Nocturnal Air Masses

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Sodar data availability and range from Aug 2008 to Oct 2016

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For the purposes of this study, all available data from the acoustic sounding of the atmosphere are used.

From 1st August to 31st October 2008 sodar measurements were made from 7:00 a.m. to 6:00 p.m. From 8th December 2008 to 27th July 2009 the sodar functioned continuously and after that until December 2009 only during the day, from 7:30 a.m. to 9:40 a.m. After December 2009 the sodar was worked continuously.”

During the years of operating mode, the manufacturer provided periodic updates to the sodar software, resulting in a gradual increase in the optimal height of the output profiles.

Types of long-term analyzes of night coastal air masses by wind direction and different conditions

1* – continuous profiles to fixed heights with simultaneous availability of 12 sodar output parameters (wind direction /WD/, wind speed and its dispersion /WS, sigWS/, vertical wind speed and its dispersion /W, sigW /, horizontal wind speed components and their dispersions /U, sigU, V, sigV/, eddy dissipation rate /EDR/, turbulent intensity /TI/ and turbulent kinetic energy /TKE/);

2 – continuous profiles with a minimum height of 110 m and simultaneous availability of 12 sodar output parameters;

3 – profiles consisting of a minimum of 3 points in height satisfying the wind direction condition and permitting an interruption only for lack of data;

All nocturnal land air masses

Averaged nocturnal land air masses characteristics with condition 3.

The color dots on the red line (averaged profile) indicates the availability of the individual profiles involved in the averaging at given altitudes and the green area indicates averaged profiles’ dispersion.

  • an approximately linear increase in the WS profile is observed up to a height of 540 m
  • positive values after 340 m are seen in the W profile and increase faster above 500 m.
  • slightly expressed peaks in the shape of the sigW, EDR and TKE profiles are observed at 430 m
  • the surface layer (SL) height is defined between 50 and 100 m with sigW, EDR, and TKE characteristics change.

Nocturnal land air masses during the warm part of the year

Averaged nocturnal land air masses characteristics during warm part of the year with imposed condition 2.
  • well expressed changes in the shape of averaged profiles after 400 meters are observed at almost all graphs.
  • the peaks in the sigW, EDR and TKE mean profiles are at the same height of 410 m above the ground
  • almost a linear decrease in the averaged TI profile is observed after the main peak at 140 m and it is interrupted by a second smaller peak at 430 m.
  • close to the ground at a height between 50 and 80 meters the SL height is defined with changes in sigW, EDR, TI and TKE profiles.

Nocturnal marine air masses during the cold part of the year

Averaged nocturnal marine air masses characteristics during cold part of the year with performed condition 3.
  • peaks are seen at the sigW and TKE profiles at a height of 300 m
  • peak is observed with the EDR profile, but at height of 340 m

The nocturnal marine air masses during the cold part of the year are defined by the winter profiles and partly by these in the transition seasons at which the synoptic conditions with eastern and northeastern wind components were dominant. Therefore, the observed sigW and TKE peaks at a height of 300 m are associated, with a slightly unstable or neutral marine PBL height, due to the relatively warm sea surface during the cold part of the year.

  • weakly pronounced positive peak at 40 – 50 m of TI
  • almost constant values of sigW up to 50 m and EDR up to 40 m
  • weak peaks of WD, WS and W at 40 m

Can be associated with the height of the IBL formed by a dominant factor the surface roughness change.

The flow of Archimedes force (buoyancy flow: β = sigW3/z) or the assessment of turbulence generation due to convection is expected and has high values near the ground and decreases with height to reach the value distinctive for the higher layer in the atmosphere. High BP values can also be expected in the interaction or entrainment zone over a convective boundary layer where transport of warmer air masses from the stable layer aloft takes place.

Averaged Buoyancy Production profiles from nocturnal marine air masses with performed conditions 1* (left) and condition 3 (right) from November to March.

Under condition 1* (calculated from 159 individual profiles):

  • decreases its values to 40 m

The negative peaks are an indicator of convective or neutral IBL and non-disturbed marine air masses over it during the night.

Under condition 3:

  • decreases its values to 50 m
  • a pronounced peak at 300 m

Thus confirming that at night in the cold season the height of the marine ABL (slightly unstable or neutral) is about 300 m.

Thermodynamic state of nocturnal air masses

The seasonal cross-sections of stability classes probabilities with height are presented here. The atmospheric stability classes according to the Pasquill-Gifford classification using the σϕ method are defined by two main types of nocturnal individual profiles with applied condition 2.

Cross-section of stability classes probabilities with height (warm season – top; cold season – down) of nocturnal land (left) and marine (right) air masses with performed condition 2.

Nocturnal land air masses during the warm part of the year

  • the slightly stable stratification (E) has probabilities mainly from 38 % to 78 % with a mean of 59 %
  • the neutral stratification (D) is the second dominant thermodynamical state with probability distribution values in the range mainly from 17 % to 29 %, a maximum of about 62 % at 570 m and 22 % mean value.
  • the stable stratification (F) is presented as the third dominant class – mean probability of about 16 %.
  • the extremely unstable (A) class is with about 2% mean probability and the moderately unstable classes (B) is below 0.7 % mean probability

Nocturnal land air masses during the cold part of the year

  • the slightly stable stratification (E) with more than 50 % probability after 70 m and more than 70 % after 200 m with a 63 % mean value throughout the acoustic sounding layer is observed.
  • the neutral stratification is the second dominant class with an average value of 20 %.
  • the stable stratification (F) is presented as the third dominant class – mean probability of about 11 %
  • the extremely unstable (A) class is with about 3% mean probability and the moderately unstable classes (B) is below 0.5 % mean probability


Nocturnal marine air masses during the warm and cold part of the year

  • the marine air masses have revealed a more unstable thermo-dynamical state of the atmosphere.
  • neutrally stratified atmosphere (D) is observed as dominant stratification in nocturnal marine air masses with minimum probability values of 33 % and mean 49 % for the warm part of the year and minimum 46 % and mean 54 % for the cold pat of the year.
  • the largest instability and the variations of thermodynamical state of all cases considered are observed in the averaged profiles of nocturnal marine air masses during the cold season.