3 YEARS OF C-BAND SIGNAL MEASUREMENTS FOR OVERWATER, LINE-OF-SIGHT LINKS IN THE MID-ATLANTIC COAST .2. METEOROLOGICAL ASPECTS OF SUSTAINED DEEP FADES

The morphology of deep and long-lived fades for line-of-sight, overwat er propagation links in the mid-Atlantic coast are examined. Such even ts, known as sustained deep fades (SDF), are analyzed employing weathe r maps, in situ measurements from radiosondes, an instrumented helicop ter, and sensors on coastal platforms. These events occurred exclusive ly from November through July over the 3 years in which fade statistic s were amassed; no SDF events were observed during the months August, September, or October. The SDF events biased the 3-year fade statistic s described in a companion paper. The results have demonstrated that s ynoptic weather conditions created by a sustained high-pressure system over the subtropical Atlantic and occasionally by a sustained low-pre ssure system whose center lies west of the coastal region result in a steady flow of warm, humid air at higher altitudes. When coupled with colder water conditions, such a flow results in a surface temperature inversion, an increase of water vapor pressure with altitude, and a po sitive lapse rate of the radio refractivity. This condition leads to s evere subrefraction resulting in fades ranging from 20 to 60 dB for du rations in excess of 2 hours and lasting as long as 2 days. Propagatio n modeling using a refractivity profile derived from sensors on board a helicopter during an SDF event predicted the same range of fades as were measured during that period.