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Uccellini and Johnson (1979)

Uccellini and Johnson (1979). Uccellini, L. W., and D. R. Johnson, 1979: The Coupling of Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms. Mon. Wea. Rev . 107, 682–703. Purpose of paper:

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Uccellini and Johnson (1979)

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  1. Uccellini and Johnson (1979) Uccellini, L. W., and D. R. Johnson, 1979: The Coupling of Upper and Lower Tropospheric Jet Streaks and Implications for the Development of Severe Convective Storms. Mon. Wea. Rev. 107, 682–703.

  2. Purpose of paper: To determine how the transverse ageostrophic circulation about an upper tropospheric jetstreak influences the intensity of the low level jet and the development of severe convection • Methods: • Theory • Numerical model • Observation Typical severe weather scenario with upper and lower level jets

  3. indirect circulation associated with the exit region of a jetstreak THEORY OUR GOAL: Derive an equation that relates the ageostropic wind on a low level theta surface to the pressure tendency. Low level pressure changes are forced by the upper level jetstreak. The equation we seek will theoretically relate an accelerating low level jet to the ageostrophic circulation induced by the upper level jet JET Ps decreasing Ps increasing Contours (vertical motion in mb/s) Arrows (ageostrophic circulation) Number under arrows (ageostrophic wind (m/s))

  4. The theoretical development in this paper is cast in isentropic coordinates We will apply the geostrophic momentum approximation in this paper: Assume that the wind can be approximated by its geostrophic value except when considering advection. The (vector) expansion of the total derivative in isentropic coordinates The geostrophic momentum approximation: We approximate U by Ug except in the advection term

  5. (1) From before: Recall in scalar notation Use vector form of momentum equation (2) Substitute from (1) into (2) Assume adiabatic flow and neglect second term

  6. With the assumptions made above, the ageostrophic wind is related to 1. the rate of change of the pressure gradient force (i.e. geostrophic wind) on a theta surface (the isallobaric component) 2. Advection/inertial forces Recall from ATMOS 403 that the geostrophic wind on a theta surface is related to the gradient of the Montgomery stream function: where the Montgomery stream function is given by Put in the isallobaric term and it becomes…..

  7. Isallobaric contribution to the ageostrophic wind Hydrostatic equation in isentropic coordinates Integration of hydrostatic equation from surface to theta surface (L) Time derivative of equation above:

  8. From last page… Find expression for this term Substitute equation for theta And take derivative Substitute in top equation: last RHS term in top cancels first RHS term on bottom

  9. Simply by using expression for Isallobaric contribution to the ageostrophic wind From a while ago Combine equations:

  10. The isallobaric component of the ageostrophic circulation on an isentropic surface L… ..is related to the gradient of the surface pressure tendency .. ..and the integrated pressure tendency Between the surface and L An existing low level jet will accelerate in this region, increasing shear and potential for severe weather JET EXIT L s,1 s,2

  11. Model Hybrid isentropic/sigma coordinate model used to simulate jetstreak circulations L rest of atmosphere =  coordinates lower 200 mb =  coordinates Initial conditions: functions designed to produce jetstreak

  12. Air exiting column Mass flux divergence ( 10 g m-2 s-1) in each quadrant of jetstreak as a function of altitude () from model ageostrophic component dominates Air entering column Solid (total), ageostrophic (dashed), geostrophic (dot dashed)

  13. Isallobaric term Inertial/advective term At the level of the jetstreak, the pressure tendency is small (weak isallobaric effect), but advection/coriolis force effects are large Streamline analysis at the level of the jet streak due to the inertial advective term

  14. At low levels, the isallobaric effect dominates, creating the ageostrophic acceleration of the winds toward the north under the exit region of the jet

  15. Ageostrophic flow forced by along stream variations in PGF at jetstream level Entrance Exit Isallobaric ageostrophic wind on a lower isentropic surface beneath jet

  16. P P Surface pressure falls in exit region lead to ascent of theta surfaces and isallobaric acceleration upward and northward along potential temp surface Heating transports mass from below to above L P on isentropic surf Cool here P on isentropic surf Heat here (latent heat in convection) Differential diabatic heating has the same effect

  17. An example case study: This study chosen because there was a jetstreak in the upper troposphere, but no low level jet in the lower atmosphere. The development of the LLJ could be attributed to the isallobaric accelerations associated with the jetstreak circulation. Surface charts for the 12 hour period

  18. Radar depictions: Cloud tops in hundreds of feet RW = rainshowers TRW=thunderstorms

  19. Jetstreak propagating eastward 330 K isotachs and Mongomery streamfunction No cross-jet flow here Downward extension of Upper jetstreak 300 K isotachs and Mongomery streamfunction SW 20 m/s LLJ between Kentucky and Pennsylvania flowing from 900 to 700 mb 300 K pressure and mixing ratio Moist air intrusion

  20. Another view of upper jetstreak and LLJ on isobaric surfaces Note angle between the isotachs and height contours: LLJ represents ageostrophic flow associated with isallobaric forcing

  21. Note the four quadrant jetstreak pattern 12Z 10 May 340 K 300 K Surface 00Z 11 May 340 K 300 K Surface Green: increasing with time Red: decreasing with time Mass tendencies in layers bounded by isentropes or surface

  22. Actual data in cross section illustrating same effect Potential temperature 325 K 305 K J J Wind speed

  23. Evaluation of the integrated pressure tendency term in the equation for the isallobaric ageostrophic wind on 300 K surface

  24. Further illustrates lower level ageostrophic flow induced by upper level jet streak Term 1: Isallobaric wind Term 3: Integrated pressure tendency Term 2: Surface pressure tendency

  25. Normal wind Implications of coupled jet streaks for convective storm development • Intensification of the LLJ by isallobaric forcing: • significantly increases lower atmospheric moisture flux • Significantly increases lower atmospheric sensible heat flux • enhances low level shear • THEREFORE: Moisture flux Sensible Heat flux enhances environment for severe storm development Cross sections from Omaha, Nebraska to Tetersboro, New Jersey

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