Key3: The Tropopause. Bernard Legras. Laboratoire de Météorologie Dynamique IPSL and ENS, Paris

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1 Key3: The Tropopause Bernard Legras Laboratoire de Météorologie Dynamique IPSL and ENS, Paris UTLS Summer School - Cargèse - 4 October 2005

2 THE TROPOPAUSE Definition and vertical structure Dynamical tropopause The height of the tropopause Trend in tropopause temperature and height

3 THE TROPOPAUSE Definition and vertical structure Temperature profiles Discovery of the tropopause The tropopause on other planets WMO definition Double tropopause structure Dynamical tropopause The height of the tropopause Trend in tropopause temperature and height

4 The tropopause is most simply identified by a discontinuity in the lapse rate (static stability) Tropical tropopause Mid-latitude tropopause Polar tropopause ERA-40 S. Malardel

5 Discovery of the tropopause Léon Teisserenc de Bort ( ) Unmaned balloon launching in Trappes, circa 1898 Discovery of the tropopause The instrument: the meteographe Glaisher & Coxwell, 1962

6 The tropopause on other planets Venus Titan

7 Ajaccio 1 October 2005

8 Standard NCEP definition of the tropopause The first tropopause (i.e., the conventional tropopause) is defined as the lowest level at which the lapse rate decreases to 2 K/km or less, and the average lapse rate from this level to any level within the next higher 2 km does not exceed 2 K/km. If above the first tropopause the average lapse rate between any level and all higher levels within 1 km exceed 3 K/km, then a second tropopause is defined by the same criterion as under the statement above. This tropopause may be either within or above the 1 km layer. A level otherwise satisfying the definition of tropopause, but occuring at an altitude below that of the 500 mb level will not be designated a tropopause unless it is the only level satisfying the definition and the average lapse rate fails to exceed 3 K/km over at least 1 km in any higher layer.

9 Simple and double tropopause structure

10 Latitudinal variation of the thermal tropopause tropopause double tropopause region

11 THE TROPOPAUSE Definition and vertical structure Dynamical tropopause Dynamical structure Potential vorticity Tracer discontinuity Tropopause folding and cyclogenesis Stratosphere-troposphere exchange The height of the tropopause Trend in tropopause temperature and height

temperature at the tropopause Temperature minima at the tropical

12 B. Randell Isentropic surfaces crossing the tropopause in the subtropics Potential temperature at the tropopause Wind and temperature at the tropopause Temperature minima at the tropical tropopause Subtropical jet winds associated with tropopause drop ERA-40 Atlas, 2005

13 The ERTEL potential vorticity f rot v 2 P= with the isentropic density = g p /, measuring static stability The potential vorticity P (PV) is a material invariant under inviscid and adiabatic approximation A discontinuity in static stability must show up equally well in PV Unit: 1PVU = 10-6 m2 s K kg-1

14 J. W. Nielsen-Gammon, A&M Univ., Texas PV crosssection (PVU) PV and zonal wind (m s-1) PV and absolute vorticity PV and potential temperature

15 Instantaneous height-latitude cross section of potential vorticity along a single longitude (55W), with the tropopause marked (in black) as the 2PVU contour. Courtesy of H. Wernli, ETH Zurich -> H. Wernli talks PV definition of the tropopause

17 Tracer discontinuity at the tropopause Ozone distribution from MOZART B. Randel

18 double tropopause associated with break near subtropical jet tropopause from aircraft profiler measurements zonal wind (from analysis) potential vorticity (from analysis) from Pan et al., JGR, 2004

19 aircraft ozone measurements B. Randel tropopause fold associated with stratospheric intrusion See also Bethan et al., QJRMS, 96

20 Flow Induced by an IPV Anomaly (Cyclonic) DYN AMI CT LESS STABLE ROP O PAU SE WARM MORE STABLE IPV COLD IR E O F E RI C A U G TON TOSPH A STR WEAKER TROPOSPHERIC STABILITY ALLOWS CONVECTION WHICH CAN LOWER OR DAMP THE IPV ANOMALY LESS STABLE COLD ADVECTION, LESS STATIC STABLITY AND ANTICYCLONIC IPV ON SURFACE VORTICITY AND STABILITY ANOMALIES TYPICALLY HAVE THE SAME SENSE AS IPV TO MAINTAIN THERMAL WIND BALANCE Hoskins, McIntyre and Robertson (1985) Fig. 9a. θ in K, PV in PV units

21 Upper-level frontogenesis and tropopause fold (I) STRATOSPHERE (HIGH PV) TROPOPAUSE (PV=2 ISO-SURFACE) TROPOPAUSE FOLD Massacand (1996) TROPOSPHERE (LOW PV)

