University of Ljubljana Faculty of Mathematics and Physics

Department of Physics Chair of Meteorology

Vanja Kovač

STRONG PRECIPITATION CASES OVER THE NORTHWESTERN SLOVENIA: 18. SEPTEMBER 2007 FLASH FLOOD

SEMINAR

Supervisor: Doc. Dr. Nedjeljka Žagar

Ljubljana, May 2008

1 Abstract Late in the morning of September 18th 2007, an extreme amount of precipitation fell over the north-western Slovenia. The amount and intensity of precipitation produced fast increase of river flow in the area, especially Selška Sora, which flooded and caused significant damage on infrastructure. Several conditions must be met to produce this kind of intensive precipitation. In the case of September 18th, the air mass was potentially unstable, which means that after air parcels were lifted by force up to some point, they continued to move upward by themselves. Convection, the release of energy, strong precipitation and discharge of electricity in the clouds were triggered by forced lifting over the Slovenian mountains. Convection took place over a rather small location; therefore the rainfall was released over the same area over and over again, producing high river currents, floods and landslides.

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2 Contents  

1  Abstract ................................................................................................................................................. 1 

3  Introduction .......................................................................................................................................... 3 

4  Convective precipitation mechanisms: Static (in)stability .................................................................... 3 

4.1  Dry air ............................................................................................................................................ 4 

4.2  Moist air ........................................................................................................................................ 4 

4.3  Conditional instability ................................................................................................................... 5 

4.4  CAPE .............................................................................................................................................. 6 

4.5  Convective Inhibition .................................................................................................................... 6 

4.6  Potential instability ....................................................................................................................... 7 

5  Orographic precipitation mechanisms (Adapted from Comet) ............................................................ 8 

5.1  Stable upslope ............................................................................................................................... 8 

5.2  Seeder‐feeder ............................................................................................................................... 8 

5.3  Sub‐cloud evaporation contrasts .................................................................................................. 9 

5.4  Upslope release of potential instability ........................................................................................ 9 

5.5  Terrain‐driven convergence ........................................................................................................ 10 

6  September 18th case study .................................................................................................................. 11 

6.1  Atmospheric processes ............................................................................................................... 11 

6.2  Observations ............................................................................................................................... 13 

6.2.1  Satellite measurements ...................................................................................................... 13 

6.2.2  Radar Observations ............................................................................................................. 15 

6.2.3  Automatic meteorological stations (AMP‐s) ....................................................................... 15 

7  Conclusions ......................................................................................................................................... 17 

8  References .......................................................................................................................................... 17 

 

 

 

 

 

 

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3 Introduction The precipitation map for Slovenia (figure 1) in comparison to the terrain (figure 2) shows the importance of orographically triggered precipitation in the area: the Julian Alps and its foothills receive the maximum accumulation of rainfall, over 3000 mm. Certain conditions must be met for the development of intense precipitation event. The most important of these are continual and abundant inflow of moist air and its lifting. The latter can occur due to forced convergence at the windward slope. All of the above happens when cyclones coming from the Atlantic cause moist southwesterly winds to rise over the western part of Slovenia, which is more than 2000 meters altitude gained in less than 100 km from the seashore. In this paper I will explain these conditions in general as well as demonstrate the background of flood case in September of 2007.

Figure 2: Orography in Slovenia Figure 1: Average annual precipitation in

Slovenia (period 1961 – 1990)

4 Convective precipitation mechanisms: Static (in)stability (Adapted from Holton, J., 1992: Dynamic Meteorology)

Buoyancy forces, produced by density variations in the atmosphere, create currents within air mass. If the parcels of air return to their equilibrium state after the displacement, the atmosphere is stable. However, if the parcels do not return to their equilibrium state but accelerate upwards, the movement is called convection and such atmosphere is unstable. To understand this process better, we will first take a closer look at some basic terms. Details of all derivations can be found in Holton (1992).

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4.1 Dry air Potential temperature (Θ) is the temperature that a parcel of dry air at pressure p and temperature T would have if it were expanded or compressed adiabatically to the standard pressure ps.

(1)

So every air parcel has a unique value of potential temperature and this value is conserved for dry adiabatic motion, which most of the synoptic-scale motions approximately are. If potential temperature in the atmosphere is constant with respect to height, the lapse rate is

(2)

But if potential temperature is a function of height, the atmospheric lapse rate will differ from the adiabatic lapse rate and

(3)

So if Θ increases with height, an air parcel that undergoes an adiabatic displacement from its equilibrium level will be positively buoyant when displaced vertically downward and negatively buoyant when displaced upward. It will oscillate about its equilibrium level and such atmosphere is said to be statically stable. If, however, Θ decreases with height, the displacement of air parcel will increase exponentially in time. The latter can be seen in equation (5), its general solution being .

