Static Stability in the Global UTLS Observations of Long-term Mean Structure and Variability using GPS Radio Occultation Data Kevin M. Grise David W. J. Thompson Thomas Birner Department of Atmospheric Science Colorado State University Extratropical UTLS Workshop Boulder, CO October 19, 2009

Overview Static stability is a fundamental quantity for atmospheric dynamics. GPS radio occultation data provides the global coverage and high vertical resolution necessary to conduct a global survey of static stability in the upper troposphere and stratosphere. Annual mean Seasonal cycle Longitudinal structure Weekly and monthly variability

CHAMP GPS Radio Occultation Data (Wickert et al. 2001) Approximately 200 daily temperature profiles since May 2001 Global coverage Independent of weather conditions Interpolated to 100-m vertical resolution Most accurate between 5 km and 30 km altitude

Static Stability Annual mean, Zonal mean Altitude-based Average Tropopause-based Average White lines highlight shading > 6.0 x 10-4 s-2

Static Stability: Extratropics Annual mean, Zonal mean Altitude-based Average Tropopause Inversion Layer (TIL): Sharp temperature inversion in narrow layer above extratropical tropopause Zonal mean climatology is well established (e.g., Birner et al. 2002; Birner 2006; Randel et al. 2007). Physical explanation for existence of TIL is still subject to some debate (e.g., Wirth 2003; Wirth and Szabo 2007; Randel et al. 2007). Tropopause-based Average White lines highlight shading > 6.0 x 10-4 s-2

Static Stability: Extratropics Seasonal mean, Zonal mean Summer polar TIL is stronger than winter polar TIL, particularly in SH. SH stratospheric polar vortex has large N2 values. White lines highlight shading > 6.0 x 10-4 s-2

Stratospheric Dynamic Variability 60°S - 90°S 2002: Sudden stratospheric warming 2006: Strong stratospheric polar vortex

Static Stability: Tropics Annual mean, Zonal mean Altitude-based Average “Tropical TIL” : Shallow N2 maximum centered at Equator (0-1 km above tropopause) Broader N2 maxima centered between 10˚ and 20˚ latitude in each hemisphere (1-3 km above tropopause) Tropopause-based Average White lines highlight shading > 6.0 x 10-4 s-2

Longitudinal Structure: Tropics

Conclusions Static stability in the extratropical UTLS is characterized by the tropopause inversion layer (TIL). Large amplitude seasonal cycle in polar regions Little coherent longitudinal structure Wintertime strength in polar regions tied to stratospheric dynamic variability Static stability in the tropical UTLS has a unique horizontally and vertically varying structure. Equatorially centered maximum immediately above tropopause Off-equatorial maxima 1-3 km above tropopause Reminiscent of equatorial planetary wave response to tropical convection

Conventional Vertical Coordinates Seasonal-mean, Zonal-mean Summer polar TIL is stronger than winter polar TIL, particularly in SH. SH stratospheric polar vortex has large N2 values. N2 in tropics is strongest during boreal winter and has local maximum in winter hemisphere.

Static Stability: Tropics Seasonal mean, Zonal mean Tropical N2 structures are strongest during boreal winter. Off-equatorial N2 maximum in winter hemisphere is slightly stronger than that in summer hemisphere. White lines highlight shading > 6.0 x 10-4 s-2

Seasonal Cycle in Polar Regions

Seasonal Cycle in Tropics

Longitudinal Structure 1 km layer average of N2 above tropopause Except for mid-latitude winter, the strength of the extratropical TIL exhibits little coherent longitudinal structure.

Intraseasonal Variability Define a TIL index to represent the temporal evolution of near-tropopause static stability values. 1 km layer average of N2 anomalies above tropopause

Stratospheric Dynamic Variability 60°N-90°N, November-April Units: 10-5 s-2 Positive (negative) N2 anomalies at leading edge of descending warm (cold) anomalies. During winter, NH polar TIL strength appears to be strongly linked to stratospheric dynamic variability.

Stratosphere-Troposphere Coupling Implications Stratosphere-Troposphere Coupling Static Stability Zonal Wind Thompson et al. (2006) The maximum in static stability near the tropopause coincides with the period after lag 0 during which stratosphere-troposphere coupling is typically observed.

Quasi-Biennial Oscillation 20°N - 20°S Descending positive N2 anomalies are associated with the easterly phase of the QBO. The QBO appears to be the dominant driver of the strength of near-tropopause N2 values in the tropics.