Sodar data availability and range from Aug 2008 to Oct 2016

Definition for extreme events of Intergovernmental Panel on Climate Change (IPCC)

“By definition, extreme weather conditions vary from place to place. An extreme weather event to a particular place, is an event that occurs “rarely” within its statistical distribution. The definition of “rare” vary but will generally be understood as less than a tenth percentile or more of the ninetieth percentile as its statistical distribution”

The probability density function of a Weibull (the maximum likelihood method) is used in that study.

Histograms of the wind speed (green bars) with fitted 2-parameter Weibull distribution (red curve) at each level of Sodar measurements based on the method of maximal likelihood and determination of the extreme wind speeds values (blue dots) through the 90th percentile of each of the fitted Weibull distributions.

Changes of wind speed probability with height (color cross-section of height and speed intervals) speed with the availability of processed data (green bars) and reference profile for extreme values of wind (black profile) determined by the 90th percentile of statistical distribution with height for the period from Aug 2008 to Oct 2016 in MO Ahtopol

Occurrence of Extreme Wind Speed profiles over the years of the total period in MO Ahtopol distributed by months (left) and time of day (right)

  • Jul 2009 – Aug 2011 “calm period” – only a few reported extreme profiles
  • Sep 2011 – Jun 2014 “windy period”
  • Apr 2015 – Oct 2016 “windy period”
  • The occurrence of extreme wind events by hours is relatively homogenous

Characteristics of the coastal boundary layer at MO Ahtopol in the presence of extreme winds

Average profiles and their dispersions from a sample of a maximum of 10,854 selected profiles defined by the time series of extreme wind speed profiles recorded during the period from August 2008 to October 2016 in MO Ahtopol. (from left to right and from top to bottom: wind direction, extreme wind speed profile, dispersion of extreme wind speed profile, vertical wind speed, vertical wind speed dispersion, horizontal component U of the extreme wind profile (western component), dispersion of horizontal component U, horizontal component V of the extreme wind profile (southern component), dispersion of horizontal component V, eddy dissipation rate, turbulent intensity, turbulent kinetic energy)

sigW: major peaks: 150 – 160 m; 440, 470 – 490 m;
EDR – values decrease sharply in the first 90 m; major peaks 370 m, 430 – 440 m;
TKE – values increase in the first 150 m; sharp increases from 320 to 390 m; major peak: 430 – 470 m;

Pasquill-Gifford classification determined by sodar

  • The mean profile corresponds to the slightly stable stability of the atmosphere (Class E)
  • Heights set by the main peak in the turbulent averaged profiles (at 440 m ÷ 470 m) are characterized by slightly shifting the values of the stability class to neutral stratification

Buoyancy Production calculations

  • sharp decline in values in the first 90 m like EDR
  • weak peaks at 100 m,150 m, and 280 m;
  • major peak matched in height with peaks of sigW and TKE (440 m ÷ 490 m)

Weibull parameters defined only by extreme profiles

Average characteristics of the coastal boundary layer at the МО Ahtopol from all measurements for the period from Aug 2008 to Oct 2016.

Weibull parameters

In the averaged profile of the scale parameter, an almost linear change in height is observed, distinguishing two weak peaks at 50 and 80 m above the surface, followed by a major peak at a height of 270 – 300 m and a slightly sharper change in the profile values at an altitude of 500 m. The dimension of this profile is the same as the speed [ms-1] and its values are proportional to the average wind speed.

In the shape parameter profile, relatively more pronounced small peaks at a height of 50 and 80 m above the surface, a pronounced peak at a height of 260 m, and a relatively sharp change in the values at a height of 500 m is observed. The low peaks at the ground could be characterized by the height of the formed Internal Boundary Layer (IBL).

The maximum in the profile of this parameter (at 50 and 80 m height) can be related to the reversal height of diurnal wind speed, which characterizes the quasi-static state of the diurnal wind speed at various thermodynamic atmospheric conditions determined by the direct influence of the surface on the adjacent air layer during the day and night. Above this layer, the atmosphere is separated from the direct influence of the surface. This height is related to the height of the IBL owing to the transformation of marine air masses overland, but no theoretical connections have been made yet for this dependence.