Characteristics of tropopause folds over Arctic latitudes

Abstract

Characteristics of tropopause folds over Arctic latitudes have been studied using VHF radar measurements supplemented by balloon measurements. The variation of the radar parameters during the passage of tropopause folds is discussed in detail. To our knowledge, these observations constitute the first spaced antenna (SA) radar measurements during the passage of tropopause folds. This allows us to compare the parameters detectable using this mode with those observed using other configurations, such as the Doppler beam swinging (DBS) technique. In general, the structural characteristics, such as the slope of folds, seem to be similar at Arctic latitudes to that at midlatitudes; however, the height of the tropopause and the axis of the jet stream (and hence the folding) are found to be lower by 1–2 km than their counterparts in midlatitudes. In the case studies the radar-derived parameters, such as the signal-to-noise ratio (SNR) and vertical shear of horizontal wind, clearly show the upper-air frontal zone. The frontal circulation, conceived from vertical velocity, including the warm conveyer belt flow and the dry intrusion, is clearly visible in the first case, whereas it is masked by high-amplitude mountain lee waves in the second case. Further, the frontal zone seems to be acting as a critical layer to mountain lee wave activity by absorbing/filtering the wave activity. The aspect angles derived from the present analysis agree well with those estimated by vertical beam spectral width but are small in comparison with those estimated by the power ratio method. The mean full correlation analysis (FCA) turbulent velocity is estimated using the ESRAD data obtained during the passage of 15 tropopause folds. The mean eddy diffusion coefficients, Kz, near the tropopause and in the upper portion of the fold, where strong turbulence is seen in case studies, are found to be 3.54 and 6.4 m2 s−1, respectively. Utilizing the mean Kz and the mean ozone gradient (obtained from ozonesondes) values, the ozone fluxes across the tropopause and topside of the fold due to turbulence are estimated as 0.71 m s−1 ppbv and 0.32 m s−1 ppbv, respectively. Although the flux across the tropopause is more than that occurring near the fold, the exchange occurring through folds could be more significant to the tropospheric chemical budget.