22 Upper-level frontogenesis and tropopause fold (II) Planetary-scale tropopause fold on the subtropical jet in the southern hemisphere Baray et al., GRL, 2000

23 Stratospheric Transport and Exchange See talks by A. Gettelman H. Wernli J.P. Cammas

24 THE TROPOPAUSE Definition and vertical structure Dynamical tropopause The height of the tropopause Convective radiative equilibrium Stratification and the tropical tropopause Extra-tropical tropopause Sharpness of the tropopause Trend in tropopause temperature and height

25 Determination of the height of the tropopause by radiative-convective equilibrium Assumptions: - tropospheric lapse rate fixed by the moist adiabatic gradient - radiative balance in the stratosphere - match upward and downward radiative fluxes at the tropopause From Manabe & Wetherald, JAS, 1967

26 Stratification and the tropical convection pdf of moist potential temperature at the surface Folkins, JGR, 1999 Convection is able to mix efficiently air from the surface to about 345K but not above. Folkins, JAS, 2002

27 Vertical mass transport in the tropical region Region of strong convective outflow Folkins, JAS, 2002

28 Tropical tropopause Folkins, JGR? 1999 Highwood & Jhoskins, QJRMS, 1998 Adapted from Folkins, JAS, 2002 See also talk by A. Gettelman

29 Height of the extra-tropical tropopause The convective-radiative equilibrium cannot explain the temperature profile of the the extratropics. Another concept needs to be substituted to convective adjustment; Adjustement to slantwise convection induced by moist symmetric instabilities Emanuel, MWR, Baroclinic adjustment: The atmosphere is maintained in a neutral baroclinic state -> dynamical constrain relation between tropopause height HT and lapse rate, which, added to the radiative constrain of matching fluxes at the tropopause, determines both quantities Held, JAS, 1982 ; Lindzen, JAS, 1993; Harnik & Lindzen, JAS, However, GCM sensitivity studies [Thuburn & Craig, JAS, 1997] do not show that the dynamical constrain is not satisfied when relevant parameters are modified.

Haynes, Scinoca & Greensdale, GRL, 2001 Relaxation state in a simple GCM Resulting state Baroclinic activity alone is able (1) to increase PV gradient at about 300

30 Haynes, Scinoca & Greensdale, GRL, 2001 Relaxation state in a simple GCM Resulting state Baroclinic activity alone is able (1) to increase PV gradient at about 300 hpa, and lat>50n, with a corresponding increase in lapse rate gradient, (2) increase isentropic PV gradient at 45N. Role of isentropic mixing <-> tropopause as a wall.

31 Schneider, JAS, 2005; Schneider & Walker, JAS (sub.), 2005; Schneider, AREPS, 2006 Isentropic mass flux balance isentropic mass flux eddy PV flux surface θ flux Dynamical constrain Ekman

32 temperature potential temperature Unstable relaxation state Schneider, JAS, 2004 GCM statistical state temperature potential temperature

33 Isentropic streamfunction ERA-40 Atlas, 2005 Schneider, JAS, 2004

34 Sharpness of the tropopopause and inversion layer anticycl JJA Inversion above the tropopause cycl DJF ERA-40 Std dev Birner, Dörnbrack & Schumann, GRL, 2002 Std dev

35 Studies by Wirth (JAS, 2004) and Lapeyre et al. (sub JPO, 2005) both suggest that PV advection and upper level frontogenesis contribute to a pic of N2 above the tropopause. Lapeyre et al., sub JPO, 2005!! Ocean-> inverted Wirth, JAS, 2004 z N2

36 THE TROPOPAUSE Definition and vertical structure Dynamical tropopause The height of the tropopause Trend in tropopause temperature and height Temperature trend Height trend Ozone column and tropopause height

Negative trend in temperature at tropopause level Angell, J.K. 2005.

37 Negative trend in temperature at tropopause level Angell, J.K Global, hemispheric, and zonal temperature deviations derived from radiosonde records. Oak Ridge National Laboratory, U.S. Department of Energy

38 Trend in tropopause height Increase of about 5 hpa, ~ 200 m over 20 years Global monthly-mean average Sausen & Santer, Meteror. Z., 2003

39 Trend and anthropogenic forcing Santer et al. Science, July 2003

40 Santer et al. Science, July 2003

41 Santer et al. Science, July 2003

42 Correlation of ozone and tropopause height on short and long time scales column ozone and tropopause height at Hohenpeissenberg WMO, 2002 updated from Steinbrecht et al 2001

Atmospheric dynamics and meteorology

Atmospheric dynamics and meteorology B. Legras, http://www.lmd.ens.fr/legras II Potential vorticity, tropopause and baroclinic instability (supposed to be known: notions on the conservation of potential