(4)

(5)

N is Brunt-Väisälä frequency and serves as a measure of the static stability of the environment. However, the above conditions are not sufficient to describe the development of precipitation and equations become more complicated if the air contains water.

4.2 Moist air We have used the parcel method to discuss the vertical stability of a dry atmosphere and the same condition ( ) also applies to parcels in a moist atmosphere when the relative humidity is less than 100 %. But if moist air is forced to rise, it will eventually become saturated and further rising will cause condensation, latent heat release and cooling.

To describe dynamics of moist air we use equivalent potential temperature ( ), which is a potential temperature that a parcel of air would have if all its moisture were condensed and the resulting latent heat used to warm the parcel. This process is irreversible and the ascent of this type is called pseudoadiabatic ascent. An approximate expression for can be derived from the entropy form of the first law of thermodynamics (see Holton, 1992).

. (6)

The equivalent potential temperature is conserved for a parcel during both adiabatic and pseudoadiabatic displacements.

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The first law of thermodynamics can also be used to derive the rate of change of temperature with respect to height for a saturated parcel undergoing pseudoadiabatic ascent. Thus, the rate of temperature change with respect to height is

, (7)

where Γd is the dry adiabatic lapse rate and Γs is the pseudoadiabatic lapse rate, which is always less than Γd.

If the lapse rate is larger than the dry adiabatic lapse rate, the atmosphere is statically unstable; if the lapse rate is smaller than the pseudoadiabatic lapse rate, the atmosphere is stable and if the lapse rate lies between the dry adiabatic and pseudoadiabatic values, the atmosphere is conditionally unstable.

4.3 Conditional instability If the atmosphere is conditionally unstable, it is stably stratified with respect to dry adiabatic displacements but unstable with respect to pseudoadiabatic displacements. The conditional stability criterion can be expressed in terms of the gradient of a field variable, defined as the equivalent potential temperature of a hypothetically saturated atmosphere that has the thermal structure of the actual atmosphere:

. (8)

The conditional stability criterion for a saturated parcel is

The release of conditional instability requires not only that but also parcel saturation at the environmental temperature of the level where the convection begins. The mean relative humidity in the troposphere is however well below 100 %, even in boundary layer. Thus, low-level convergence with resulting forced layer ascent or vigorous vertical turbulent mixing in the boundary layer is required to produce saturation. So how does the parcel travel upwards in unstable atmosphere? As an unsaturated air parcel located near the surface rises, its temperature will follow a dry adiabat. Its dew point will follow a constant mixing ratio line. When the dry adiabat crosses the constant mixing ratio line, the air becomes saturated and condensation commences. This is the lifted condensation level, or LCL. As the parcel continues to rise, it follows the moist adiabat. As a result, the rising air parcel becomes warmer and less dense than the surrounding air. So long as the ascent path along the moist adiabat is warmer than the environment, the air parcel will remain less dense than its surroundings and will continue to rise.

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4.4 CAPE CAPE is the abbreviation for the Convective Available Potential Energy, which is a particularly useful measure for the susceptibility of a given temperature and moisture profile to the occurrence of deep convection. It provides a measure of the maximum possible kinetic energy that a statically unstable parcel can acquire.

(9)

Here B is the maximum kinetic energy per unit mass that a buoyant parcel could obtain by ascending from a state of rest at the level of free convection to the level of neutral buoyancy near the tropopause. When calculating CAPE, we normally lift the parcel that reflects the mean values of the temperature and moisture in the lowest 50 to 100 hPa. This layer represents the average heat and moisture conditions fueling convective storms. The environmental CAPE for convective storms is often in the range of 1000-2000 J/kg. However, values higher than 5000 J/kg sometimes occur.

Figure 3: An example of the skew T - log p diagram: thick green line indicates dew-point temperature, thick red line the temperature of the environment and thin yellow line the temperature of a rising parcel. Orange area represents convective available potential energy.

