Ewa Bednorz

Comparison and Validation of Selected Evapotranspiration Models for Conditions in Poland (Central Europe)

The FAO-56 Penman-Monteith (PMF56) model is accepted as the standard method for estimating reference crop (grass) evapotranspiration. However, poor data availability limits the use of this method in many regions. This study aimed to overcome this limitation (1) by evaluating the goodness-of-fit of selected simple evapotranspiration methods in relation to the PMF56 model and (2) by adjusting four of these models and developing a multiple regression equation for conditions in Poland using the PMF56 model as a reference. Using daily meteorological data, four simple models were calibrated by the generalised reduced gradient (GRG) method; moreover, a multiple regression equation was developed on the basis of Cochrane-Orcutt estimation because the error term of the ordinary least squares model was autocorrelated. When radiation data were lacking and sunshine duration data were available, the PMF56 method with a calibrated Ångström-based estimation of solar radiation was the most accurate method (relative error, RE = 0.096, for Sulejów). When neither radiation nor sunshine duration data were included in the dataset, the calibrated Penman method performed best (RE = 0.254). Moreover, when only temperature or pan evaporation data were available, the calibrated Hargreaves method provided the most accurate results (RE = 0.275). Much poorer results were obtained using the calibrated pan coefficient method (RE = 0.435). Ultimately, we developed a multiple regression method in this study that exhibited good performance (RE = 0.170). Taking into account the data limitations, the methods calibrated in this study are recommended for estimating evapotranspiration in Central Europe. This approach would substantially decrease the errors produced by the recommended non-calibrated equations.

Climatology of Hail in Central Europe

Abstract Taking into consideration the spatial variability of the occurrence of hail in central Europe in the warm season (April - September) during the years 1966-2010, five hail regions were distinguished by Ward’s group hierarchy method. For this purpose the daily data of hail occurrence were used. Circulation patterns responsible for hail precipitation at each cluster of stations were analysed using Reanalysis data (NCEP/NCAR). Composite maps of the sea-level pressure, 500 hPa geopotential height and 850 hPa-level temperature means were constructed for the days with hail. Additionally, anomalies of the values of each parameter were presented. It was found that more than 65% of hail events were recorded from April to June, with May as the hail-peak month. In all five distinguished regions hail precipitation was associated with negative anomalies of SLP and 500 hPa heights over Europe, most often with low-pressure systems developing within colder than usual air mass.

Synoptic conditions of snow occurrence in Budapest

Daily circulation patterns responsible for snow occurrence in Budapest were analysed. Three types of circulation patterns were distinguished, using a Principal Component Analysis and clustering technique, for days with snowfalls and snow cover persistence. Low pressure systems located over the Italian Peninsula (south-west of Budapest) are characteristic for days with snowfalls. Such lows coming from the west generate warm fronts and are a source of humidity necessary for the snowfalls. At the same time, higher pressure extends north or east of the Carpathian Basin and brings polar continental air masses and consequently low temperature. Snow persistence also requires temperature below zero. This condition is fulfilled at the airflow direction from the east (or south east) which brings frosty air from the interior of the continent. A winter high pressure system extending over the Carpathian Basin also generates low temperature, mainly due to the nocturnal radiation conditioned by a clear sky.

Snow depth in eastern Europe in relation to circulation patterns

Abstract Rotated principal components of the 500 hPa geopotential heights in the Euro-Atlantic sector are used as indicators of circulation pattern intensity. Daily snow-cover depth data for the years 1951–95 from 71 eastern European stations are examined. Maps of linear correlation coefficient between monthly change in snow depth and rotated principal components are presented. The positive and negative extremes of each circulation pattern are analyzed, and positive and negative snow-depth signals indicated. A daily analysis of relationships between snow depth and circulation pattern is performed for three locations. The strongest impact of the atmospheric circulation on changes in snow depth is observed in the south and west of the study area, where the eastern European (EE) and central European circulation patterns are found to have the greatest impact. The North Atlantic Oscillation (NAO) impact on the snow depth in eastern Europe is limited to the beginning and the end of winter. Snow cover has low variability in northeastern Europe (where the Scandinavian (SC) pattern is of greatest importance) and low sensitivity to change in the atmospheric circulation. The decrease in snow-cover depth observed in spring is related to the NAO, SC and EE patterns, the latter being important for snow-cover depth fluctuations over northeastern Europe in April.

