Complex Approach for Assessment of Dry Wind and Droughty Spells in Bulgaria

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Complex Approach for Assessment of Dry Wind and
Droughty Spells in Bulgaria  
BULGARIAN ACADEMY OF SCIENCES National Institute
of Meteorology and Hydrology

• Anelia Gocheva, Lyubov Trifonova, Tania
  Marinova, Lilia Bocheva
• Sofia, BULGARIA

International Conference on "Water Observation
and Information System for Decision Support
Ohrid, Republic of Macedonia, 23 26 May 2006
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Introduction In many regions all over the world
the extreme droughts and dry wind are severe
climate phenomena which occur frequently and
cause great damages and losses. The way these
hazardous events occur in Bulgaria is the subject
of this study. The paper presents a first
attempt for climatic systematization of the
available meteorological data concerning the
occurrence of dry winds and droughty spells on
the territory of the country, assessed on the
basis of Temperature Humidity Complex (THC).
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Criteria

• Dry wind (t-f-v)
• t ? 25?C, f ? 30, v
  ? 5 m/s

• Temperature-humidity (t-f) droughty spells
• t ? 25?C, f ? 30

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Figure 1. Location of the meteorological
stations used for investigation on the dry
winds and droughty spells in Bulgaria
Meteorological Data Period 19612000 Stations
249 All available t-f information was
carefully examined.
Part of them are inappropriate for the research
because of closure (especially after 1990) or
movements, frequent interruptions, short
available data periods, lack of data in the warm
season. In addition dry winds do not occur in
many costal and mountain stations. The total
number of stations was reduced 150 130
(basic) 20 (subsidiary with less than 25
years period)
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General characteristics
Table 1 Distribution (in ) of the dry winds by
duration and months
Dry winds (1961-2000) about 150 cases of dry
winds, registered in more than 50 stations, with
total duration above 550 days. The dry wind
season is April September with greatest
frequency of the phenomenon in August and July.
The cases with duration 3 4 days prevail but
events with duration 6 8 (even 9) days are also
registered. An increase of the events with
duration 4 5 days and a slide shift of the dry
wind season from April September to May
October is observed on the background of the
tendency of increasing of total annual number of
dry winds in the recent years.
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Table 2 Distribution (in ) of the tf droughty
spells in North (N) and South (S) Bulgaria
Droughty spells (1961-2000) about 2600 cases
observed during more than 10 300 days. The
season of tf droughty spells is April - October
but while in North Bulgaria about 25 of the
cases are observed in August and July, for the
territory of South Bulgaria this figure is about
40. Besides, about 2/3 of all cases are
registered on the territory of south part of the
country. The values of observed maximum duration
of droughty spells in North and South Bulgaria
are again ?in favor of the South? in proportion 9
15 based on (1961-1990) data period and 11 17
during the last decade (19912000).
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Figure 2. Distribution of dry winds and t f
droughty spells by years
A tendency of increasing in total annual number
of t f droughty spells (DS) can be seen during
the last decades although not so expressed like
in the field of dry winds (DW). The smooth curves
on Fig. 2 (based on relative frequencies)
represent polynomials of second (DS) and third
(DW) order and the 95 confidence intervals are
shown by dash lines.
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Statistical extrapolation
Statistical extrapolation was carried out using
Fisher-Tippet type II function. Dry winds About
10 to 12 events annually can be expected on the
territory of the country at least once in 30 and
50 years. Droughty spells The number of
consecutive days with t-f droughty conditions
referring to the same return periods are about 12
15 for North and 25 30 for South Bulgaria.

Figure 3. Number of dry winds and days with
tf droughty spells extrapolated by means of
Fisher-Tippet type II function
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Spatial distribution
Dry winds The spatial distribution of the dry
winds in Bulgaria in their capacity of ?climatic
anti-resources? is analyzed using an effective
method, applied in climatic aspect for assessment
of ?climatic resources? (Kobysheva, Ilina,
2001).
Figure 4. Risk of dry winds by administrative
districts in Bulgaria
The obtained results show as regions mostly in
danger the districts of Burgas and Pernik
followed by the districts of Yambol and Kardjali,
Sliven, Kyustendil and Blagoevgrad, etc.
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Droughty (t - f) conditions
The existing correlation between D and f14s,
revealed on the basis of hourly synchronous air
temperature and relative humidity records for an
approximately 10-years period (19601968) is
used. It is analytically presented by means of
Chebishev?? polynomials (1)
Figure 6. Regions in Bulgaria with extreme t f
droughty spells (t?25?C, f?30)
The obtained results are used for revealing the
regions at greatest risk of extreme tf droughty
spells (Struma river valley, Thracian lowland,
WesternCentral Danube plain)
(1)
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Combined (t?f?wr) conditions
The distribution of t f droughty spells over
territory follows in general the air temperature
spatial peculiarities. The rainless periods
(wr) have some own typical territorial
specialities. The regions at greatest risk are
those subjected to a greatest extent to the
combined influence of the prolonged radiation
overheating as well as of the prolonged lack of
precipitation, in their complex interaction.
Figure 8. Risk of both extreme rainless periods
and t-f droughty spells by districts
These regions are determined juxtaposing the
combined tfwr effect by districts using the
methodology of (Kobysheva, N., O. Ilina, 2001)
and equal weights for tf and wr. According to
the obtained results, the combined
temperature-humidity and ?rainless? influence
turns out to be extremely unfavourable in the
districts of Blagoevgrad and Haskovo, followed by
the district of Kardjali, these ones of Yambol,
Smolyan and Kyustendil, etc.
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• Droughty spells
• weather
• (2600 cases, 10300 days)
• mean air temperature about 32?C
• maximum air temperature 37?C (till 44?C on some
  places)
• mean deficit of air humidity about 35 mb (between
  32 and 40 mb)
• maximum deficit above 50mb (and even more)
• mean wind velocity about 2 4 m/s (or calm) and
  maximum wind velocity about 20 m/s.

Figure 9. General meteorological conditions
during dry winds and droughty spells by
administrative districts

• mean air temperature about 32?C
• maximum air temperature till 43?C
• mean deficit about 35 mb, maximum deficit about
  30 40 mb (even above 50 mb again)
• mean wind velocity is about 6 7 m/s in western
  and 8 9 m/s in eastern part of the country
• the maximum wind velocity is about 12 17 m/s
  (West BG) and 17 21 m/s (East BG)

Dry wind weather (150 cases, 550 days)
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Typical synoptic situations
Figure 11a Satellite Image 14.08.2000 1200 IR
Figure12a Satellite Image 21.08.2000 1200 IR
Combined situations sometimes lead to extreme
cases of droughty spells prolonging up to 8-10
days or even more. Example of typical
combination of two synoptic situations is the
period from 11.08 till 23.08.2000. More
detailes - in ?Climatological Analysis of the
Synoptic Situations Causing Dry Wind and Droughty
Spells in Bulgaria? by Trifonova, L, A.Gocheva,
T.Marinova,L. Bocheva (FP-044, BALWOIS 2006).
Figure 11b Geopotential height dm on 1000
hPa (14.08.2000 NCEP data)
Figure 12b Geopotential height dm on1000
hPa (21.08.2000 NCEP data)
Figure 11c Geopotential height dm on 500 hPa
(14.08.2000 NCEP data)
Figure 12c Geopotential height dm on 500 hPa (
21.08.2000 NCEP data)
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During the last decade of the 20-th century the
droughty spells, as well as the dry wind periods
are increased and the obtained results confirm
the conclusion for changes in weather conditions
in the recent decades and increasing of the
climate extreme events.
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Thank you for your attention!