4.5 Convective Inhibition  (Adapted from Meted, Comet program)

Until now, we have ignored the common occurrence of a capping inversion, the strength of which is measured by Convective Inhibition (CIN). This lid can prevent near-surface parcels from reaching their level of free convection. As a result, this lid can prevent storms from forming, even with high instability aloft. In the presence of a capping inversion, an additional mechanism

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is required to initiate convection. In the absence of mesoscale lifting, there are three common mechanisms for overcoming a capping inversion:

1. Heating 2. Moistening 3. Synoptic-scale lifting

In the case of heating, the presence of a lid can prevent convection while the surface temperature climbs. The instability, measured by CAPE, continues to grow until daytime heating eliminates the capping inversion. The stronger the lid, the less likely convection is to occur. However, if storms do form, they are more likely to be severe if the CIN is high. CIN can be calculated by the following equation:

(10)

Moistening shifts the ascent path on the skew-T diagram far enough to the right so that a rising air parcel remains buoyant throughout its ascent, eliminating convective inhibition. Moistening in the lower atmosphere can occur through either low-level advection of moister air into the region or locally through evaporation from a local moisture source such as a lake or irrigated field. Low-level moist advection can produce large changes in a short time to overcome relatively large amounts of CIN. Advection can be important even when the air is initially moist.

Synoptic-scale ascent is caused by the passage of a short wave or front. These processes act to lift and weaken the inversion layer. On the skew-T diagram, this effectively eliminates the inversion, shifts the inversion to the left, or some combination of the two. In either event, the inversion no longer acts to cap the ascent path of a rising air parcel. Because this process acts rather slowly on its own, it will be most effective if it coincides with daytime heating and/or moistening of the boundary layer. (Meted)

4.6 Potential instability The term potential instability describes an atmospheric layer being unstable if and only if forcing is present. It can't be realized as real instability without forcing. Potential instability occurs when there is dry mid-level air over warm and moist air in the lower troposphere. Convective instability is released when dynamic lifting from the surface to mid-levels produces a moist adiabatic lapse rate of air lifted from the lower troposphere and a dry adiabatic lapse rate from air lifted in the middle troposphere. The potential for instability is only realized when the thermal column ascends, most often up a slope of a mountain (orographic uplift), up an air mass front (frontal lifting) or by convergence at Earth's surface, and reaches saturation. The best way to analyze potential instability is by the use of a Skew-T diagram. A rapid decrease of dew-point with height will exist at the boundary between the near saturated lower troposphere and dry mid-levels. There will often be an inversion separating the dry air aloft and the moist air near the surface. This inversion is important because heat, moisture and instability can increase under this "capping" inversion during the day. Once the cap breaks, explosive convection can result.

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5 Orographic precipitation mechanisms (Adapted from Comet) The interaction of a flow with topography depends strongly on the nature of the topography (an isolated mountain or a mountain barrier, the angle of the slope …) and the nature of the incident flow (the strength and direction of the wind, static stability). The topic of this section is very wide and will therefore only be discussed briefly.

5.1 Stable upslope The most basic question when considering the characteristics of flow interacting with topography, is whether the air will go up and over a mountain or will be forced to go around it. If we compare this question to a marble rolling up a hill, we notice that if we want the marble to roll over the hill, its kinetic energy must be greater that the potential energy needed to rise to the top of a barrier. For atmospheric flows, the gravitational potential energy is represented through the static stability and the momentum of kinetic energy is the incident wind velocity. To represent the ratio of the kinetic energy to the potential energy, meteorologists use the Froude number, given by:

, (11)

where U is the wind velocity, hm is the height of the mountain and N is the Brunt – Väisälä frequency. If the Froude number is greater than 1, the air will rise over the mountain, but if it is less than 1, the flow is blocked by topography and must either go around it or turn back. If Froude number is greater than 1, stable air mass is forced to lift over the mountain, peak or ridge. If it is sufficiently moist, precipitation can occur. However, if the layer of moist air is not deep enough, non-precipitating clouds develop. If operating alone, this method is not very efficient.

5.2 Seeder­feeder If there is a non-precipitating orographic cloud over the mountain in the low level and there is an additional “seeder” cloud at a higher level, rain (or snow) from the seeder cloud can fall through the feeder cloud and enhance additional precipitation. This mechanism is the introduction of ice from above into a lower level liquid or supercooled liquid cloud. This introduction of ice provides condensation nuclei, thus initiating precipitation from the low level cloud layer. The low cloud layer may consist of liquid droplets, supercooled liquid droplets, or the cloud may glaciate and produce snow. The resulting precipitation type is, of course, dependent upon the thermal profile from the cloud to the surface as well as temperatures of exposed surfaces in the case of freezing rain.

