Short Communication A synoptic analysis of the diurnal cycle of thunderstorm precipitation in Kraków (Southern Poland)

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1 INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 30: (2010) Published online 26 May 2009 in Wiley InterScience ( DOI: /joc.1960 Short Communication A synoptic analysis of the diurnal cycle of thunderstorm precipitation in Kraków (Southern Poland) Robert Twardosz* Department of Climatology, Jagiellonian University, ul. Gronostajowa 7, Kraków, Poland ABSTRACT: A study was made of the relationships between the diurnal pattern of thunderstorm precipitation in Kraków and the types of air masses and atmospheric fronts prevailing over southern Poland during the period The results of these investigations contribute to a better understanding of the physical processes taking place in the atmosphere that are associated with convective rainfall and may be used to improve the forecasting of the temporal distribution of convective precipitation. The elementary parameters of the daily precipitation cycle were identified, such as the amplitude and phase of the frequency and total thunderstorm precipitation. A separate analysis was performed of air mass and frontal precipitation. The 24-h thunderstorm precipitation cycle depends on the synoptic situation that determines the thermal and humidity properties of air masses. There is an afternoon peak in Kraków s general diurnal pattern of thunderstorm precipitation that is primarily linked to cold fronts and to air mass precipitation occurring in polar maritime air. Thunderstorm precipitation linked to warm fronts is less frequent and is concentrated in the evening. Copyright 2009 Royal Meteorological Society KEY WORDS hourly precipitation; thunderstorm; synoptic situation Received 23 October 2007; Revised 21 April 2009; Accepted 22 April Introduction The studies available concerning diurnal precipitation patterns confirm their considerable regional and seasonal diversity. A variety of patterns is reported by American (Wallace, 1975; Winkler, 1987; Winkler et al., 1988; Dai, 2001), Japanese (Fujibe, 1988; Oki and Musiake, 1994) and European researchers (Svensson et al., 2002; Twardosz, 2007a, 2007b). The literature on the subject, however, provides scant documentation of any relationship between daily precipitation and synoptic situations. The studies in this area include those by Faiers (1988, 1993) and Faiers et al. (1994) dealing with the intensity and frequency of hourly precipitation events in Louisiana compared with the synoptic situations prevailing in the corresponding parts of North America. The authors concluded that atmospheric fronts played a leading role in the shaping of the 24- h precipitation patterns. Svensson et al. (2002) analysed the characteristics of heavy precipitation at Eskdalemuir (Scotland) in relation to Mayes weather types (Mayes, 1991). The authors demonstrated that there were links between the morning peak precipitation and warm fronts and between the afternoon peak and cold fronts. Three recent publications by Twardosz (2005, 2007a, 2007b) * Correspondence to: Robert Twardosz, Department of Climatology, Jagiellonian University, ul. Gronostajowa 7, Kraków, Poland. r.twardosz@uj.edu.pl about the influence of the synoptic situations identified by Niedźwiedź (1981, 2007) on diurnal precipitation patterns were based on the 117-year-long series of pluviographic records made in Kraków that provide a good representation of the climatic conditions in southern Poland. The studies offer a wide discussion of the diurnal precipitation cycle broken down by seasons, precipitation types, air masses, atmospheric fronts and circulation types. The present study is restricted to thunderstorm precipitation, i.e. precipitation indicative of convection activity, and aims to analyse Kraków s hourly precipitation data in the period spanning The author focuses on two issues: (1) determination of the amplitude and phase of the diurnal thunderstorm precipitation cycle and (2) an investigation of the relationship between these parameters on the one side and the thunderstorm type (air mass or frontal) on the other. None of the three publications by Twardosz cited above took a selective view of thunderstorm precipitation from a synoptic perspective. They only considered overall 24-h thunderstorm precipitation cycles in terms of their frequency and totals during the period (Twardosz, 2007a). When relating daily synoptic situation to precipitation frequency or total, it is helpful to know whether precipitation occurs throughout the day, or mainly only during part of it, and what precipitation-producing mechanisms may be at work (Svensson et al., 2002). Benefits from such a study include an insight into the efficiency of such Copyright 2009 Royal Meteorological Society

