in the water vapor flux (evapotranspiration) between wet and dry years were observed. It demonstrates that the scenario of positive feedback between wetland carbon release and climate change could be realistic and supports the need of natural wetland preservation or rewetting. Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219 and University of Lodz under project 4/IDUB/DOS/2021. The authors thank the authorities of the Biebrza National Park for allowing the continuous measurements in the area of the Park.

Peatlands have often been neglected in Earth System Models (ESMs). Where they are included, they are usually represented via a separate, prescribed grid cell fraction that is given the physical characteristics of a peat (highly organic) soil. However, in reality soils vary on a spectrum between purely mineral soil (no organic material), and purely organic soil, typically with an organic layer of variable thickness overlying mineral soil below. They are also dynamic, with organic layer thickness and its properties changing over time. Neither the spectrum of soil types nor their dynamic nature can be captured by current ESMs. Here we present a new version of an ESM land surface scheme (Joint UK Land Environment Simulator, JULES) where soil organic matter accumulation - and thus peatland formation, degradation and stability – is integrated in the vertically-resolved soil carbon scheme. We also introduce the capacity to track soil carbon age as a function of depth in JULES, and compare this to measured peat age-depth profiles. This scheme simulates dynamic feedbacks between the soil organic material and its thermal and hydraulic characteristics. We show that draining the peatlands can lead to significant carbon loss along with soil compaction and changes in peat properties. However, negative feedbacks can lead to the potential for peatlands to rewet themselves following drainage. These ecohydrological feedbacks can also lead to peatlands maintaining themselves in climates where peat formation would not otherwise initiate in the model, i.e. displaying some degree of resilience. The new model produces similar results to the original model for mineral soils, and realistic profiles of soil organic carbon for peatlands. In particular the best performing configurations had root mean squared error (RMSE) in carbon density for peat sites of 7.7–16.7 kgC m−3 depending on climate zone, when compared against typical peat profiles based on 216 sites from a global dataset of peat cores. This error is considerably smaller than the soil carbon itself (around 30–60 kgC m−3) and reduced by 35–80 % compared with standard JULES. The RMSE at mineral soil sites is also smaller in JULES-Peat than JULES itself (reduced by ~30–50 %). Thus JULES-Peat can be used as a complete scheme that simulates both organic and mineral soils. It does not require any additional input data and introduces minimal additional variables to the model. This provides a new approach for improving the simulation of organic and peatland soils, and associated carbon-cycle feedbacks in ESMs, which other land surface models could follow.

In the global methane budget, the largest natural source is attributed to wetlands, which encompass all ecosystems composed of waterlogged or inundated ground, capable of methane production. Among them, northern peatlands that store large amounts of soil organic carbon have been functioning, since the end of the last glaciation period, as long-term sources of methane (CH4) and are one of the most significant methane sources among wetlands. To reduce uncertainty of quantifying methane flux in the global methane budget, it is of significance to understand the underlying processes for methane production and fluxes in northern peatlands. A methane model that features methane production and transport by plants, ebullition process and diffusion in soil, oxidation to CO2, and CH4 fluxes to the atmosphere has been embedded in the ORCHIDEE-PEAT land surface model that includes an explicit representation of northern peatlands. ORCHIDEE-PCH4 was calibrated and evaluated on 14 peatland sites distributed on both the Eurasian and American continents in the northern boreal and temperate regions. Data assimilation approaches were employed to optimized parameters at each site and at all sites simultaneously. Results show that methanogenesis is sensitive to temperature and substrate availability over the top 75 cm of soil depth. Methane emissions estimated using single site optimization (SSO) of model parameters are underestimated by 9 g CH4 m−2 yr−1 on average (i.e., 50 % higher than the site average of yearly methane emissions). While using the multi-site optimization (MSO), methane emissions are overestimated by 5 g CH4 m−2 yr−1 on average across all investigated sites (i.e., 37 % lower than the site average of yearly methane emissions).

Water storage plays an important role in mitigating heat and flooding in urban areas. Assessment of the capacity of cities to store water remains challenging due to the extreme heterogeneity of the...