Synoptic conditions of heavy snowfalls in Europe

Abstract Synoptic conditions of heavy snowfalls at four stations in urope (slo, remen, molensk and udapest) were analyzed using daily data of snow cover depth from 50 winter seasons (1960/1961–2009/2010). Composite maps of the sea level pressure and 500 geopotential height means and anomalies were presented for the days with an increase in snow depth by ≥10 cm, for each station separately. In the same way, maps were presented for the 850-hPa temperature and the precipitable water content. Additionally, 48 h back trajectories of air masses for the chosen days with the highest amount of snowfall were constructed, using the model. Negative anomalies of sea level pressure and 500-hPa heights, which correspond to low‐pressure systems spreading over urope, are prerequisites for the occurrence of heavy snowfalls at each analyzed location. In udapest, heavy snowfalls appear as a result of cyclonic cold fronts from the editerranean ea. Another typical location of snow‐bringing low‐pressure systems is the altic ea region. Negative pressure anomalies over the orth tlantic with a low‐pressure centre over the orwegian ea cause snowfalls in slo. There are two main sources of air masses bringing abundant snowfalls in urope: the tlantic region and editerranean. According to back trajectories, the humid air masses are transported over long distances and on the way they are shifted from the low troposphere in the source regions to the upper layers.

Warm spells in Northern Europe in relation to atmospheric circulation

This study describes warm spells in Northern Europe and determines the synoptic situations that cause their occurrence. In this article, a relatively warm day was defined as a day when the maximum temperature exceeded the 95th annual percentile, and a warm spell (WS) was considered to be a sequence of at least five relatively warm days. In the analysed multiannual period and within the investigated area, 24 (Kallax) to 53 (Oslo) WSs were observed. The occurrence of WSs was mainly connected with positive anomalies of sea level pressure and a 500-hPa isobaric surface, displaying the presence of high-pressure systems. This occurrence was also accompanied by positive T850 anomalies.

A white christmas or a christmas thaw? : Changes in snow cover depth in German-Polish lowlands during the last decade of December against daily circulation patterns

Snow cover occurrence in the German-Polish lowlands during the third decade of December in years 1971-2000 was investigated. Synoptic maps and anomaly composite maps for the days with snowfalls and for the thaw days have been constructed based on the daily sea level pressure and 500 hPa geopotential heights data. The decrease in the snow cover depth is related to lows over the east Atlantic and west Europe, causing south-westerly airflow. Three types of circulation favourable for snow cover formation have been distinguished, using the k-means cluster analysis. Two of them cause air advection from the north. The third type is characterized by high pressure gradient and a dynamic, deep low shifting from the west to central Europe.

The occurrence of coreless winters in central Spitsbergen and their synoptic conditions

Ten warm-core winters were distinguished in the period 1975/76–2007/08, drawing on data from the Svalbard Airport weather station in central Spitsbergen and using the conditions t m 1t m+1 (t is the monthly mean temperature while m 1, m and m+1 denote subsequent winter months) and the parameter of a coreless rate, c r (the ratio of the second, a 2, to the first, a 1, harmonic of the annual temperature wave). Composite maps of sea-level pressure (SLP) and z 500-hPa means, and anomaly maps, were constructed separately for the mid-winter warm events and for the cold months before and after warming. Using the clustering method, different pressure patterns were recognized among the days of mid-winter warm spells. The occurrence of coreless winters in central Spitsbergen seems to be largely controlled by the position, extent and intensity of large-scale atmospheric systems, mainly the Icelandic Low. When the low spreads to the east and its centre is over the Barents Sea the inflow of air masses from the northern quadrant is observed over central Spitsbergen. This inflow of cold air of Arctic origin to the island takes place during the months preceding and following warm mid-winter events. Different circulation conditions appear when the Icelandic Low gets deeper than usual and shifts northwards, which coincides with warm spells. Sometimes a secondary cyclonic centre is located over Fram Strait and its warm sector encompasses Svalbard.

Atmospheric conditions of intense thaws in the Polish lowlands

Synoptic conditions of daily changes in the snow cover depth by ≥ 5 cm were analyzed. Negative pressure anomalies appear over the North Atlantic and Scandinavia, which means low pressure systems moving over Europe along a northerly path. At the same time, positive pressure anomalies appear over the Mediterranean, indicating the expansion of the Azorean High. Such distribution of anomalies increases the horizontal baric gradient, which results in the intensif cation of the western and south western f ow and stronger than usual winds from the SW quadrant. Transport of warm and humid air masses from the south-west brings about thawconducive conditions in central Europe: an increase in temperature and a considerable increase in the content of precipitable water, which means abundant precipitation. Different circulation types favorable for intense thawing were distinguished using the method of the hierarchical grouping. Two of them are characterized by the presence of deep and widespread cyclonal systems located north, north-west or west of the researched area. Less frequently, the decrease in the depth of snow cover occurs in the presence of the high located west over the Atlantic Ocean, which brings air masses from the north western direction. Such situations occur most frequently towards the end of winter.

Review of Polish contribution to snow cover research (1880–2017)

Abstract The purpose of this article is to present the development of multifaceted research on snow cover conducted by Polish researchers in various parts of the world since the end of the 19th century up to the modern times. The paper describes Polish studies on physical and chemical properties of snow cover, its long-term changes, relationships between snow cover and climate, impact of snow cover on environmental conditions and human activity. This work is also an attempt to show the contribution of Polish snow-related research to the international achievements in this fields.