2 DIURNAL CYCLE OF THUNDERSTORM PRECIPITATION IN KRAKÓW 1009 mechanisms (Twardosz, 2007b) that is useful in forecasting heavy rainfall. Other natural scientists in addition to climatologists, for instance hydrologists and meteorologists, also investigate this element of the climate. They all need detailed precipitation data, including its variation in time. Information on the variation of thunderstorm precipitation with time is also an essential input for water management and flood prevention in urban areas such as the city of Kraków. 2. Sources and methods The study is based on a long and consistent record of pluviographic measurements taken at the Jagiellonian University s Astronomical Observatory in Kraków (ϕ = N, λ = E, h = 206 m a.s.l.) and on the documentation of synoptic situations based on the collection of sea level synoptic maps covering the period that was built up by Niedźwiedź (2003, 2007). Days with thunderstorm precipitation, i.e. featuring lightning and thunder, were selected for the study. Data covering 00 : : 00 Central European time (CET) were used for the analysis to determine if a thunderstorm had occurred at any time during the day. This assumption is consistent with the definition of a thunderstorm found in the Secretariat of the World Meteorological Organization (1992). Determination of the diurnal thunderstorm precipitation cycle involved identification of its main parameters, i.e. amplitude (A) and phase (T max ), as related to precipitation frequency and totals. In this study, precipitation frequency is determined hourly and refers to the number of events in which the total precipitation was at least 0.1 mm in that hour. In each 1-h interval, precipitation frequency was presented as a relative value defined as a percentage of the total number of precipitation events 0.1 mm h 1. Similarly, the diurnal cycle of precipitation totals is presented in relative values, i.e. the hourly precipitation values normalised by the 24-h precipitation total. The amplitude of the frequency of occurrence and of the total precipitation is defined as the difference between the maximum and average values, and is calculated according to the following formula: A = P max P P 100% where P max is the maximum value within 1-h intervals and P is the 24-h average value of a given precipitation characteristic. This definition of the normalised amplitude is a simplified adaptation of the method used by Wallace (1975) and Dai (1999), where the normalised amplitude is obtained after fitting harmonics to the diurnal series. In this way, the amplitude, expressed as a percentage, provides information about the magnitude of the diurnal cycle, as compared with the 24-h average. The error margins of the author s amplitude estimation formula are also quoted, while further details about the amplitude determination can be found in his recent publications (Twardosz, 2005, 2007a, 2007b). Phase T max of the diurnal cycle means the peak hourly precipitation frequency and total in a 24-h period and is expressed in CET (coordinated universal time, UTC +1). Out of the extensive hourly data set, two subsets were defined representing thunderstorm precipitation type: air mass and frontal. These were identified using Niedźwiedź s (2003, 2007) synoptic situation documentation reflecting atmosphere dynamics over southern Poland, i.e. between N and E. This documentation is a catalogue of air mass and atmospheric fronts prevailing on each single day from 1951 to Surface synoptic charts of Europe from 00 : 00 to 12 : 00 UTC provided the basis for this classification. The author used Polish synoptic maps, published by the Institute of Meteorology and Water Management, and the German Europäischer Wetterbericht and Täglicher Wetterbericht. The air mass types subjectively identified by Niedźwiedź (2003) are associated with the classification of the air mass source area (known as a geographical classification ) generally accepted in meteorological services, i.e. arctic, polar maritime, polar continental and tropical. Cases in which more than one air mass was present during a 24-h period were treated separately. Niedźwiedź identified four types of fronts in his atmospheric front classification: (1) warm, (2) cold, (3) occluded and (4) stationary. Again, he treated days with more than one front separately. He did not differentiate between the different types of occlusion in occluded fronts. Each thunderstorm precipitation day was attributed a relevant air mass or an atmospheric front from the catalogue of weather situations by Niedźwiedź. Only days featuring a single air mass and a single front were analysed. The air mass and atmospheric front data were received directly from Professor Niedźwiedź asa computer file. (This file is available at the University of Silesia, Department of Climatology, Sosnowiec, Poland.) 