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Mixing-layer height

The annual and diurnal variation of the mixing-layer height is an important parameter in air quality studies and in satellite retrieval algorithms. It is the height up to which atmospheric pollutants are rapidly mixed in the vertical.
MLH 2002 Hannover
Diurnal variation of the mixing-layer height from different remote sensing methods (top: SODAR, middle: RASS, bottom: ceilometer).

A temporal coverage with high resolution of the daily variation of the MLH is possible only by remote sensing techniques. At IMK-IFU three techniques are available: SODAR (acoustic, detects temperature fluctuations (turbulence) and temperature inversions), ceilometer (optical, detects vertical profiles of the aerosol distribution), and RASS (electro-magnetic, detects directly the temperature profile). The combined use of two or three of these techniques gives a more complete picture of the structure of the atmospheric boundary layer. Algorithms for the determination of MLH from SODAR and ceilometer are described in Emeis and Türk (2004) and Emeis et al. (2007). A general overview on the determination of MLH from remote sensing is given in Emeis et al. (2008). A first validation of ceilometr data by RASS data is presented in Emeis et al. (2009).

 

 

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Examples from remote sensing measurements

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This first example (from Emeis and Schäfer 2006) gives an example for the detection of the MLH from a SODAR (black and red symbols) and a ceilometer (blue triangles). Due to a limited height range there are no SODAR data above about 1200 m. Prominent features are a shallow nocturnal stable boundary layer (0 - 6 a.m. and 6 to 12 p.m.), a convective boundary layer between 6 a.m. and 6 p.m., and a residual layer above the nocturnal stable layer. The black vertical bars indicate inversions analysed from radiosondes ascents nearby.

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time-height section (42 h, 1300 m) of acoustic backscatter intensity from a SODAR (red: high backscatter, green: low backscatter)

time-height section (42 h, 1300 m) of the horizontal wind vector from as SODAR (arrow length: wind speed, arrow direction: horizontal wind direction)

 

This second example shows the development of a convective boundary layer over 42 hours from 12 noon on the first day to 6 a.m. the third day from SODAR data. The upper fringe of the red colour (left frame) indicates the top of the mixing layer. On the middle day wind speed and wind direction within the mixing layer is charcteristically different from the wind conditions above (right frame).

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This third example displaying a ceilometer measurement of a one-day time-height cross-section from a sunny spring day in an urban environment shows a stable nocturnal boundary layer with high particle concentrations between 0 and 9 a.m., and then the development of a well-mixed convective boundary layer between 9 a.m. and 6 p.m. Later in the evening a new stable surface layer forms. The lines plotted in the image show the mixing-layer height determined with an automated algorithm relying on the vertical gradient of the optical backscatter intensity.

 

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This fourth example of a potential temperature measurement (temperature scale from blue (5°C) over green/yellow (20°C) to purple (35°C)) with a RASS (see also Emeis et al. (2009)) starts at midnight and ends 78 hours later and is typical for a period with low winds and few clouds. During the nights near-surface air cools due to outgoing longwave radiation and a strongly stable stratification of the atmosphere forms. During daytime the atmospheric boundary layer is well mixed due to the heating of the surface by solar insolation.

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Contact: Stefan Emeis