| description abstract | This paper presents a climatology of coherent disturbances detected during 1991?95 by a network of barometers with a diameter of about 50 km located in a very flat terrain centered on the Flatland Atmospheric Observatory in east-central Illinois. An automatic, wavelet-based adaptive filter is used to extract the waveforms of all disturbance events with amplitudes larger than a frequency-dependent threshold. The extracted events cover characteristic temporal scales from about 30 min to 6 h, that is, the range that includes mesoscale disturbances that affect the weather and the forecasts. The analysis resulted in two classes of events. One class, called coherent events, or CEs, consists of disturbances that propagated coherently through the barograph network and for which the phase propagation velocity, dominant period, and horizontal wavelength could be estimated with good accuracy. The propagation directions of 97% of the CEs were between 0° and 180° (i.e., had an eastward component) and the speeds of 96% were between 10 and 50 m s?1 with a mode at 25?30 m s?1. The other class, called incoherent events, or IEs, consists of disturbances that had significant amplitudes but that did not propagate coherently across the network, so that the propagation velocity could not be estimated. This class consists of localized disturbances and wave packets with short periods and/or wavelengths, or with pressure signatures that were too different at the network stations. The extracted events are attributed to gravity waves, wave packets, gravity currents, pressure jumps, solitary waves, bores, etc. The rate of occurrence of events had a strong seasonal dependence, with a maximum in fall and winter and a minimum in summer. The CEs occurred about 20%?21% of the total time in fall and winter and 12% in summer, while all events occurred 34% in both fall and winter and 23% in summer. The seasonal dependence of events confirms the strong relation of these disturbances to the baroclinicity of the atmosphere. Concurrent vertical velocity fluctuations observed by the 50-MHz radar at the Flatland Atmospheric Observatory showed that many of the large-amplitude events extended up to at least 7 km, the highest altitude reliably observed by the radar. | |
