Characteristics of the sporadic Na layers observed during the Airborne Lidar and Observations of Hawaiian Airglow/Airborne Noctilucent Cloud (ALOHA/ANLC-93) campaigns

We summarize the characteristics of the 43 sporadic Na (Nas) layers observed by the airborne Na density and the Haleakala Na wind/temperature lidars during the Airborne Lidar and Observations of Hawaiian Airglow/Airborne Noctilucent Cloud (ALOHA/ANLC‐93) campaigns. Nas layers are clearly not a local phenomenon since the horizontal extents varied from approximately 25 to almost 1600 km with a mean of 440 km. For the majority of the Nas layers observed above Haleakala, the temperatures were significantly warmer than the mean. The average enhancement was 12.9°K or about 6%. The temperature variations typically led the Nas density variations in time and on average reached their maximum values approximately 0.7 km below and 23 min before the Nas maxima. However, the maximum temperatures and Nas densities were only weakly correlated. Strong vertical shears in the horizontal winds were observed during the majority of the Nas events. The average maximum total shear was approximately equal to the shear instability limit of 42 m s−1 km−1. The maximum shears were located on average about 0.5 km below the Nas layer. In most cases the meridional shears were significantly stronger than the zonal shears. The maximum total shears were generally directed to the N‐NE and to the south. The strong temperature and wind perturbations observed during the majority of the events suggest that gravity waves play an important role in Nas formation, although the exact nature of that role remains unclear.