Characteristics of tropopause folds over Arctic latitudes

Abstract

[1] 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, K z , 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 m 2 s À1 , respectively. Utilizing the mean K z 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 À1 ppbv and 0.32 m s À1 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.

Tropopause fold occurrence rates over the Antarctic station Troll (72° S, 2.5° E)

“…As was shown by previous studies (in the Northern Hemisphere Rao and Kirkwood, 2005, and in the Southern Hemisphere Mihalikova et al, 2012), folds can be positively identified in radar data by static-stability features which agree closely with layers of enhanced ozone measured by colocated in situ measurements by ozonesonde. This makes radar observations a powerful source of information about the occurrence rates of tropopause folds in areas without the possibilities of targeted ozonesonde measurement campaigns.…”

“…Among direct observation methods, VHF radars in particular have proved to be a very useful tool in the study of tropopause folds. In the Northern Hemisphere, folds at mid-latitudes were studied with the help of Vaughan et al (1994), Hocking et al (2007) or at Arctic latitudes by Rao and Kirkwood (2005). Improved knowledge of tropopause folding processes is needed for understanding of the stratospheric source in the long-term development and budgets of tropospheric ozone (O 3 ).…”

Turbulence for different background conditions using fuzzy logic and clustering

“…Strong vertical shears of horizontal wind associated with jet streams give rise to Kelvin-Helmholtz instabilities(KHI) which are important sources for the generation of turbulence. Strong turbulence is observed bordering jet streams over both tropical (Rao et al, 2001;Das et al, 2010) and high latitude (Rao and Kirkwood, 2005) sites while Pepler et al (1998) found that there is widespread turbulence throughout the jet. Turbulence generated in the lower troposphere is either due to static instability or dynamic instability.…”

“…There is less data available at the higher altitudes as the SNR becomes very low in dry, neutrally stable air masses which are common in the upper troposphere (see e.g. Rao and Kirkwood, 2005).…”

The Coupled Effect of Mid-Tropospheric Moisture and Aerosol Abundance on Deep Convective Cloud Dynamics and Microphysics

“…This phenomenon has been extensively studied during the TOGA COARE IOP [6,57,58]. Recent studies have revealed dry layers in other regions, such as over the West African monsoon area [7], the Arctic [59], the United Kingdom [60] and the tropical Atlantic [61,62]. Further work is required to determine whether the effect of aerosol on cloud dissipation could have wider implications for large-scale cloud processes and climate.…”