Wetlands, even if cover a relatively small fraction of Earth’s surface, play an important role in global carbon cycle. They are the main terrestrial source of methane (CH4), but due to anaerobic conditions they accumulate significant part of captured in photosynthesis carbon dioxide (CO2). Due to the progressive climate change these ecosystems are exposed to different climate-inducted extreme events. One of them are fires that can significantly affect the carbon-storage potential of the wetlands.In this study we analyze the impact of a great fire on one of the largest mid-European wetlands in Biebrza Valley (northeastern Poland) on the CO2 net ecosystem exchange (NEE). Over 5,500 ha of landscape of the Biebrza National Park burned down during this event in April 20-25, 2020. In the north-east edge of the core of this fire, there was an eddy-covariance measurement site, where greenhouse gas fluxes (CO2, CH4, H2O) had been continuously recorded since 2013. The measurement system suffered to some extent, but flux measurements were resumed after repair works in approximately 2 weeks. Almost the entire source area of eddy-covariance system was affected by the fire. Thus, post-fire measurements show the dynamics of NEE for an ecosystem recovering from a fire.In the flux measurements period (2013-2020) the studied ecosystem was affected not only by the above fire event but also by severe droughts in 2015 and 2018-2020. In consequence in non-fire years the annual totals of CO2 flux followed the mean ground water table level (WTL) and spanned from -990 gCO2∙m-2∙yr-1 (CO2 sink) in the wettest year to +1020 gCO2∙m-2∙yr-1 (CO2 source) in the driest year 2019. However, even taking into account the influence of WTL and temperature fluctuation we observed clear impact of the spring fire on CO2 exchange. Shortly after the fire, in May, the wetland was in average a source of CO2 (positive monthly total of NEE), which had not happened before even in the driest years. However, already in the second half of May, the absorption of CO2 began to predominate over the emissions. From the mid-June to the end of July we observed very intensive growth of plant cover and exceptionally strong absorption of CO2, much higher than in other years with similar thermo-hydrological conditions. Consequently, the total CO2 flux in the post-fire period (May-December) was negative, while in remaining dry years the strong emission of CO2 was observed for the same part of year. Acknowledgements: Funding for this research was provided by the National Science Centre, Poland under project UMO-2020/37/B/ST10/01219. The authors thank the authorities of the Biebrza National Park for allowing the continuous measurements in the area of the Park.

Automatic processing and verification of data obtained in experiments have an essential role in modern science. In the paper, we discuss the assessment of data obtained in meteorological measurements conducted in Biebrza National Park in Poland. The data is essential for understanding the complex environmental processes, such as global warming. The measurements of CO2 flux brings a vast amount of data but suffer from drawbacks like high uncertainty. Part of the data has a high-level of credibility while, others are not reliable. The method of automatic evaluation of data with varied quality is proposed. We use LSTM networks with a weighted square mean error loss function. This approach allows incorporating the information on data reliability in the training process.

The generally-accepted paradigm of wetland response to climate change is that water table drawdown and higher temperatures will cause wetlands to switch from a sink to a source of atmospheric carbon. However, it is hard to find a multi-year, ecosystem scale dataset representative of an undisturbed wetland that clearly demonstrates this paradigm on an annual total basis. Here we provide strong empirical confirmation of the above scenario based on six years of continuous eddy-covariance CO2 and CH4 flux measurements in Biebrza Valley, north-eastern Poland. In wet years the mire was a significant sink of atmospheric carbon (down to −270 ± 70 gC-CO2 m−2 yr−1 against +21.8 ± 3.4 gC-CH4 m−2 yr−1 in 2013) whereas in dry years it constituted a substantial carbon source (releasing up to +130 ± 70 gC-CO2 m−2 yr−1 and +2.6 ± 1.4 gC-CH4 m−2 yr−1 in 2015). Our findings demonstrate that the scenario of positive feedback between wetland carbon release and the present climate change trajectory is realistic and support the need of natural wetland preservation or rewetting. Our findings also indicate that conclusions drawn regarding a wetland's response to changing climate can depend strongly on the chosen period of analysis.

We compare over 1 million sounding measurements with ERA-Interim reanalysis for the 38-yr period from 1979 to 2016. The large dataset allows us to provide an improved insight into the spatial and temporal distributions of the prerequisites of deep moist convection across Europe. In addition, ERA-Interim is also evaluated. ERA-Interim estimates parameters describing boundary layer moisture and midtropospheric lapse rates well, with correlation coefficients of 0.94. Mixed-layer CAPE is, on average, underestimated by the reanalysis while the most unstable CAPE is overestimated. Vertical shear parameters in the reanalysis are better correlated with observations than CAPE, but are underestimated by approximately 1–2 m s−1. The underestimation decreases as the depth of the shear layer increases. Compared to radiosonde observations, instability in ERA-Interim is overestimated in southern Europe and underestimated over eastern Europe. High values of instability are observed from May to September, out of phase with the climatological pattern of wind shear, which peaks in winter. From September to April, favorable conditions for thunderstorms occur mainly over southern and western Europe with the peak location and higher frequency shifting to central and eastern Europe from May to August. For southern Europe, the annual cycle peaks in September with high values of inhibition suppressing thunderstorm activity in July and August. The area with the highest annual number of days with environmental conditions favorable for thunderstorms extends from Italy and Austria eastward through the Carpathians and Balkans.

We investigate the area-averaged sensible heat flux ( $$Q_{H}$$ ) obtained with a scintillometer along a 3.1-km path length over the city centre of Łodź, Central Poland. The annual cycle of $$Q_{H}$$ peaks in June but is lower by the middle of summer. In winter, due to a large amount of anthropogenic heat input, $$Q_{H}$$ remains positive all day long, with positive night-time fluxes also found during months with frequent cold advection, e.g., June 2010. In the diurnal cycle of this flux, several features specific to urban areas are seen: the peak shifts 1–2 h after noon, the heat flux turns from positive to negative 1–2 h after sunset. In Łodź $$Q_{H}$$ was observed during inflow from the north and north-west, i.e. from the city centre. As this area is mostly covered with impervious materials, most of the heat exchanged between the ground and the overlying air is in the form of sensible heat flux. Under the conditions of inflow from the east and south-east, the maximum heat flux is approximately $$100\,\hbox {W}\,\hbox {m}^{-2}$$ lower than during the inflow from the city centre, since more vegetation exists to the east and south-east of the scintillometer path. Cold and warm advection are found to be a vital factor in the observed heat-flux variability in the centre of Łodź.

