1. Contact persons: Carmen J. Nappo:nappo@atdd.noaa.gov; phone 865 576 1252; fax 865 576 1327 David L. Auble :auble@atdd.noaa.gov; phone 865 576 2344; fax 865 576 1327 2. Data description. The ATDD-microbarograph network was designed to monitor the temporal and spatial characteristics of gravity waves and other coherent, i.e., non-turbulent atmospheric pressure perturbations. These perturbations are obtained as pressure differences between the static atmospheric pressure and a reference pressure as described in Section 3. 3. Instrument description. The ATDD differential microbarograph records the difference, ëP(t), between the static atmospheric pressure, Pa(t), and the pressure in a thermally insulated closed reference chamber, Pc, where t is the time. Thus, ëP(t) = Pa(t)-Pc. The differential pressure gauge (SETRA model 264) has a range of ñ2.5 mb, and over a course of a day, especially during frontal passages, the change in atmospheric pressure can exceed this value. When this happens, the pressure gauge saturates. To prevent instrument saturation, the reference chamber is periodically set to atmospheric pressure by opening a valve. If the valve is opened at time t0, then the differential pressure is given by ëP(t+t0) = Pa(t+t0)-Pc(t0). During CASES-99, the reference pressure was set to atmospheric pressure every 15 minutes. The recorded data are ramp-like structures each 15 minutes in length beginning at ëP÷0 and either ascending or descending depending on the atmospheric pressure tendency. These raw data are labeled T, for example T-276.dat is the raw data file for Julian day 276. To construct a continuous string of differential pressure, the 15-minute segments are joined end-to-end. These files are labeled P, for example P-276.dat. 4. Locations, times and units. Microbarographs were located at CASES-99 towers 1, 4, 5, 7, 8, and 9. In the data files these locations are labeled S1, S4, S5, S7, S8, and S9 respectively. Differential pressure data were recorded continuously from 3 October to 31 October, 1999. Data sampling was at 10 Hz from which 10-second averages were calculated and recorded. The times in the data files are in fractions of day, and the differential pressure units are in microbars. 5. Data usage. Coherent pressure signals can be obtained from the P-files by band-pass filtering, wavelet analyses, etc. Since the characteristics of these digital filters are user-specified, we provide here only the elementary data.