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Figure 4: Stable upslope precipitation event Figure 5: Upslope precipitation event with seeder - feeder

5.3 Sub­cloud evaporation contrasts The layer in which sub-cloud evaporation takes place is shallower over the mountains than over lowland, so in threshold cases precipitation may reach the ground in the mountains, but not over the plain. Furthermore, orographic ascent increases relative humidity, resulting in less evaporation over mountains compared to evaporation over plains.

Figure 6: Shallower sub-cloud evaporation over mountains due to shallower layer.

Figure 7: Shallower sub-cloud evaporation due to increased relative humidity.

5.4 Upslope release of potential instability If the thermodynamic profile is potentially unstable, forced uplift triggers convection over the windward slopes and over the top of the mountain ridge. The release of potential instability is realized when air parcels are orographically lifted to the level of free convection. This mechanism gives the highest precipitation rates. The preferred synoptic environment where this release can occur is the post cold-frontal sector or warm sector in front of the cold front, as seen in figure 9. The latter conditions are even more favorable if there is a pre-frontal surge of cold air aloft. Convection may be deep or shallow, but both can result in substantial precipitation enhancement. This mechanism is very effective over relatively small hills, particularly if only a small amount of lift is needed to release instability.

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Figure 8: Upslope release of potential instability

Figure 9: Favorable conditions for the

release of potential instability

5.5 Terrain­driven convergence Terrain-driven convergence typically occurs in the flow that has low Froude number (less than 1) and therefore tends to move around barriers rather than over them – opposite to chapter 5.1. There are many different types of terrain-driven convergence, but since in meteorology air is considered incompressible, the idea is that the additional mass (due to convergence) has to move upwards to satisfy continuity equation and this lifting triggers precipitation.

Figure 10: An example of terrain-driven convergence – Olympic Mountains (Washington, USA)

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6 September 18th case study

6.1 Atmospheric processes  On September 18th the Eastern Alps were hit by a cold front that spread over the entire Europe, starting at the cyclone over the Northern Scandinavia, crossing the Eastern and Central Europe, the Alps, the Iberian Peninsula and ended over the Atlantic (figure 11). The front passed Slovenia in the evening. In the prefrontal area, the winds were westerly and rather strong, bringing warm, moist and unstable air (figure 12), while at the upper levels, the new air mass was rather cold (figure 13), thus producing very unstable conditions, ideal for the convection development. Furthermore, the wind direction rotated to the right with height and wind shear also produces favorable conditions for stationary convection. The automatic meteorological station on Vogel measured 303 mm of rainfall in 24-hour period, whereas unofficial stations reported up to 485 mm.

Figure 11: The weather situation over Europe on the 18.9.2007 at 12 UTC (ECMWF model at 00 UTC 18.9.2007, 12 h integration).

The model forecast fields are used here to describe general flow features over Slovenia on September 18th, especially in the lower troposphere where the influence of orography plays a dominant role for the location and intensity of precipitation. Details of the model performance in forecasting precipitation as well as an inter-comparison of different models on this case would be the subject of the follow-on seminar.

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Figure 12: The temperature, relative humidity and geopotential at 850 hPa at 03 UTC.

Figure 13: The temperature and geopotential at 500 hPa at 03 UTC.

Figure 14: The wind speed and direction at 700 hPa at 03 UTC.

Figure 15: The wind speed and direction at 925 hPa at 03 UTC.

The wind direction was mainly westerly at higher altitudes (see figure 14), but more south-westerly closer to the ground (figure 15), thus enabling the advection of warm and moist air from above the sea.

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Figure 16: Skew-T diagram for Udine at 00 UTC, September 18th 2007.

Figure 17: Same as Figure 16, but with temperature of 1000 m lifted air parcel (green).

Figure 16 shows the sounding results in Udine, Italy. It can be seen that the temperature lapse rate is smaller than the one of the dry adiabat, so according to this criteria the air is statically stable, but the lapse rate is very similar to moist adiabat. It can still be estimated as smaller in general, but at some levels the lapse rate is in between the moist and dry adiabat and in those areas the air mass is only conditionally stable. The calculated CAPE is 21.5 J/kg. Greater numbers appear only if the mass is already unstable. Convective inhibition is moderate with 122 J/kg. On figure 17 the green line represents the air that is lifted by force for 1000 m, pointing out the amount of available energy if such lifting occurred.

By observing the weather situation we can say that the conditions were favorable for the development of strong convective precipitation. The air mass was potentially unstable, warm at lower levels and cold at higher. The wind shear produced stationary convection, the advection of moisture caused more intense rainfall and upslope lift released the potential instability.

