Part II - Continuous Observations

Surface Layer



Microbarometers-Sonic

During the BLLAST field campaign (14 June to 8 July), three microbarometers PAROSCIENTIFIC (Model 6000-16B) will be deployed in supersite 1 (See Fig. 7). The configuration will be forming a triangular array of 150m aprox. and at 1m a.g.l.. In order to avoid contamination from wind speed, static pressure ports (GILL 230-61002) are connected to the microbarometers, so that almost all the dynamic perturbation produced by the wind is filtered. These high precision digital instruments can detect very small pressure perturbations, of the order of thousandths of hPa. During BLLAST, a sampling rate of 2Hz is chosen as a compromise between having a good temporal resolution and registering small enough pressure perturbations (with this sampling rate the resolution is around 0.002 hPa).
The objective is to study the small scale static pressure fluctuations produced in the atmospheric boundary layer. These fluctuations can be due to turbulent motions (high frequency) and also to the propagation of waves of different types (gravity waves, Kelvin-Helmholtz instabilities, etc), produced by different mechanisms such as orographic forcing, fronts, convective forcing, geostrophic adjustment, shear instability, etc. The typical amplitudes of the pressure fluctuations related to waves is in the range of 0.01-0.1 hPa with a periodicity ranging from 1 to 40 minutes, although some episodes of intense mesoscale gravity waves with amplitudes up to 2-3 hPa have been reported.
The array configuration of the three microbarometers can be used during specific periods of the field campaigns, to detect and characterize wave events by means of lag analysis, cross-correlation or methods based on wavelets decompositions (wavelength, phase speed and direction of propagation of these waves can be evaluated).
Additionally, an Ultrasonic Wind Sensor, uSonic-3 Scientific (previously USA-1) (METEK) will be deployed above the Microbarometer A, at z=2.4m agl measuring the 3 velocity components (u, v and w) and sonic temperature (T) at 20 Hz sampling rate.


Fig. 7: Deployment of the microbarometers array in BLLAST (site 1)


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