In 1992, a surface-based, mesoscale microbarograph array with four pressure sensors was installed near the Hohenpeissenberg, Southern Germany and has since been in continuous operation (Fig. 1).
The network was designed for the detection of short gravity waves with wavelengths in the range of 2 - 30 km. It consists of an array of 4 pressure sensors with an average separation of about 1.2 km. The sensors are further developed versions of a type originally developed at the former Heinrich-Hertz Institut für Atmosphärenforschung und Geomagnetismus, Berlin. Sensors of this type have been successfully operated for many years. Special features of these instruments are: (i) a capacity microphone exposed to the differential pressure between the ambient air and an internal reservoir, (ii) an adjustable needle valve controlling the air flow between reservoir and environment, and (iii) the frequency modulation of a 2 kHz carrier signal as the pressure output signal. Data are sampled at 1 Hz and are transmitted on-line to a central data-processing unit.
Figure 1. Location of the microbarograph network Lichtenau in Southern Germany.
Pressure variations of period ranging from 2 sec to approximately 30 minutes can be resolved with a resolution of 3 mbars. To prevent temperature influences on the pressure signal, the sensors were thermally insolated in a container of 1.5 m height which was buried in the ground and mounted flush with the surface. A well-tested calibration method yields the sensitivity and the time constant of the sensor. A detailed description of the network is given in [1].
The data processing includes a wavelet analysis of the data from each sensor to isolate the events of interest in time and period from either background fluctuations or superimposed processes of various origin. Cross-correlation techniques are used to determine the propagation speed.
Figure 2. Time series of pressure variations. The arrows and numbers below the indicated time intervals correspond to the calculated propagation velocity vectors. (For clarity the different curves are vertically shifted.)
There is a strong coherence between the four sensors signals nearly all the time. Fig. 2 displays the 4 pressure variation time series for a 12 hour time period. It was found that the indicated pressure disturbances, some of which appear as short gravity wave trains, originate from southerly directions. Since the air flowed over the Alps, orographic forcing of the waves is suggested.
Although it was intended to study gravity waves, the network is actually sensitive to other processes as well. Five different modes are identified in a purely phenomenological way by their typical pressure signatures: (i) a basic mode with strong coherence but of a less wave-like character which is likely due to drifting density structures, (ii) short gravity waves of a few oscillations, (iii) solitary waves of either suppression or elevation type, (iv) fronts, and (v) uncorrelated fluctuations due to turbulence.
The observations made with the network show enhanced appearance of short gravity waves before approaching fronts, near thunderstorms and during Foehn-situations. Future investigations will include a statistical analysis of the gravity wave occurence and characteristics in relation to the synoptic situation.
The high pressure resolution of the network of only a few mbar and its high sampling rate make it an ideal instrument to analyse the surface pressure signature of lower tropospheric mesoscale processes. The network operates autonomously and does not require much maintenance. Use of the information gained from the network is increased if it is combined with other data, such as wind or radar data. This suggests that the system would best operate in a combined mode. For vertically profiling remote sensing systems, such as a wind profiler, the microbarograph network can add valuable information on the horizontal spatial structure and propagation speed of observed 3-d structures.
Hauf, T., U. Finke, G. Bull, J. Neisser, J.-G. Stangenberg, 1995:
A ground based network for atmospheric pressure fluctuations.
(submitted to the J. Atmos. Oceanic Technol.)
MAP Data Centre - April '05 - MAP WebMaster