Atmospheric surface-layer processes during the total solar eclipse of 11 August 1999

The central path of the total solar eclipse (TSE) of 11 August 1999 mostly passed through a region of active monsoon in India, with the eclipse ending around local sunset. Measurements in the surface layer (SL) were carried out close to the eclipse axis at Akola (20degrees42' N, 77degrees2' E) in central India. The global radiation flux reaching the surface vanishes around totality at 1803:24 (LT), followed by a small recovery before again dipping to zero at sunset. The temperatures in the SL, and subsoil at 50-mm depth, show a local minimum with a lag of about 10 min after the second contact, whereas the lag appears to vanish when the temperature series is detrended. The SL exhibits near-neutral, though generally stable, conditions from about 1500 hr itself due to heavy cloud cover followed by the eclipse-induced cooling of the surface. The wind component across the eclipse axis vanishes at totality, the wind vector aligning with the azimuth of the traversing moon shadow. The deceleration of the mean flow can be due to the combined effect of the colder surface and downward heat flux, where the locally altered horizontal temperature gradients may cause, as in this instance, the cessation of the cross flow. The oscillations in temperature and wind that show significant peaks, around 90-100 min as well as the semi-period of the eclipse near 60 min, persist for several hours past the eclipse event. A fresh analysis of the published data on the TSE of 7 March 1970 shows spectral peaks in the temperature nearly coincident with those already reported from the surface pressure records. The oscillations in the SL fields during the two TSE events are very similar implying that the source mechanism is also likely to be the same in both the cases.