The sensible heat flux (H) is determined using large-aperture scintillometer (LAS) measurements over a city centre for eight different computation scenarios. The scenarios are based on different approaches of the mean rooftop-level $$(z_{H})$$ estimation for the LAS path. Here, $$z_{H}$$ is determined separately for wind directions perpendicular (two zones) and parallel (one zone) to the optical beam to reflect the variation in topography and building height on both sides of the LAS path. Two methods of $$z_{H}$$ estimation are analyzed: (1) average building profiles; (2) weighted-average building height within a 250 m radius from points located every 50 m along the optical beam, or the centre of a certain zone (in the case of a wind direction perpendicular to the path). The sensible heat flux is computed separately using the friction velocity determined with the eddy-covariance method and the iterative procedure. The sensitivity of the sensible heat flux and the extent of the scintillometer source area to different computation scenarios are analyzed. Differences reaching up to 7% between heat fluxes computed with different scenarios were found. The mean rooftop-level estimation method has a smaller influence on the sensible heat flux (−4 to 5%) than the area used for the $$z_{H}$$ computation (−5 to 7%). For the source-area extent, the discrepancies between respective scenarios reached a similar magnitude. The results demonstrate the value of the approach in which $$z_{H}$$ is estimated separately for wind directions parallel and perpendicular to the LAS optical beam.

Observational data on greenhouse gases exchange between ecosystems and the atmosphere are crucial in understanding the global climate mechanisms. Among different methods of estimation of this exchange, the eddy-covariance (EC) technique provides a direct measure of the net flux density across the atmosphere-ecosystem interface. Still, such data are highly scarce for Central European wetlands. In this work, we present the results of two years (2013–2014) of continuous open-path EC measurements of methane (CH 4 ) and carbon dioxide (CO 2 ) fluxes at the wetlands of the Biebrza National Park in northeastern Poland – one of the biggest coherent lowland wetland area in Central Europe. The measurement site (53°35′30.8″N, 22°53′32.4″E) was located near the Kopytkowka river in a fen peatland, whose soils are slightly decomposed due to dehydration. The mean annual sum of CH 4 release equaled 29 ± 4 g CH 4 m −2 yr −1 in wetter year 2013 and 20 ± 1 g CH 4 m −2 yr −1 in drier year 2014. The mean annual uptake of CO 2 reached 980 ± 150 g CO 2 m −2 yr −1 and 560 ± 130 g CO 2 m −2 yr −1 in 2013 and 2014, respectively. Both fluxes show a clear annual pattern with maximum CH 4 release in June and July (at a level of 130 nmol m −2 s −1 ) and maximum CO 2 uptake in June (at a level of 4.2 μmol m −2 s −1 ). The considerable C–CO 2 uptake in comparison to C–CH 4 emissions suggests that the Biebrza river wetlands are a significant carbon sink, but the net contribution of the Biebrza mires to the climate forcing is unclear because of different metrics which can be used to specify the relative weights of CO 2 and CH 4 .

Continuous eddy-covariance measurements of turbulent methane exchange between the ground and the atmosphere were carried out during the period from July 2013 to September 2015. The measurement sites were located in areas characterized by different types of use: the city (Łodź, central Poland) and the wetlands of Biebrza National Park (northeastern Poland). Regardless of the type of surface, such long-term, continuous measurements of the turbulent fluxes of methane are rare. Our aim was to investigate the temporal variability of the turbulent methane exchange in the surface–atmosphere system, while considering the impact of land use. Because cities are a huge source of atmospheric carbon dioxide, we compared the intensity of methane emissions from the city as compared to the wetland, which is considered to be the most intense natural source atmospheric methane. In both cases, the results show a clear prevalence of positive as compared with negative fluxes. This shows that both areas are net sources of methane to the troposphere. The measurements also demonstrated the existence of a clear annual cycle of turbulent methane flux in the centre of the city (average values in winter ranged from ∼40 to 60 nmol m−2·s−1 and were significantly greater than values measured in summer) as well as in the wetlands, where maximum values were observed during the warm part of the year (approximately 80 nmol m−2·s−1 or more). The different times of maximum CH4 flux (FCH4) occurrence resulted from the differentiation of processes that determine methane emissions: in the city these are anthropogenic emissions (strongest in winter); in the wetland, natural processes dominate. The diurnal variability of FCH4 was faintly detectable, except for the cold half of the year in the city and the warm half of the year in the wetland. The studied area of the centre of Łodź, in turn, is also characterized by a weekly cycle of methane exchange: the values measured on working days were higher by 6.6% (winter) to 5.6% (summer) than those observed on weekends. Differences in the annual exchange of methane in 2014 were insignificant: the centre of Łodź emitted 17.4 g m−2 net, while the wetland emitted about 18.7 g m−2 net. The results of the measurements performed during the period from January to September 2015, however, suggest that a very dry summer, drying of the soil and, consequently, a reduced water level in the fen significantly reduced the ability of the wetland to emit methane. This pattern was not observed in the city.

