Seasonal climatologies of persistent layers of enhanced refractive index structure parameter C2N were developed for the lower stratosphere from VHF radar observations at White Sands Missile Range, New Mexico, for the period January 1991 to September 1996. Knowledge of the nature of enhanced refractivity layers is of high interest to the atmospheric sciences, propagation, and remote sensing communities. The layers reported have C2N enhanced at least 7 dB above the background continuously for at least 11 hours and migrate vertically no more than one radar range gate (150 m) over 1 hour. The cumulative frequency of the lengths (11–37 hours) of the 259 persistent layers identified shows that 25% of the layers last over 17 hours. Comparisons of profiles of wind speeds, variances of the wind components, vertical shear of the horizontal wind, Doppler spectral width, temperature, Brunt-Vaisala frequency, and Richardon's number for times with and without persistent layers at 17 km show that wind speed at 5.6 km in addition to spectral width, wind shear, and vertical velocity variances at 17 km are stronger during enhanced layer episodes than during nonlayer periods. Possible sources for the persistent layers are suggested, and the shortcomings of each hypothesis are discussed. Several case studies of radiosonde ascents during persistent layers give no obvious indication of the source of these layers.
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