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COST 733, Brussels, Belgium, 6-7 March 2008

Synoptic condition for Snowstorms in Bulgaria

Latin Latinov

National Institute of Meteorology and Hydrology,

66 Tzarigradsko chaussee blvd, 1784 Sofia, Bulgaria,


Snowstorms, snowblowings and snowdrifts formation

They lead often to disruptions in communications, great material damages, and in some cases even victims. These phenomena happen in definite synoptic conditions.

After analyzing of cases during the period from 1971 to 2003, they are grouped in four types. It is a prerequisite temperatures at H-850 hPa to be lower than minus 2°C. When temperatures at 850 hPa are below minus 10°C the frequency of snowstorms sharply decreases.


Snowstorms, snowblowings and

snowdrifts formation • First type. Combined influence of a high, north from the

country and a low, passing through Greece and Turkey.

Cases from this type are most of all. • Second type. Cold air is rushing from northeast. The

Siberian high is active – a very cold air is rushing in its periphery, over Bulgaria.

• Third type. A low is crossing the country. At high levels, a deep trough from north is moving very slowly from west to east.

• Fourth type. A cold front is passing from northwest.


Type I (Snowstorms). Scheme of the tracts of the highs (a) and the lows (b). Lines separating the dominant baric field: c – persistent; d – temporary.

Combined influence of a high, north from the country and a low, passing through Greece and Turkey. Cases from this type are most of all.

a - b - c - d -


Type II (Snowstorms). Cold air is rushing from northeast. The Siberian high is active – a very cold air is rushing in its periphery, over Bulgaria.

a - b - c - d -


Type III (Snowstorms). A low is crossing the country. At high levels, a deep trough from north is moving very slowly from west to east.

a - b - c - d -


Type IV (Snowstorms). A cold front is passing from northwest.

.

a - b - c - d -


• For the period 2004-2006 a test was made of the results obtained. In this period there are nine synoptic situations when snowstorms are registered in Bulgaria. The total number of days in these cases is 23.

• The same data was used as for the typification of the synoptic conditions, leading to snow storms:

• synoptic and part of meteorological records of NIMH – BAS;

• fields of geopotential, temperature and wind on different levels in height from the archives of http://www.weter3.de and http://wetterzentrale.de;

• surface atmospheric pressure is analyzed using the geopotential in H – 1000 mb.

http://www.weter3.de/

http://wetterzentrale.de/

http://wetterzentrale.de/


• To determine the geometrical similarity of the fields in comparison, parameters and are used, and for their calculation differences are defined:

• , ( 1 )• where Xφi are the values of the element in point with coordinates

(φ,i), and is the mean value for Хφ for parallel (φ). The similarity of and is determined by the coincidences of and in sign: for parallels and for the field all over.

• , ( 2 )• where k is the number of points on the parallel, • , ( 3 )

• where n is the number of points in the whole field.• The parameter is varying from 0 to 1. When >0.6 there

is a similarity, and if <0.4 there is contradiction.

1Х 2Х

1Х 2Х nn


• For similar synoptic situations a condition was imposed also: the temperature at level H – 850 hPa to have negative value in points: 22.5° E longitude, 42.5° N latitude; 25° E longitude, 42.5° N latitude; 27.5° E longitude, 42.5° N latitude.

• Two areas were chosen, in which to determine the coefficient of similarity:

• From 40° W long. to 50° E long. and from 30° N lat. to 75° N lat.

• From 5° W long. to 40° E long. and from 30° N lat. to 60° N lat.• The coefficients of similarity during the period 1974- 2003, in

most cases, are bigger than 0.7 for circulations in one type and less than 0.4 for different type circulations. The means from the coefficients of similarity for circulations from a same type lay between 0.6 and 0.7, and for different types – between 0.3 and 0.5. A relatively bigger dispersion of the coefficients of similarity is noticed for the greater area, which could be explained with the contribution for similarity or non-similarity of areas far away from the Balkan Peninsula.


• For all synoptic situations there is at least one day that satisfies the condition for similarity with the defined characteristic types, but this condition does not fit for all days. There are some cases with days with atmospheric circulation, having high coefficients of similarity with the main types, but without snowstorms. The reason for these deviations is the very low temperatures at level H-850 hPa, and in these cases the snow precipitation is insignificant.

• Table 1 shows the correlation between the days with concurrence and non-concurrence when different expectations (F1 – for snowstorm expected and F2 – for not expected) and realization (D1 – snowstorm occurred and D2 – no snowstorm) for the period 1971-2003.


Table 1. Correlation between predicted and observed snowstorms during the period 1971-2003 in Bulgaria for the area (in percents):

The sum of concurrence N, where( 4 )

For the period 1974-2003 N is 87% for the bigger area and 91% for the

smaller.

2211 DFDF NNN


• For all synoptic situations with registered snowstorms in Bulgaria during the period 2004-2006 (9 cases) there is at least one day with a coefficient of similarity greater than 0.6, but there is one case with a high coefficient of similarity, but without snowstorm, that is – forecast is false. The correlation of the days with concurrence and non-concurrence when different expectations for the period 2004-2006 is shown in Table 2 (the designations are the same as in Table 1).


Table 2. Correlation between predicted and observed snowstorms during the period 2004-2006 in Bulgaria for the area (in percents):

For the period 2004-2006 N decreased and is, respectively, 82%for the bigger area and 86% for the smaller. The results from the test of independent data are worse than those obtained from dependent, but are, however, completely acceptable. In meteorology, this result is accepted as a good one, and is a premise for being used when analyzing the atmospheric situation. Still more, it can be got automatically, and to suggest the forecaster to be aware of snowstorms in our country.


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