Mesoscale Atmospheric Systems. Surface fronts and frontogenesis. 06 March 2018 Heini Wernli. 06 March 2018 H. Wernli 1

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1 Mesoscale Atmospheric Systems Surface fronts and frontogenesis 06 March 2018 Heini Wernli 06 March 2018 H. Wernli 1

2 Temperature (degc) Frontal passage in Mainz on 26 March March 2018 H. Wernli 2

3 Pressure (hpa) 06 March 2018 H. Wernli 3

4 Windspeed (m/s) Winddirection 06 March 2018 H. Wernli 4

5 Rapid surface frontogenesis over the US surface isotherms (F) and wind barbs 12 UTC 20 Jan UTC 21 Jan 1959 Carlson 1998, Fig March 2018 H. Wernli 5

6 Concept of frontogenesis due to confluent flow Confluence occurs in flows with strong deformation (see later) Note: confluence convergence! Bluestein (in Rao 1985, Fig. 9.7) 06 March 2018 H. Wernli 6

7 Vertical cross-section across front isentropes, normal wind velocity Sanders 1955 Carlson 1998, Fig March 2018 H. Wernli 7

8 What is a front? front = elongated zone of strong temperature gradient elongated: length ~1000 km; width ~ km strong: ~5 K (100 km) -1 i.e., an order of magnitude larger than typical background baroclinicity ~5 K (1000 km) -1 Other definitions (less appropriate): temperature discontinuity boundary between tropical and polar air masses 06 March 2018 H. Wernli 8

9 Importance of fronts ubiquitous weather phenomenon in extratropics strongly varying meteorological conditions across front severe weather associated with frontal passage (heavy rain, thunderstorms, strong winds) upper-level fronts (see later) often associated with clearair turbulence and stratosphere-troposphere exchange 06 March 2018 H. Wernli 9

10 Kinematics: frontogenesis function Early concepts by Petterssen (1936) Miller (1948) Basic idea: Consider =? (1) 06 March 2018 H. Wernli 10

11 Theory of frontogenesis Horizontal version of (1) in quasi-geostrophic limit: g (2) is the Q-vector and E = Dθ/Dt is the diabatic heating (latent heating in clouds, radiation) Caveat: These equations describe evolution of horizontal temperature gradient along motion of fluid parcel but not for the front itself! 06 March 2018 H. Wernli 11

12 Radiative effects on frontogenesis Frontogenesis with stratus cloud in cold air Frontolysis with stratus cloud in warm air Carlson 1998, Fig March 2018 H. Wernli 12

13 Theory of frontogenesis Consider adiabatic limit (E=0) of surface frontogenesis (i.e., consider only horizontal gradient of θ): where D total deformation = (α 12 +α 22 ) δ angle between dilatation axis and isentropes Note: there is a deformation term and a divergence term (which is zero in the q.g. limit) 06 March 2018 H. Wernli 13

14 Effect of deformation on fronts frontogenetic (b=δ < 45 ) frontolytic (b=δ > 45 ) Bluestein (in Rao 1985, Fig. 9.9) 06 March 2018 H. Wernli 14

15 Quasi-geostrophic frontogenesis Structure of front as evolved from QG deformation frontogenesis Unrealistic aspects: frontal zone does not tilt with height regions of static instability are produced Remark about lecture notes: Chap. 1.1 and 1.2: what we discussed so far Chap : a detailed mathematical excursion using semi-geostrophic theory Chap 2: what we do next Stone 1966 (in Rao 1985, Fig.9.12) 06 March 2018 H. Wernli 15

16 Fronts in baroclinic waves fronts develop typically within growing baroclinic waves cold fronts are typically 2-dimensional, warm fronts 3-dim Idealized experiment of baroclinic instability: baroclinic zone (i.e., upper-level jet) and finite amplitude upper-level perturbation (i.e., positive PV anomaly) Hoskins et al March 2018 H. Wernli 16

17 Idealized experiments of fronts developing in intensifying extratropical cyclone Day 0 initial perturbation: upper-level trough (induces weak surface pressure minimum) north-south temperature gradient 06 March 2018 H. Wernli 17

18 Simultaneous development of surface cyclone and fronts Day 1 06 March 2018 H. Wernli 18

19 Simultaneous development of surface cyclone and fronts Day 2 06 March 2018 H. Wernli 19

20 Simultaneous development of surface cyclone and fronts Day 3 06 March 2018 H. Wernli 20

21 Simultaneous development of surface cyclone and fronts Day 4 06 March 2018 H. Wernli 21

22 Quasi-geostrophic frontogenesis: the dynamical picture Equations for vorticity, vertical motion and the horizontal temperature gradient g g g where the material derivative is only along the geostrophic flow g 06 March 2018 H. Wernli 22

23 Frontogenesis in evolving extratropical cyclone (dry, E=0) Q-vectors and divq at the surface on day 2 Wernli et al March 2018 H. Wernli 23

24 Frontogenesis in evolving extratropical cyclone Q-vectors and divq at the surface on day 4 06 March 2018 H. Wernli 24

25 Frontogenesis in evolving extratropical cyclone Strong difference between warm and cold fronts with respect to correlation of ζ (gray colors) and divq (dashed contours): WHY? 06 March 2018 H. Wernli 25

26 Frontogenesis in evolving extratropical cyclone Flow of air parcels in regions of cold, warm, and bent-back warm front; dashed lines show regions of large divq parcel positions on day 2 day 3 day 4 cold frontal air parcel moves with front warm frontal air parcels move along front and cross max divq region 06 March 2018 H. Wernli 26

27 Temporal evolution of vorticity along trajectories vorticity 06 March 2018 H. Wernli 27

Atmospheric dynamics and meteorology

Atmospheric dynamics and meteorology B. Legras, http://www.lmd.ens.fr/legras III Frontogenesis (pre requisite: quasi-geostrophic equation, baroclinic instability in the Eady and Phillips models ) Recommended