3. Results Thunderstorm precipitation occurs on average on 20 days per year in Kraków during the period and accounts for 12% of all precipitation days. Thunderstorms produced 190 mm of rain a year on average and contributed 28% to the annual total. The greatest seasonal concentration of thunderstorm precipitation is in the summer occurring on 32% of the precipitation days within that period. This type of rainfall is caused by deep convection leading to a build-up of Cumulonimbus clouds and to intensive showers. Upwards air movement depends on the diurnal solar radiation patterns. The diurnal pattern of thunderstorm precipitation frequency and totals has a characteristic T max in the afternoon around 15 : : 00 CET (Figure 1). Similar patterns of high-intensity rainfall were also reported by British (Svensson et al., 2002) and Japanese researchers

3 1010 R. TWARDOSZ Table I. Number of days with thunderstorm precipitation by air masses and front types, Air masses Fronts Arctic 16 (3.6) Warm 99 (16.2) Polar maritime 292 (64.7) Cold 408 (66.7) Polar continental 115 (25.5) Occlusion 68 (11.1) Tropical 28 (6.2) Stationary 37 (6.0) Total 451 (100) Total 612 (100) Percentage values are given in parenthesis Figure 1. Diurnal distribution of thunderstorm precipitation. (Fujibe, 1988). The precipitation totals T max coincides in time with the frequency T max. Nearly half of the total thunderstorm precipitation is concentrated within 6 h (14 : : 00 CET). The overall picture of the 24- h thunderstorm precipitation pattern, shown on Figure 1, is basically produced by a combination of the summer pattern and that of spring and autumn. In summer, the long days favour atmospheric convection, and thunderstorm precipitation tends to last until late afternoon. In spring and autumn, the shorter day and the related earlier peaking of received solar radiation create the best convective conditions in the early afternoon (Twardosz, 2007a). In the light of the amplitude values (Figure 1), the 24- h cycle is much more pronounced in the thunderstorm precipitation totals than in their frequency. The 24-h amplitude of the totals is more than double the 24-h average. According to Easterling and Robinson (1985), normalised amplitude values more than 100% represent a well-defined time of diurnal maximum with little occurrence at other times. In areas with features of a continental climate, the 24-h precipitation cycle is mainly driven by free thermal convection. In uniform air masses, the occurrence of thermal thunderstorms in the afternoon is one of the manifestations of convective activity. The location of the study area within the polar front zone causes most precipitation events to be driven by cyclonic systems moving from west to east and carrying extensive systems of atmospheric fronts. Frontal thunderstorm precipitation was found to account for 58% of the total thunderstorm precipitation days in Kraków. The remaining 42% of cases represent air mass precipitation. Air mass thunderstorm precipitation occurs primarily in polar maritime air mp (65% cases) followed by much rarer cases of polar continental air cp (25%) (Table I). Frontal thunderstorm precipitation, on the other hand, is mostly linked with cold fronts (67%), whereas warm front cases represent a minority of cases (16%) (Table I). The prevalence of the cold front linked thunderstorm precipitation is a result of the frequency of occurrence of cold fronts that is twice as high as that of warm fronts over southern Poland (Twardosz, 2005). The likelihood of precipitation developing on a cold front is also much higher because of the stronger convection dynamics linked with this type of front. The main characteristics of the two types of thunderstorm precipitation are shown in Table II. Obviously, frontal precipitation events tend to be longer and yield more rain, whereas the average intensities show no statistically significant differences. The greater yield from frontal thunderstorms comes mainly from intensive thermal and dynamic vertical air currents developing in the frontal zone. Moreover, rain cloud systems in this zone tend to be much more protracted, especially on cold fronts, than the isolated systems of Cumulus clouds forming as a result of free thermal convection. Both types of thunderstorm precipitation have their daily T max of frequency and totals at 15 : : 00 h CET (Figure 2). Air mass thunderstorms tend to yield most precipitation in the afternoon, as manifested in greater amplitudes of both the precipitation characteristics