The work presents a historical outline and main achievements of Lodz researchers in the field of urban climate. Although regular meteorological measurements in Lodz began at the beginning of the 20th century, the development of urban climatology, understood as the city's influence on the field of meteorological elements, dates back to the 1950s. The last decade of the 20th century and the beginning of the present century brought a special intensification of activities in the field of urban climatology in Lodz. Apart from studies on the classic problems of the urban heat island or solar relations, unique measurements of the heat balance, surface exchange of greenhouse gases (CO2 and CH4) or radon concentration in the city were carried out during this period; the innovative measurement methods (scintillometry) were used and original numerical models were developed.

This paper presents eddy covariance measurements of evapotranspiration and sensible heat flux in wetland area. The rate of evapotranspiration is characterized by a distinct seasonal pattern and assumes the highest values during summer. From April to September, the latent heat flux is characterized by a distinct diurnal pattern with the highest average values (at the noon hours) at a level of ca 200 W·m−2. Moreover, in the period from June to August the values of latent heat flux far outweigh those of sensible heat flux (the Bowen ratio in the noon hours ranging between 0.4 and 0.6) and account for 50–60 % of radiation balance. The average daily totals of evapotranspiration vary from about 0.5 mm·d−1 during the winter months to about 3–3.5 mm·d−1 in summer.

To investigate temporal variability of methane (CH4) fluxes in an urban environment, air–surface exchange fluxes of CH4 were continuously measured using eddy covariance techniques at a city-centre site in Łódź, Poland, from July 2013 to August 2015. In the immediate vicinity of the measurement site, potential methane sources include vehicle traffic, dense sewerage infrastructure and natural gas networks. Sensible and latent heat fluxes have also been measured since 2000 and carbon dioxide fluxes since 2007 at this site. Upward CH4 fluxes dominated during the measurement period, indicating that the city centre is a net source of CH4 to the troposphere. The highest monthly fluxes were observed in winter (2.0 to 2.7 g m−2 month−1) and the lowest in summer (0.8 to 1.0 g m−2 month−1). Fluxes on working days were around 6 % higher than on weekends. The cumulative flux indicates that the city centre emitted a net quantity of nearly 18 g m−2 of CH4 in 2014. Stable values of the FCO2∕ FCH4 ratio in months (minimum 2.41 × 10−3, maximum 5.3 × 10−3) and the lack of a clear annual course suggest comparable magnitude of both fluxes.

The full surface energy balance of the sea water of the Arctic fjord was analysed for the period from May to November 2014 in Hornsund (SW Spitsbergen). The sensible and latent turbulent fluxes were measured with the aid of the open-path eddy covariance method. The measurement site was located in the Wilczek Peninsula (Wilczekodden) on the rocks, right at the seafront. At this location, and with the wind system observed there, the source area of turbulent fluxes was spread over the sea water of the Hornsundfjord. The turbulent fluxes were calculated for 1-h intervals with a standard methodology. Stationarity was checked by three independent tests, and two datasets were analysed independently: data approved by all three tests, and data approved by at least one of the tests. The sensible heat flux, Q H , undergoes a clear seasonal regularity, with downward heat transport (negative fluxes) from July to September (with mean ranges from −30 to −15 Wm−2), reaching close to zero in October, then upward in the other months of the analysed period, reaching maximum in November (50 Wm−2). The latent heat flux, Q E , was mostly positive (more than 90 %) and more intensive. The highest mean values of Q E were recorded in July and November (around 135 Wm−2), with the lowest in May, June and August (around 70 Wm−2). Very intensive latent heat fluxes, above 200 Wm−2, in extreme cases exceeding 500 Wm−2, were observed in all months.

A Met Office/Natural Environment Research Council Joint Weather and Climate Research Programme workshop brought together 50 key international scientists from the UK and international community to formulate the key requirements for an Urban Meteorological Research strategy. The workshop was jointly organised by University of Reading and the Met Office.

Peatlands store substantial amount of carbon, are vulnerable to climate change. To predict the fate of carbon stored in peatlands, the complex interactions between water, peat and vegetations need more attention. This study describes a modified version of the ORCHIDEE land surface model for simulating the hydrology, surface energy and CO2 fluxes of peatlands on daily to annual time scales. The model, referred to as ORCHIDEE-PEAT, includes a separate soil tile in each 0.5° grid-cell, defined from a global peatland map and identified with peat-specific soil hydraulic properties. Runoff from non-peat vegetation with a grid-cell containing a fraction of peat is routed to this peat soil tile, which maintains shallow water tables. The water table position separates oxic from anoxic decomposition. The model is evaluated against eddy-covariance (EC) observations from 30 northern peatland sites, with the maximum rate of carboxylation (Vcmax) being optimized at each site to match the peak of growing season gross primary productivity (GPP), derived from direct EC measurements. Regarding short-term variations from day to day, the model performance was good for the variations in GPP (r2 = 0.76, Nash-Sutcliff modeling efficiency, MEF = 0.76), with lesser accuracy for latent heat fluxes (LE, r2 = 0.42, MEF = 0.14) and Net ecosystem CO2 exchange (NEE, r2 = 0.38, MEF = 0.26). Seasonal variations in GPP, NEE and energy fluxes on monthly scales showed moderate to high r2 values ranging from 0.57 to 0.86. For spatial across-sites gradients of annual mean GPP, NEE and LE, r2 of 0.93, 0.27, and 0.71, respectively, were achieved. The water table variations are not well predicted (r2 < 0.1), likely due to the uncertain water input to the peat from surrounding areas. However, when using the observed water table in the carbon module to define the fraction of oxic and anoxic decomposition instead of the modeled water table, ORCHIDEE-PEAT shows a small improvement in reproducing NEE. Moreover, we found a significant relationship between optimized Vcmax and the latitude (temperature), which can better reflect the spatial gradients of annual NEE than using an average Vcmax value. In a future version of ORCHIDEE-PEAT, the influences of water table on photosynthesis and depth-dependent influences of soil temperature on respiration may be included.

