Marine Air Flow

The long-term sodar observations at Ahtopol provided the opportunity to start statistical studies of the complex structure of the coastal boundary-layer and in particular to define the scales of the internal boundary layer (IBL) which is the reason for high pollution levels in coastal cities

Sodar data availability and range from 20 July 2008 – 10 October 2012

For the purposes of this study, only data from 20 July 2008 – 10 October 2012 are used covering 1454 days,

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 applied analyzes with different conditions

1 – continuous profiles to fixed heights;

2 – simultaneous availability of 12 sodar output parameters;

3 – profiles with a minimum height of 100 m;

* profiles start from 50 m;

  • 18769 profiles of marine air masses were registered posing condition that the profiles reach at least 100 m height.
  • during the day (9 am to 5 pm) 11432 marine profiles were registered posing condition 3 and 7160 of them during the warm part of the year (May to September).
  • within the marine air masses, the daytime cases prevail over the nighttime cases, because of the presence of local circulation in the study region.

Distances from the coast to the sodar at different wind directions of marine airflow in the vicinity of the observation site south of Ahtopol.

Generalizing the data from the fan-shaped diagram

  • the biggest number of profiles (2485) of marine air masses is recorded for 390-m to 584-m fetch over land and wind direction of 35-80 degrees
  • about twice less, 1883 marine profiles correspond to 980-m to 1170-m fetch
  • only 124 marine profiles correspond to 1620-m to 2480-m fetch.

The wind direction of 35-80 degrees is typical for the quasi-stationary conditions during sea breeze situations at the site.

The IBL grows nearly parabolic with the distance from the shore inland, which causes complex surface of elevated inversion at complex coastline and land cover

The sodar measurements at Ahtopol allow to assess some features of the IBL based on statistics.

As seen in the fan-like diagram, the marine air flow passes different distances (400 to 2500 m) before reaching the sodar site, which means internal boundary layer height in the range 50 – 200 m, depending also on the other basic parameters – air temperature, wind speed, turbulent fluxes of heat and momentum over land and the thermodynamic stratification over sea.

The thermal IBL is characterized with intensive turbulent conditions because of fast land surface heating in summer days. It is kept by the stably stratified and less turbulent marine air mass above.

The IBL height is usually determined based on temperature profiles, but the sodar provides profiles of turbulence characteristics, which is used in this study for the first time to assess the IBL development at the Bulgarian Black Sea coast. The observation site is situated about 400 m perpendicular to the coastline, which suggests thermal IBL in the range of 50-100 m for air flows from the sea.

Illustration (cross hеight-time cross-sections) of wind speed and direction, dispersion of the vertical wind speed, and turbulent kinetic energy is presented for a typical summer sea breeze day – 5 August 2008 at Ahtopol.

The height of the thermal IBL can be detected up to 100-120 m in the graph of the vertical wind speed dispersion and turbulent kinetic energy during the period 1-4 pm.

During this period the wind direction is constantly from east and the wind speed of 3-6 ms-1.

The high values of the turbulent parameters show intensive convection during this hot summer day.

All marine air masses and condition 3

Averaged sodar profiles of all marine air masses during the period 20 July 2008 – 10 October 2012 with applied condition 3. The color legend denotes the number of profiles reaching a specific height.

The mean values and their dispersion of 12 sodar’s parameters of all marine cases indicate IBL height of 50 – 80 m in the profiles of:

  • dispersion of the vertical wind speed (sigW)
  • turbulent kinetic energy (TKE)
  • eddy dissipation rate (EDR)
  • turbulence intensity (TI).

Other features of the complex structure of the coastal boundary layer seen in this figure are:

  • a height of 600 m for the marine boundary layer in the graphs of sigW and TKE
  • the sea breeze cell core height with maximal wind speed at 250 – 260 m in the graphs of wind speed (WS), sigW, and TKE.

Marine air masses during the day

Reducing the profiles to only daytime (9 am to 5 pm) cases the statistics indicate:

  • IBL height of 70 -130 m in sigW, EDR, TI, TKE graphs;
  • marine boundary layer height of 500 – 520 m in the graphs of sigW and TKE
  • sea breeze cell core height with maximum wind speed at 250 – 260 m in WS, sigW, TKE profiles.

Daytime marine air masses profiles during the warm part of the year

Marine air during the day and warm part of the year with applied condition 3
  • the IBL height of 70 – 150 m can be detected in sigW, EDR, TI, TKE graphs;
  • marine boundary layer height of 500-520 m in the graphs of sigW and TKE
  • sea breeze cell core height with maximum wind speed at 250 – 260 m in WS, sigW, TKE profiles.
  • confirming the comments made so far about observed peaks altitudes in the averaged profiles and their dispersions.
  • high BP values can also be expected in the interaction or entrainment zone over a convective boundary layer where transports of warmer air masses from the higher stable layer takes place

The data and analysis presented in this study are unique and the results original, as they provide insight into the complex structure of the coastal boundary layer.

Thermal IBL height of 70 -150 m is detected in the profile of several turbulence parameters: dispersion of the vertical wind speed (sigW), eddy dissipation rate (EDR), turbulence intensity (TI), and turbulent kinetic energy (TKE) for all marine air masses, for the daytime cases and for the daytime warm period cases.

The height of the marine boundary layer (usually with stable or stable-t-neutral stratification) is estimated to 600 m for all marine air masses and 500-520 m for the daytime marine air masses from the profiles of the dispersion of the vertical wind speed (sigW) and the turbulent kinetic energy (TKE).

In the profiles of wind speed (WS), dispersion of the vertical wind speed (sigW), and of turbulent kinetic energy (TKE) the prominent feature of the sea breeze cell, namely the core with maximum wind speed was located at 250 – 260 m. This feature is very important for the planning and the risk assessments for high constructions in the coastal areas.