studied. As much as 45% of air mass thunderstorm precipitation falls within a 5-h range, 12 : : 00 CET. In uniform air masses, vertical currents develop as a result of radiation and thermal factors resulting in daily air temperature fluctuations. Free air mass convection develops very rapidly, particularly in the summer months, and the resulting isolated systems of Cumulus clouds pose no significant barrier to a further development of the ascending currents. In the late afternoon, the conditions for the development of convection quickly disappear, as does the likelihood of a wet thunderstorm. Indeed, there is a considerable drop in the daily precipitation pattern after 17 : 00 CET that is very visible in the totals. The parameters of the 24-h air mass thunderstorm precipitation cycle may differ considerably from those Table II. Selected characteristic of thunderstorm precipitation, Thunderstorm type Mean duration (min) Mean depth (mm) Mean intensity (mm h 1 ) Air mass 140 ± ± ± 0.2 Frontal 194 ± ± ± 0.2

4 DIURNAL CYCLE OF THUNDERSTORM PRECIPITATION IN KRAKÓW 1011 Figure 2. Diurnal distribution of thunderstorm precipitation for the frontal (top) and air mass precipitation (bottom). described above depending on the type of air mass (Figure 3). In a cp air mass, the amplitude of both characteristics is more than twice as high and T max falls smoother than that of mp air. In a cp air mass, the cloud formation and precipitation formation processes are much slower because of low air humidity, despite insolation conditions conducive to the development of convection. The related thunderstorm precipitation is limited to a short period in the afternoon. Nearly half of the precipitation totals falls within 3 h, 14 : : 00 CET. There is a sharp T max at 16 : : 00 CET. This later timing can be explained by the longer period of time necessary for the development of ascending currents in continental air due to the higher elevation of the condensation level. The dry cp air masses are transported to southern Poland primarily by eastern and south-eastern circulation (Niedźwiedź, 1981). The polar maritime air, mp, coming mainly from the Atlantic, is more humid and therefore more unstable. This creates conditions for precipitation events to occur at any time of day or night. In the afternoon, a thermal stimulus from the heated ground greatly speeds up the development of raindrops. Nevertheless, the 24-h precipitation cycle of mp air is much smoother than that of cp air. Figure 3. Diurnal distribution of thunderstorm precipitation for the polar continental, cp (top), and for the polar maritime air masses, mp (bottom). The 24-h frontal thunderstorm precipitation cycle (Figure 2) features a noticeable T max during the hours 15 : : 00 CET. There is also a high concentration of this precipitation type, as opposed to air mass thunderstorms, in the evening. This is explained by differences between the 24-h cycles of cold and warm fronts, especially in the case of T max (Figure 4). The type of atmospheric front mainly determines the slope of the frontal surface and the related build-up of rain clouds. Cold-front thunderstorms feature a sharper daily cycle of both precipitation characteristics. Dynamic convection on the cold front leads to the development of storm clouds and to high-intensity precipitation. The latter can still increase if the cold front meets heated ground in the afternoon because the clouds ahead of the front do not restrict insolation. The pattern is different when considering warm fronts, with the most intensive precipitation developing in the evening and a characteristic T max of the totals during the hours 20 : : 00 CET. A well-developed multi-layered frontal cloud system has a retarding effect on the formation of free thermal convection during the day. In contrast, the ground effect acts as a stimulus and increases towards the end of the day. This type of warm front precipitation distribution primarily occurs in Kraków in connection with

5 1012 R. TWARDOSZ because of the much smaller role it plays in the overall thunderstorm precipitation pattern, it has a far smaller impact on the overall 24-h pattern. The climatography of thunderstorm precipitation is an essential input for better forecasting of this phenomenon. To achieve a fuller knowledge in this area, further research on convective precipitation should link such events with instability indices, such as convective available potential energy and convective inhibition. Acknowledgements I am deeply grateful to Professor Tadeusz Niedźwiedź from Silesia University for making the catalogue of air mass and atmospheric front types for available to me. I appreciate the constructive comments by three anonymous reviewers. I thank Mr Pawet Pilch and Dr Martin Cahn for reviewing the English. This work was partially supported by the Polish Ministry of Science and Higher Education under grant No. N N Figure 4. Diurnal distribution of thunderstorm precipitation for cold (top) and warm (bottom) front. western and north-eastern air mass advection (Twardosz, 2007b). 