Monday, 13 May 2002: 2:15 PM
A case study of atmospheric conditions at 2-19 km over Vandenburg AFB during passage of a cyclone
Continuous observations from a 50 MHz radar of the vertical profiles of winds, turbulent kinetic energy density (TKE), refractivity turbulence structure constant (Cn2), and the variance of the vertical velocity are combined with routine rawinsonde ascents to characterize conditions at 2-19 km over Vandenburg Air Force Base, California, during 21-27 July 2001. Early and late in this period the flow aloft was relatively light and turbulence levels very small. For about 36 hours around 25 July the flow at mid-levels (about 600-200 mb) was disturbed by the passage of a vigorous cyclone. Cn2 intensities were enhanced by over 15 dB during this event. The enhancements of Cn2 are highly correlated with the relative enhancements of TKE and vertical velocity variance, as well as with the meridional wind speed. It is tentatively suggested that the increased turbulence intensities are due to in situ processes such as shear instabilities or geostrophic adjustment associated with the strong wind of the cyclone. It seems unlikely that they are due to waves upwelling from the surface because throughout this period a strong low-altitude inversion was observed by the 12-hourly radiosonde ascents; the top of the inversion layer was typically about 1 km above the surface. In the lower stratosphere, above 16 km, winds show a periodic oscillation with downward phase progression with dominant period less than 24 hours.