We present the turbulence spectra and cospectra derived frommore than five years of eddy-covariance measurements at two urban sites inŁód´z, central Poland. The fast response wind velocity components were obtained using sonic anemometers placed on narrow masts at 37 and 42 m above ground level. The analysis follows Kaimal et al. (Q J R Meteorol Soc 98:563–589, 1972) who established the spectral and cospectral properties of turbulent flow in atmospheric surface layer on the basis of the Kansas experiment. Our results illustrate many features similar to those of Kaimal et al., but some differences are also observed. The velocity (co)spectra from Łód´z show a clear inertial subrange with −2/3 slope for spectra and −4/3 slope for cospectra. We found that an appropriate stability function for the non-dimensional dissipation of turbulent kinetic energy calculated from spectra in the inertial subrange differs from that of Kaimal et al., and it can be satisfactorily estimated with the assumption of local equilibrium using standard functions for the non-dimensional shear production. A similar function for the cospectrum corresponds well to Kaimal et al. for unstable and weakly stable conditions. The (co)spectra normalized by their spectral values in the inertial subrange are in general similar to those of Kaimal et al., but they peak at lower frequencies in strongly stable conditions. Moreover, our results do not confirm the existence of a clear “excluded region” at low frequencies for the transition from stable to unstable conditions, for longitudinal and lateral wind components. The empirical models of Kaimal et al. with adjusted parameters fit well to the vertical velocity spectrum and the vertical momentum flux cospectrum. The same type of function should be used for longitudinal and lateral wind spectra because of their sharper peak than occurs for the Kansas data. Finally, it should be stressed that the above relationships are well-defined for averaged values. The results for individual 1-h periods are very scattered and can be significantly different from the generalized functions. Funding for this research was provided by the Polish Ministry of Science and Higher Education (State Committee for Scientific Research) under grant no. N306 276935 for the years 2008–2012 and grant no. N306 519638 for the years 2010–2013.

Although there are many reports of scintillometer measurements in natural environments, there are few studies concerning the application of scintillometry in urban areas. The main aim of this paper is to present the results of turbulent sensible heat flux (H) measurements with a large aperture scintillometer (LAS) in ?ód?, Central Poland, conducted during the period September 2009?May 2012. The scintillometer data (HLAS) were compared with the results from two eddy covariance towers (HEC) located in the eastern and western parts of the old city core. HLAS was computed with the free and mix convection assumptions. Overall, HLAS and HEC compare well to each other (R2 higher than 0.84). Large discrepancies were found when characteristics of the source areas differed between LAS and EC, in particular for the western EC station under southern, southwestern and to some extent western inflow conditions. The diurnal cycle of H peaks in June but is smaller by the middle of the summer, which was relatively wet in 2010 and 2011. The good performance of LAS even in winter demonstrated its applicability for long-term monitoring of H in urban areas.

This paper presents the Semi-empirical URban-canopY parametrization SURY, which bridges the gap between bulk urban land-surface schemes and explicit-canyon schemes. Based on detailed observational studies, modelling experiments and available parameter inventories, it offers a robust translation of urban canopy parameters containing the three-dimensional information into bulk parameters. It is extremely suitable for an intrinsic representation of canopy-dependent urban physics in existing bulk urban land-surface schemes of atmospheric models. At the same time, it delivers high efficiency in terms of computational cost for long-term climate modelling and numerical weather prediction. SURY enables versatility and consistency in choosing between the urban canopy parameters from bottom-up inventories and bulk parameters from top-down estimates. SURY is tested for Belgium at 2.8 km resolution with the COSMO-CLM model (version 5.0_clm6) that is extended with the bulk urban land-surface scheme TERRA_URB (version 2). The model reproduces very well the urban heat islands observed from in-situ urban-climate observations, satellite imagery and tower observations, which is in contrast to the original COSMO-CLM model without an urban land-surface scheme. As an application of SURY, the sensitivity of the COSMO-CLM model in terms of land-surface temperatures, air temperatures and associated urban heat islands is quantified for the urban canopy parameter ranges from the Local Climate Zones classification system. On the one hand, their city-scale effect shows that additional urban canopy information has potential for improving regional atmospheric modelling. On the other hand, the model performance and its sensitivity to the different urban canopy parameters largely depend on the temperature quantity considered. Such an ambiguity demonstrates that a multi-variable model evaluation is a requirement for improving and comparing online urban atmospheric modelling strategies.