4. Conclusions The study shows the great utility of stratifying hourly precipitation events into associated synoptic situations to achieve a better understanding of the physical mechanisms creating diurnal patterns. Differences between the processes involved in the development of air mass and frontal precipitation were discussed and documented together with the differences in their 24-h patterns. Analysis of the 56-year homogenous pluviographic records of Kraków reveals that the 24-h diurnal cycle of the thunderstorm precipitation depends on the synoptic situation that determines the thermal and humidity properties of air masses. This is manifested by the varied amplitude of the cycle and the timing of its peak values. The 24-h cycle of the thunderstorm precipitation totals is sharper than that of the frequencies. Thunderstorm precipitation features a single-modal distribution, regardless of the air mass and atmospheric front type. The overall 24-h pattern of thunderstorm precipitation, with its characteristic afternoon peak, is mainly driven by cold-front precipitation, but is also affected by air mass precipitation occurring in polar maritime air. The warm front precipitation peaks at a different time, but References Dai A Observed and model-simulated diurnal cycles of precipitation over the contiguous United States. Journal of Geophysical Research 26(3): Dai A Global precipitation and thunderstorm frequencies. Part II: diurnal variations. Journal of Climate 14: Easterling DR, Robinson PJ The diurnal variation of thunderstorm activity in the United States. Journal of Climate and Applied Meteorology 24: Faiers GE A synoptic weather type analysis of January hourly precipitation Charles, Louisiana. Physical Geography 9: Faiers GE Diurnal distribution of hourly rainfall events during January by synoptic weather types at Lake Charles, Louisiana. Texas Journal of Sciences 45: Faiers GE, Keim BD, Hirschboeck KK A synoptic evaluation of frequencies and intensities of extreme three- and 24-hour rainfall in Louisiana. Professional Geographer 46(2): Fujibe F Diurnal variations of precipitation and thunderstorm frequency in Japan in the warm season. Papers in Meteorology and Geophysics 39(3): Mayes JC Regional airflow patterns in the British Isles. International Journal of Climatology 11: Niedźwiedź T Synoptic situations and its influence on spatial differentiation of selected climatic elements in upper Vistula River Basin. Rozprawy Habilitacyjne UJ 58: (in Polish with English summary). Niedźwiedź T Frequency of air-masses in southern Poland in the second half of 20th century. Prace Geograficzne IGiPZ PAN 188: (in Polish with English summary). Niedźwiedź T Catalogue of Daily Air Masses and Atmospheric Fronts over Southern Poland, , Computer file. (The file available at University of Silesia, Department of Climatology, Sosnowiec, Poland). Oki T, Musiake K Seasonal change of the diurnal cycle of precipitation over Japan and Malaysia. Journal of Applied Meteorology 33: Secretariat of the World Meteorological Organization International Meteorological Vocabulary, 2nd edn. WMO, No. 182, Secretariat of the World Meteorological Organization: Geneva; 784. Svensson C, Jakob D, Reed DW Diurnal characteristics of heavy precipitation according to weather type at an upland site in Scotland. International Journal of Climatology 22: Twardosz R The Synoptic and Probabilistic Aspects of Diurnal Precipitation Variation in Cracow ( ). Institute of Geography and Spatial Management of Jagiellonian University: Kraków; 123 (in Polish with English summary). Twardosz R. 2007a. Seasonal characteristics of diurnal precipitation variation in Kraków (South Poland). International Journal of Climatology 27:

6 DIURNAL CYCLE OF THUNDERSTORM PRECIPITATION IN KRAKÓW 1013 Twardosz R. 2007b. Diurnal variation of precipitation frequency in the warm half of the year according to circulation types in Kraków, Southern Poland. Theoretical and Applied Climatology 89: Wallace JM Diurnal variations in precipitation and thunderstorm frequency over the conterminous United States. Monthly Weather Review 103: Winkler JA Diurnal variations of summertime very heavy precipitation in the Eastern and Central United States. Physical Geography 8(3): Winkler JA, Skeeter BR, Yamamoto PD Seasonal variations in the diurnal characteristics of heavy hourly precipitation across the United States. Monthly Weather Review 116: