1. Stratospheric Forcing of Mid-tropospheric Blocking Anticyclogenesis
Stephen J. Colucci
Cornell University

2. Motivation
Sudden Stratospheric Warmings (SSWs) are occasionally preceded by tropospheric blocking (e.g. O’Neill and Taylor, QJRMS,1979; Quiroz, JGR, 1986).
However, there is no meaningful relationship between SSWs and antecedent or eventual blocking (Taguchi, JAS, 2008).
Nevertheless, can stratospheric processes influence the onset of tropospheric blocking anticyclogenesis?

3. Procedure
Study a tropospheric blocking episode during January 1987 when there was a downward propagating stratospheric warming event (Baldwin and Dunkerton, JGR, 1999).
Use ERA-40 data (Z, T, u, v, ) on 23 pressure levels (1000 mb through 1 mb) at 6-h time resolution and 2.5-degree horizontal resolution.
Supplement these data with NNRP diabatic heating fields (long- and short-wave heating, condensational warming) on 28 sigma surfaces (0.995 to ) at 6-h time resolution, every degrees in longitudinal direction and on a 94-point Gaussian grid from pole to pole.

4. Evolution of January 1987 blocking episode

5. Analyzed 72-h 500-mb height change during block onset

6. Diagnostic Model for 500-mb Height Tendency
Vertically integrate hydrostatic, thermodynamic energy equation between bottom pressure Pb and top pressure PT :
∂Zb/∂t = ∂Zt/∂t Pb
- (Rd/g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P Pt
[ similar to Hirschberg and Fritsch (JAS, 1991) ]

7. (Diagnostic Model continued)
Set Pb = 1000 mb and PT = 500 mb and rearrange to get 500-mb height tendency equation in terms of lower tropospheric processes:
∂Z500/∂t = ∂Z1000/∂t
1000
+ (Rd/g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P
500

8. (Diagnostic Model continued)
Set Pb = 500 mb and PT = 20 mb* to get a 500-mb height tendency equation in terms of upper-level (including stratospheric) processes:
∂Z500/∂t = ∂Z20/∂t
500
- (Rd/g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P 20
*vertical motion data are unreliable in the upper stratosphere,
according to Wohltmann and Rex (Atm. Chem. Phys., 2008)

9. (Diagnostic Model continued)
Combine previous two equations for 500-mb height tendency to obtain:
∂Z500/∂t = [ ∂Z20/∂t + ∂Z1000/∂t ]/2
1000
+ (Rd/2g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P
500
- (Rd/2g) ∫ [ (1/Cp) dh/dt - VH . p T + P /Rd ] dP/P 20

10. Total contribution to 500-mb height tendencies from lower tropospheric (left) and upper-level (right) processes during block onset

11. Analyzed and predicted 500-mb anticyclogenesis from lower-level (left) and upper-level (right) processes

12. Area-averaged (10 deg X 10 deg) total calculated 500-mb height tendencies (upper forced versus lower forced) following maximum analyzed 500-mb height rise

13. Area-averaged advective versus adiabatic 500-mb height tendencies, following maximum analyzed 500-mb height rise

14. Area-averaged radiatively versus condensationally forced 500-mb height tendencies, following maximum analyzed 500-mb height rise

15. Conclusions
The 500-mb anticyclogenesis associated with the January 1987 blocking case critically depended upon upper-level (mostly stratospheric) adiabatic cooling.
Lower tropospheric adiabatic and advective warmings assisted the 500-mb anticyclogenesis.
Condensational warming, mostly from the lower troposphere and associated with a sea-level cyclone (not shown), assisted with the maintenance of the 500-mb anticyclone, once it was established.

16. Question: Is there a stratospheric influence on sea-level cyclones and anticyclones? Answer: Yes. See manuscript by Colucci (in review by JAS, 2008) for details (available upon request).
Briefly: stratospheric warm-air advection helped force sea-level cyclogenesis in two cases; stratospheric adiabatic cooling initiated a spectacular sea-level anticyclogenesis case.
Important question: What is the typical thermodynamic structure of the stratosphere above tropospheric weather systems?