6.2 Observations Three types of observations can be used: satellite, radar at Lisca and automatic meteorological stations. All of these show that rainfall that day varied greatly in space as well as in time.

6.2.1 Satellite measurements 

The satellite images in visible spectra (see figures 18 - 22) show the development of very bright clouds over Slovenia. Bright color indicates the cloud’s thickness and height. The higher and denser the cloud is and the more water it contains, the brighter it appears on satellite images. Looking from the ground, these clouds are darker because the light reflects on them rather than passes through. Local triggering can also be spotted: the clouds show granular structure, indicating smaller convective cells.

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Figures 18 to 22: Satellite (Meteosat) measurements of visible radiance, taken on September 18th 2007 at 7, 9, 11, 13 and 15 UTC.

Judging from a set of satellite observations in visible spectra, taken every 15 minutes, the strongest convection occurred between 10:30 and 12:00 UTC. The clouds on figure 20 (11 UTC) are the brightest and cover the biggest area.

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6.2.2 Radar Observations 

The mayor precipitation band spread over the north of Slovenia with its maximum in the mountains of Northwestern Slovenia (Figure 23). The bands run from SW to NE in the direction of the air flow. The convection cells triggered over the east Italy and west Slovenia and were carried east with the main flow. There are at least two problems in radar detection of the accumulated precipitation: the distance between the radar and the precipitation maximum and the surrounding hills that shade radar’s beams. The last can be seen clearly on Figure 23. This and the lack of thicker AMP net contribute to rather poor observation data. Nonetheless the correlation between total precipitation and topography of Slovenia (Figure 2) can still be noticed.

Figure 23: Precipitation accumulation over Slovenia on September 18th from 00 UTC to 24 UTC.

6.2.3 Automatic meteorological stations (AMPs) 

The data gathered at automatic stations from all over Slovenia is the largest indicator of the diversity in precipitation distribution. If we compare the stations at higher elevation: Krvavec and Rudno polje, we cannot notice similarity in time distribution or total rainfall. Furthermore, even for the stations close by, such as Krvavec and Brnik, there is no time distribution or total accumulation match.

Name Latitude [°] Longitude [°] Elevation [m] 24-h precipitation [mm] Lesce 46,36556 14,17917 515 182,04 Rudno polje 46,34667 13,92861 1347 217,70 Gorenja vas 46,08722 14,185278 530 86,27 Dvor 46,06250 14,34972 341 46,50 Krvavec 46,29778 14,53861 1740 153,40 Brnik 46,21778 14,4775 364 181,70

Table 1: The accumulated (24-hour) precipitation on September 18th for several AMP-s.

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As seen in Figure 24, the maxima at the given stations are very varied. Neither in time distribution nor in the amount of rainfall is there a correlation. While the precipitation in Lesce increased to its maximum and then decreased with no other remarkable local maximum, the precipitation at other stations had several maxima. It can also be seen that the maxima sequences are correlated neither to latitude nor to longitude of the stations.

0 5 10 15 20 250

1

2

3

4

5

6

7

UTC [h]

5-m

inut

e ac

cum

ulat

ed p

reci

pita

tion

[mm

]

LesceRudno poljeBrnikKrvavecGorenja vasDvor

Figure 24: The evolution of rainfall on September 18th. The maxima of the observations are marked with asterisks. By gathering all available data on rainfall accumulation, the Environmental Agency of the Republic of Slovenia (ARSO) formed a daily rainfall map of Slovenia for September 18th. Due to lack of observations the lines are probably too smooth, but the general precipitation shape is clear enough to once again notice the correlation with Slovenian relief.

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Figure 25: Daily accumulated precipitation map for Slovenia on September 18th.

7 Conclusions The convection in a potentially unstable air mass can be triggered by several mechanisms: the warming of lower layers, the advection of moist in lower layers, local convergence or forced uplift. In the case study all of the above were present, which led to very intensive rainfall. Its amount varied greatly in location as well as in time. It can be assumed that the topography played significant role in location of precipitation maximum. More thorough investigation and its results will be presented in the author’s diploma.

8 References • Holton, J. R. 1992. Dynamic Meteorology, Third edition. Academic press, San Diego. • www.meted.ucar.edu (April 17th, 2008) • Žagar, M. 2007. Vremenska situacija ob poplavah 18. Septembra 2007 (Seminar) • www-k12.atmos.washington.edu (March 16th, 2008) • www.theweatherprediction.com (March 16th, 2008) • www.arso.gov.si (March 16th, 2008) • www.zrc-sazu.si (March 16th, 2008)

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