Analysis of extreme precipitation in different time intervals using moving precipitation totals

Tiina Tammets1, Jaak Jaagus2

1 Estonian Meteorological and Hydrological Institute

2 Department of Geography, University of Tartu

Outline

• Introduction, precipitation regime and extremes in Estonia• Overview of the characteristics used for description of

precipitation extremes• Objectives of the study• Using of moving precipitation totals as characteristics of

precipitation regime• Extreme precipitation in Estonia in dependence of a

number of days and months in observed period• Extreme precipitation in Estonia in case of very wet and

dry conditions during 1957-2006• Trends in precipitation extremes in Estonia during 1957-

2006 • Conclusions

Annual curve of monthly mean, maximum and minimum precipitation in 1966-2009, mean of

the meteorological stations in Estonia

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cip

ita

tio

n,

mm

average min max

Precipitation in July and August 1966-2009, mean of the meteorological stations in

Estonia

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August July

2002

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JUNE

JULY

AUGUST

PRECIPITATION 2009

IN ESTONIA

Characteristics used for description of precipitation extremes

• quantiles of precipitation amounts;• maximum number consecutive wet days (R>= 1 mm; 10

mm);• maximum number of consecutive dry days with the

threshold of 0.1 mm; 1 mm precipitation; • number of heavy precipitation days in a month or year

(R>=10 mm);• number of very heavy precipitation days (R>=20 mm);• number of dry and wet days in a month or in a year with

chosen threshold;• mean wet-day and dry-day persistencies;• hydrothermical coefficient.

hydrothermical coefficient (HTK) :

HTK = Precipitation / 0,1* Temperature

Hydrotermical coefficient, average of Estonian meteorological stations June-August 1961-2009

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HT

K,

mm

/°C

HTK = R/0,1*T

to show how much has been rained till this day

not to show how much it rains in a day

but

For many purposes it is essential

A day will be extreme day, if the amount of precipitation till this day has been too small or too large.

Objectives

• To define characteristics describing continuous dry and wet spells with various duration, which doesn’t divide time into months or 10-day periods

• To elaborate a method characterising climatology of extreme precipitation totals for any time periods (number of following days, months or years using moving totals)

• To detect the most severe wet and dry spells in Estonia during last 40-50 years and to analyse trends in days with precipitation extremes

Mathematically the sequence of moving totals (averages){sj(n),1 j N-n+1}

is derived from a sequence {ai, 1 i N} obtained by taking the totals

(averages) of the subsequent n terms:

sj(n)= aj (by moving averages sj

(n)= aj),where N is the total number of days

in the precipitation time series and n the number of days through which the moving average is calculated. We find drought and wet days by calculating sj

(n)

with time period n for each day i in the time series and choosing the days with

values of sj(n)

, that are smaller or larger than the given threshold t.

CALCULATING MOVING TOTALS (AVERAGES)

To find the number of extreme days we have to calculate the moving total or average of precipitation time series

1ni

ij

1ni

ijn1

Counting of moving total

np

n

N days

days days1 12 23 34 45 56 67 78 89 9

10 1011 1112 1213 1314 1415 1516 16

17 1718 1819 1920 20

21 2122 2223 23

if the average of precipitations on

a day and

previous n days is M mm and

more the day has been counted as

a

wetday

if there are no precipitation on

a day and

previous n days the day

has been counted as a

droughtday

Presentation of the extreme precipitation in any number of days, months or years

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number of days

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Jõgeva maxJõgeva minPoly. (Jõgeva max)Poly. (Jõgeva min)

Extreme totals of precipitation for any number of days during 1966-2009 has been found in Tallinn, Vilsandi, Väike-Maarja, Võru Tartu and Pärnu stations

Precipitation stations in Estonia

Extreme precipitation totals in Estonian climate stations in 1966-2009

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number of days

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TALLmax V-MAARmax PÄRNUmax TÕRAmax

VILSmax VÕRUmax TALLmin V-MAARmin

PÄRNUmin TÕRAmin VILSmin VÕRUmin

Dependence of maximum and minimum precipitation on the number of successive days at six stations in Estonia in 1966-2009

Extreme precipitation totals 1966-2009 in Estonian climate stations

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pärnumax pärnumin

v-maarjamax v-maarjamin

tallinnmax tallinnmin

võrumax võrumin

vilsandimax vilsandimin

tõraveremax tõraveremin

1 year

2 years

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5 years

Dependence of maximum and minimum precipitation on the number of successive months at six stations in Estonia in 1966-2009

In agrometeorological studies of Estonia, the criterion of extremely wet conditions - mean daily precipitation 10 mm or more during successive 10 days is used.

If the moving average for a 10-day period sj (10)>= 10 mm,

then the last day of the period is regarded as a wet day. Extremely dry conditions for field plants mean that there is no precipitation during successive 20 days; then sj

(20) = 0

and the last day of the period has been counted as a dry day.

Extremely wet and dry days for Estonia

criteria:

• extremely wet conditions: when the mean daily amount of precipitation is 10 mm and more during 10 consecutive days

• extremely dry conditions: no precipitation during 20 consecutive days

number of day

wet dry

123456789101112 average 13 10 mm in a day without14 precipitation1516

17 1. wet day

18192021

22 1. dry day

232425262728

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wetdays days with precip.10mm and more

Monthly relative number of wet days and days with precipitation ≥10 mm during 1957-2006, mean of

the meteorological stations in Estonia

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drydays days without precipitation

Monthly relative number of dry days and days without precipitation during 1957-2006, mean of

the meteorological stations in Estonia

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1 5

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9

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Maximum number of dry days in Estonia from May to August during 1957-2006

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relative number of extremes

Linear (relative number of extremes)

Number of wet and dry days (mean of 56 stations) and relative number of extreme (wet+dry) days in

1957-2006

The method of moving precipitation totals allows to

1. present maximum and minimum precipitation in different periods (from 1 day to 4-5 years) on graph, which gives complete information about precipitation extremes of a station. It allows also to compare extreme precipitation amounts in different stations for every time period.

2. connect precipitation extremes with the dynamics other characteristics of hydrological regime (for example soil moisture content, ground water level, river runoff etc.) and find the best predictants of precipitation regime for extreme situations of environment

3. extract the time intervals of extreme precipitation events to relate them to atmospheric circulation

Conclusions

Conclusions

Using of the method of moving precipitation totals for the analysis of precipitation extremes in Estonia demonstrated that

• maximum and minimum precipitation in case of any number of successive days and months on the coast of open sea (Vilsandi) is significantly lower than in the continental Estonia

• occurrence of wet days is the highest in July and August while the number of dry days is maximal only in August and much lower in July

• maximum number of days without precipitation has been observed in May

• total number of extreme (wet and dry) days in Estonia has increased significantly during 1957-2006