This paper presents the Semi-empirical URban canopY parametrization (SURY) v1.0, which bridges the gap between bulk urban land-surface schemes and explicit-canyon schemes. Based on detailed observational studies, modelling experiments and available parameter inventories, it offers a robust translation of urban canopy parameters – containing the three-dimensional information – into bulk parameters. As a result, it brings canopy-dependent urban physics to existing bulk urban land-surface schemes of atmospheric models. At the same time, SURY preserves a low computational cost of bulk schemes for efficient numerical weather prediction and climate modelling at the convection-permitting scales. It offers versatility and consistency for employing both urban canopy parameters from bottom-up inventories and bulk parameters from top-down estimates. SURY is tested for Belgium at 2.8 km resolution with the COSMO-CLM model (v5.0_clm6) that is extended with the bulk urban land-surface scheme TERRA_URB (v2.0). The model reproduces very well the urban heat islands observed from in situ urban-climate observations, satellite imagery and tower observations, which is in contrast to the original COSMO-CLM model without an urban land-surface scheme. As an application of SURY, the sensitivity of atmospheric modelling with the COSMO-CLM model is addressed for the urban canopy parameter ranges from the local climate zones of http://WUDAPT.org. City-scale effects are found in modelling the land-surface temperatures, air temperatures and associated urban heat islands. Recommendations are formulated for more precise urban atmospheric modelling at the convection-permitting scales. It is concluded that urban canopy parametrizations including SURY, combined with the deployment of the WUDAPT urban database platform and advancements in atmospheric modelling systems, are essential. ispartof: Geoscientific Model Development vol:9 issue:9 pages:3027-3054 status: published

Atmospheric measurements over 5 years (2005–2010) at two sites in Łodź, central Poland have been analyzed to develop a better understanding of turbulence in urban areas. Fast response wind velocity, temperature, humidity and CO2 concentration were measured using sonic anemometers and gas analyzers, placed on narrow masts at 37 and 42 m above the ground. The measurements were used to calculate standard deviations of each parameter, and were then normalized according to local Monin–Obukhov similarity theory and plotted as a function of stability parameter ζ = z′/L. Results for the wind components show typical scaling with a power law with exponent ±1/3 in the free convection limit, and that approaches a constant value close to neutral stratification. For stable conditions, the constant value in the neutral limit remains the same for stability parameters lower than 0.1–0.2, then increases. The normalized standard deviation of temperature fits the −1/3 law in the free convection limit, approaching a constant value within a stable limit. However, it exhibits hyperbolic characteristics for close to neutral stratification. The normalized standard deviations for humidity and CO2 concentration exhibit scaling similar to the wind components in the unstable regime and remain constant in the stable domain. The results for the wind components and for temperature are in the range of various functions found in other studies. The absolute values for humidity and CO2 concentration seem to be slightly higher, but only single examples of such investigations can be found in the literature.

Coreless winters in the European sector of the Arctic and their synoptic conditions The coreless winters (i.e. not having a cold core) were distinguished in four stations within the European sector of the Arctic. Anomalies of the frequency of the Niedźwiedź's (2011) circulation types were calculated separately for the mid-winter warm months and for cold months preceding and following the warm-spells. Furthermore, composite and anomaly maps of the sea level pressure as s well as anomaly maps of the air temperature at 850 gpm (geopotential meters) were constructed separately for the mid-winter warm events and for the cold months before and after warming. Different pressure patterns were recognized among the days of mid-winter warm spells, using the clustering method. The occurrence of coreless winters in the study area seems to be highly controlled by the position, extension and intensity of large scale atmospheric systems, mainly the Icelandic Low. When the Low spreads to the east and its centre locates over the Barents Sea the inflow of air masses from the northern quadrant is observed over the North Atlantic. This brings cold air of Arctic origin to the islands and causes an essential drop in the air temperature. Such situation takes place during the cold months preceding and following the warm mid-winter events. During the warm spells the Icelandic Low gets deeper-than-usual and it is pushed to the northeast, which contributes to the air inflow from the southern quadrant.

The primary aim of this paper was to present the results of turbulent sensible heat flux (QH) measurements with Scintillometer Scintec BLS900 (LAS). The theoretical background of scintillation method has been presented as well as two different ways of sensible heat flux computation from LAS. The measurements presented here were conducted from April to December 2011. Diurnal and partially annual variability of QH has been analyzed, moreover the mean diurnal course of QH in considered months has been prepared. In order to choose the optimal method of QH computation from LAS the results obtained with three different Monin-Obuchov similarity theory functions as well as free convection assumption has been compared. In first case it has been proved that sensible heat flux differs significantly depending on chosen universal function. In turn in case of free convection assumptions it has been revealed that it is valid only for very unstable conditions. Therefore it does not apply to long-time heat flux measurement in case of Łódź. Nevertheless, this study has shown that scintillation method is valuable and reasonable source of path-averaged QH estimates in considered city.

