The trend analysis demonstrated an overall increase in the values of air temperatures as well as an increase in the occurrence of extremely hot days, but a decrease in the number extremely cold days. At the same time a statistically significant increase in the precipitation intensity in both mean and extreme values has been detected (Figures 2-8). The historical observation data show that the increase in the number of heavy precipitation events (B) has been evident for the last ~80 years, but the number of summer days (A) has significantly increased since the middle of the past century, especially during the last decade (Figures 9-10).

12th EMS Annual Meeting & 9th European Conference on Applied Climatology (ECAC)Łódź, Poland, 10 – 14 September 2012

Trends in the frequency of extreme and hazardous weather events in LatviaZanita Avotniece1, Valery Rodinov2, Lita Lizuma1, Agrita Briede3, Māris Kļaviņš2,

1 Latvian Environment, Geology and Meteorology Centre, 165 Maskavas str., Riga, LV-1019, Latvia2Department of Environmental Science, University of Latvia, Raiņa blvd. 19, LV- 1586, Riga, Latvia, e-mail: maris.klavins@lu.lv

3Department of Geography, University of Latvia, Raiņa blvd. 19, LV- 1586, Riga, Latvia

INTRODUCTIONIn this study the long-term variability of extreme climate event indicators and such hazardous weather events as glazed ice, rime, thunderstorms and hail in Latvia was investigated. To assess the trends of changes of extreme climate events, indexes, such as the number of extremely hot days, frost days or the number of days with heavy precipitation (altogether 19 indexes) were used. The trend analysis of long-term changes of extreme climate events demonstrates a significant increase in the number of meteorological events associated with an increased summer temperature and a decrease in the number of events associated with extreme temperature events in winter time. There is also an increase in the number of days with heavy precipitation. Trends in the changes of the annual amount of hazardous weather events showed spatial differences between the observation stations included in this study. A statistically significant decreasing tendency for the annual number of thunderstorm days was detected, however, in some observation stations there has been an increase in the occurrence of hail as well as in the intensity of thundershowers, which could be an indicator of the increase of the overall thunderstorm intensity.The present study is based on daily air temperature, precipitation and weather phenomena data series for 14 meteorological stations in Latvia obtained from Latvian Environment, Geology and Meteorology Centre. An ensemble climate change indices (Table 1), that follow the definitions recommended by the Expert Team on Climate Change Detection and Indices, were derived from daily temperature and precipitation data and calculated by using the RClimDex 1.0 software, which runs in the R platform and besides the computation of indices also includes a simple quality control of the data.

Table 1 A list of indices used in this study

Trends in meteorological event time series were analysed by using non-parametrical Mann-Kendall test, which was applied to each variable at each site. The trend was considered as substantial at a significance level of p≤ 0.1 if the test statistic was greater than 1.6 or less than -1.6, as statistically significant at a significance level of p≤0.01 if the test statistic was greater than 2.6 or less than -2.6 and as very significant at a significance level of p≤0.001 if the test statistic was greater than 3.3 or less than -3.3.

Index name

Explanation Value

TX Annual or monthly mean of daily maximum temperature °C

TN Annual or monthly mean of daily minimum temperature °C

TNn Annual or monthly minimum value of daily minimum temperature °C

TNx Annual or monthly maximum value of daily minimum temperature °C

TXn Annual or monthly minimum value of daily maximum temperature °C

TXx Annual or monthly maximum value of daily maximum temperature °C

FD Frost days (annual count when daily minimum temperature <0ºC) Days

ID Ice days (annual count when daily maximum temperature<0ºC) Days

SU Summer days (annual count when daily maximum temperature >25ºC)

Days

TR Tropical nights (annual count when daily minimum temperature >20ºC)

Days

CSDI Cold spell duration indicator (Annual count of days with at least 6 consecutive days when minimum temperature <10th percentile)

Days

WSDI Warm spell duration indicator (Annual count of days with at least 6 consecutive days when maximum temperature >90th percentile)

Days

Ptot Annual total precipitation amount in wet days (precipitation amount ≥ 1mm)

mm

SDII Simple daily intensity index (annual total precipitation divided by the number of wet days (precipitation amount ≥ 1.0mm) in a year)

mm/ day

CDD Consecutive dry days (annual maximum number of consecutive days with precipitation amount <1mm)

Days

CWD Consecutive wet days (annual maximum number of consecutive days with precipitation amount ≥1mm)

Days

R10 Annual number of heavy precipitation days (precipitation amount ≥10 mm)

Days

R95p Very wet days (annual total precipitation when precipitation amount>95th percentile)

mm

R99p Extremely wet days (annual total precipitation when precipitation amount>99th percentile)

mm

Figure 1 Weather stations used in this study

Figures 2-8 Long-term changes in the extreme climate events in Latvia over the

period 1950-2010 (Mann-Kendall test statistics)

A B

Figures 9-10 Long-term changes in the number of A: summer days (maximum air temperature > +25ºC) and B: heavy precipitation (daily precipitation amount ≥ 10

mm) in Riga over the period 1881-2010

Figures 11-14 Annual mean number of days with glazed ice, rime, thunderstorms and hail in Latvia over the

period 1960-2010

CONCLUSIONS•Since the middle of the past century significant changes in the extreme climate events have been observed in Latvia: there has been a significant increase in the number of meteorological events associated with an increased summer temperature and extreme precipitation, and a decrease in the number of events associated with extreme temperature events in winter.•Historical observation data show that that the increase in the number of heavy precipitation events has been evident for the last ~80 years, but the number of summer days has significantly increased since the middle of the past century, especially during the last decade. •Due to the wintertime warming, the annual number of glazed ice and rime has decreased. The annual number of thunderstorms has also decreased, but the increase in the number of hail events and thundershower intensity could be an indicator of the increase in the thunderstorm intensity.

For the analysis of the long-term changes in the hazardous weather events in Latvia the annual number of days with glazed ice, rime, thunderstorms and hail was calculated. The annual mean number of these weather hazards (Figures 11-14) shows that the winter season phenomena as glazed ice and rime occurs more often in the eastern part of Latvia, while the summer season phenomena as hail and thunderstorms are more characteristic for the central part of the country. The trend analysis of these phenomena (Table 2) revealed spatial differences, however due to the observed warming in wintertime temperatures in most of the observation stations the annual number of glazed ice and rime has significantly decreased. There has also been a decrease in the annual number of thunderstorms, however in some observation stations a significant increase in the annual number of hail has been detected. The changes in the mean and maximum precipitation amount during thunderstorms were also analysed during this study, and in some observation stations the analysis showed a significant increase in the intensity of thundershowers.

  Glazed ice

Rime Thunderstorm Hail

Alūksne 0,34 -1,74* -2,36* 3,19***

Daugavpils -0,65 2,74** -3,36*** -3,29***

Dobele 2,21* -1,36 -0,81 -0,32

Kolka 0,20 0,12 -2,73** 1,11

Liepāja -3,97*** -1,57 -0,81 -0,23

Mērsrags -3,06** -1,06 -0,89 2,22*

Priekuļi 0,22 0,04 -1,71* 1,74*

Rīga -2,80** -2,08* -3,09** -2,93**

Rūjiena -5,03*** -4,16*** -2,82** 1,46

Skrīveri -1,07 -1,82* -3,61*** -0,97

Skulte 0,37 -2,52* -3,79*** -1,91*

Stende 0,50 -1,21 -1,00 -1,19

Ventspils 2,42* 1,08 -1,51 0,20

Zosēni -6,15*** -4,92*** -4,06*** -0,40

Table 2 Trends in the changes of hazardous weather events in Latvia over the period 1960-2010 (Mann-

Kendall test statistics)

Glazed ice Rime Thunderstorms Hail