Continuous measurements of carbon dioxide turbulent flux F carried out with the eddy covariance method have been made in Łodź since F July 2006. The measurement point (Lipowa Station) is located in the west part of the densely built-up city centre, where artificial surfaces clearly prevail over natural terrain. The measurement system includes a Kipp and Zonen's (Delft, Holland) CNR1 net radiometer, RMYoung (Traverse City, Michigamn, USA) 81000 sonic anemometer and Li-cor (Lincoln, Nebraska, USA) 7500 open path H2O/CO2 infrared gas analyser. Sensors are installed on the high tower 20 m above the roof of the building and 37 m above the ground, so the measurement height exceeded the urban canopy layer more than twice. The diurnal and annual variability of carbon dioxide flux for the period July 2006 to August 2008 is analysed in the article. The results show the characteristic features in diurnal and annual courses of F. Independently from the season, positive (upward) fluxes of the order of 0–15 µmol m−2 s−1 prevail in the data. During the cold season, an increase in turbulent CO2 exchange is observed (F quite often exceeded 30 µmol m−2 s−1). This can be attributed to anthropogenic CO2 emissions, which are particularly strong in winter due to, among other things, mineral fuel combustion during domestic heating. The average monthly fluxes are positive in all seasons, which means that emission of CO2 in the surroundings of the measurement point prevails over its uptake. Apart from the season, the maximum flux occurred during the day and the minimum during the second part of the night. Wintertime monthly averaged fluxes are much higher than summertime ones. The observed increase in CO2 exchange during weekdays in comparison with weekends can be caused by the weekly rhythm of traffic in the surroundings of Lipowa Station. Copyright © 2010 Royal Meteorological Society

Urban land surface schemes have been developed to model the distinct features of the urban surface and the associated energy exchange processes. These models have been developed for a range of purposes and make different assumptions related to the inclusion and representation of the relevant processes. Here, the first results of Phase 2 from an international comparison project to evaluate 32 urban land surface schemes are presented. This is the first large-scale systematic evaluation of these models. In four stages, participants were given increasingly detailed information about an urban site for which urban fluxes were directly observed. At each stage, each group returned their models' calculated surface energy balance fluxes. Wide variations are evident in the performance of the models for individual fluxes. No individual model performs best for all fluxes. Providing additional information about the surface generally results in better performance. However, there is clear evidence that poor choice of parameter values can cause a large drop in performance for models that otherwise perform well. As many models do not perform well across all fluxes, there is need for caution in their application, and users should be aware of the implications for applications and decision making. Copyright  2010 Royal Meteorological Society

Extra-tropical cyclone frequency and intensity are Currently under intense scrutiny because of the destruction recent windstorms have brought to Europe, and because they are a major meridional heat transport mechanism that may respond to differential latitudinal warming trends. Several studies using reanalysis data covering the second half of the 20th century Suggest increasing storm intensity in the northeastern Atlantic and European sector. Fewer analyses cover a longer time period but show different trends or point towards the dominance of interdecadal variability instead of ally clear trends. Hence, it is relevant to analyse cyclone variability over as long a period as possible. In this Study, we analyse interdecadal variability in cyclone activity over northwestern Europe back to AD 1780 by combining information from eight storminess indices applied in all Eulerian framework. These indices, including four new approaches towards gauging cyclone activity, use the series of thrice-daily sea level pressure observations at Lund and Stockholm. We find pronounced interdecadal variability in cyclonic activity but no significant overall consistent long-term trend. The major interdecadal-scale variability common to all indices is in good agreement with geostrophic wind reconstructions for NE Atlantic and NW Europe, and with variations in the North Atlantic oscillation (NAO). Our results show that the reanalysis studies cover a time period chiefly coinciding with a marked, but not exceptional in our 225-year perspective, positive variation in the regional cyclone activity that has more recently reversed. Because of the interdecadal variations, a near-centennial time perspective is needed when analysing variations in extra-tropical cyclone activity and the associated weather conditions over northwestern Europe. Copyright (C) 2009 Royal Meteorological Society

The paper focuses on the absorption of shortwave radiation in an urban street canyon. To test the effective albedo of the canyon an analytic solution of the multiple reflection problem is applied. The infinitesimally long canyon is divided into slices and a matrix of view factors for the slices is defined. Incoming shortwave radiation includes direct and diffuse parts and shadowing effects are included in the analysis. The model is validated against Aida’s (1982) scale model data and measurements in a real canyon. The results demonstrate a rapid decrease of the effective albedo as the canyon aspect ratio (its height to width, H/W) are increased. It is also shown that diurnal changes of the effective albedo can be very complex depending on the particular combination of H/W ratio, surface reflectivity and canyon orientation.

Surface properties, such as roughness and vegetation, which vary both within and between urban areas, play a dominant role in determining surface–atmosphere energy exchanges. The turbulent heat flux partitioning is examined within a single urban area through measurements at four locations in Łódź, Poland, during August 2002. The dominant surface cover (land use) at the sites was grass (airport), 1–3-story detached houses with trees (residential), large 2–4-story buildings (industrial), and 3–6-story buildings (downtown). However, vegetation, buildings, and other “impervious” surface coverage vary within some of these sites on the scale of the turbulent flux measurements. Vegetation and building cover for Łódź were determined from remotely sensed data and an existing database. A source-area model was then used to develop a lookup table to estimate surface cover fractions more accurately for individual measurements. Bowen ratios show an inverse relation with increasing vegetation cover both for a site and, more significant, between sites, as expected. Latent heat fluxes at the residential site were less dependent on short-term rainfall than at the grass site. Sensible heat fluxes were positively correlated with impervious surface cover and building intensity. These results are consistent with previous findings (focused mainly on differences between cities) and highlight the value of simple measures of land cover as predictors of spatial variations of urban climates both within and between urban areas.

Energy fluxes have been measured over an area near the centre of the city of Łodź, Poland, since November 2000. The site was selected because the building style (surface cover and morphology) is typical of European cities, yet distinct from the majority of cities where energy balance observations have been studied thus far. The multi-year dataset permits consideration of temporal changes in energy balance partitioning over a wide range of seasonal and synoptic conditions and of the role of heat storage and anthropogenic fluxes in the energy balance. Partitioning of net radiation into the turbulent fluxes is consistent in the two years, with the largest differences occurring due to differing precipitation. The monthly ensemble diurnal cycles of the turbulent fluxes over the two years are similar. The largest differences occur during the July–September period, and are attributable to greater net radiation and lower rainfall in 2002. The latent heat flux accounts for approximately 40% of the turbulent heat transfer on an annual basis. The average daily daytime Bowen ratio and its variability are slightly reduced during the summer (growing) season. Anthropogenic heat is a significant input to the urban energy balance in the winter. The fluxes observed in this study are consistent with results from other urban sites.

Data from two automatic stations in Łodź (one urban and one rural) for the period 1997–2002 are analyzed to reveal urban–rural contrasts of such parameters as air temperature, relative humidity, water vapour pressure and wind speed. Under favourable weather conditions the highest temperature differences between the urban and rural station exceeds 8 °C. Relative humidity is lower in the town, sometimes by more than 40%. Water vapour pressure differences can be either positive (up to 5 hPa) or negative (up to −4 hPa). Wind speed at the urban station is on average lower by about 34% in night and 39% during daytime. Regression analysis shows that for rural winds lower than 1.13 m s−1 urban winds can be stronger than rural speeds. Attention has also been paid to singularities in the course of the analyzed parameters over 24 hour periods. It is shown that the typical course of the urban heat island intensity under favourable conditions is similar in all season. Four stages of this course have been distinguished. Wind speed differences also seem to change in a typical way. Case studies show that humidity contrasts, unlike temperature, can evolve in different ways under fine weather conditions. Types of relative humidity evolution are proposed.

The role of net heat storage ΔQS and anthropogenic heat QF are considered in the surface energy balance for a downtown area in Łodź, Poland, for a 2 year period. Eddy covariance measurements provide estimates of the turbulent heat fluxes and radiometric measurements of the net all-wave radiation. A method to determine ΔQS based on representative surface temperature sampling is employed and compared with results from two other models. Results show that ΔQS is an important flux on the scale of hours to days and that it can be more than 10 W m−2, on average, for periods of a week or more. By incorporating ΔQS estimates over hourly intervals, QF was then determined as the residual of the energy balance. Using the approach, QF averaged 32 W m−2 from October to March (60% of available energy), and −3 W m−2 from June to August. The physically unrealistic negative values for the summer period may suggest underestimation of turbulent fluxes, but no causal factor was identified. Although energy balance closure was close to 100% throughout the year, there was weaker agreement in the winter. This is attributed to errors in estimates of ΔQS and variation in QF. Results highlight the need for future investigations of the urban surface energy balance to incorporate more complete measurements and estimates of ΔQS. Copyright © 2005 Royal Meteorological Society.

The investigations which have been conducted out for many years permit the statement that in the climate of Poland the occurrence of the UHI is the most important feature of urbanized areas. Over 80% of nights are characterized by surplus heat in towns, amounting to 2–4°C, and sporadically to 8°C and more. A distinct annual course of the UHI intensity is observed; the greatest differences occur during summer nights when skies are clear. In winter, favorable conditions for the existence of the UHI occur less frequently, and the intensity of the UHI is smaller. In the daytime the city is very often colder than its surroundings. Two types of spatial structure of the heat island have been observed: multi-cellular type, occurring on windless conditions (0–1 m/s), and a more simple general type with the 2–4 m/s wind. The distribution of temperature on the night of February 2/3 and 5/6 1996 is presented; the UHI reached an extremely high intensity then (up to 12°C).

By uni- and bimodal mean vector analysis it is concluded that the decadal pattern of surface-wind-direction frequencies in Lund, southern Sweden, has changed substantially between 1741 and 1990. Trend analysis shows that statistically significant decreasing trends for north and north-east winds are present during all seasons, whereas a decline in easterly wind is most prominent in July-September. These trends are paralleled by increases of south-east (January-May and August-November) and south-west winds (April-September and November-January). These trends promote milder autumns, winters, and springs and a somewhat cooler summer. The investigation period is divided into three phases on the basis of similar flow patterns, which in general terms agree with Scandinavian temperature records: (i) 1741–1840-a continental low-zonality period featuring extreme bimodal flow of wind with relatively high frequencies of east winds, which indicate a continental climate. Warm summers and cold winters are produced by this flow pattern. The severity of winters was enhanced by a high frequency of north and north-east winds. (ii) 1841–1910-a transition period with decreased continentality and low yearly temperatures. This circulation type caused a temperature drop except during winter. (iii) 1911–1990-a maritime high-zonality period featuring further strengthening of the westerly flow. A warm maritime phase with least expressed warming in summer. Severe winters are further counteracted by low frequencies of north and north-east winds.