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Chapter 1: What is the Mesoscale? Mesoscale energy sources.

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1 Chapter 1: What is the Mesoscale? Mesoscale energy sources

2 Gage and Nastrom (1985) [shifted x10 to right] Note two spectral extremes: (a) A maximum at about 2000 km (b) A minimum at about 500 km 1 10010 1000 wavelength [km] (1) Scales of atmospheric motion inertial subrange (Kolmogorov 1941) power spectrum units: m 2 s -2 per wavenumber (m -1 ) bin k = 1/

3 FA=free atmos. BL=bound. layer L = long waves WC = wave cyclones TC=tropical cyclones cb=cumulonimbus cu=cumulus CAT=clear air turbulence From Ludlam (prior to Gage/Nastrom) energy cascade mesoscale Big whirls have little whirls that feed on their velocity; and little whirls have lesser whirls, and so on to viscosity. -Lewis Fry Richardson

4 Scales of atmospheric motion Air motions at all scales from planetary-scale to microscale explain weather: – planetary scale: low-frequency (10 days – intraseasonal) e.g. MJO, blocking highs (~10,000 km) – explains low-frequency anomalies size such that planetary vort adv > relative vort adv hydrostatic balance applies – synoptic scale: cyclonic storms and planetary-wave features: baroclinic instability (~3000 km) – deep stratiform clouds size controlled by =df/dy hydrostatic balance applies – mesoscale: waves, fronts, thermal circulations, terrain interactions, mesoscale instabilities, upright convection & its mesoscale organization: various instabilities – synergies (10-500 km) – stratiform & convective clouds time scale between 2 /N and 2 /f hydrostatic balance usually applies – microscale: buoyant eddies (cumuli, thermals), turbulence: static and shear instability (1-5 km) – convective clouds Size controlled by entrainment and perturbation pressures no hydrostatic balance buoyancy: 2 /N ~ 2 /10 -2 ~ 10 minutes inertial: 2 /f = 12 hours/sin(latitude) = 12 hrs at 90°, 24 hrs at 30°

5 Fig. 1.1

6 Eulerian vs Lagrangian Eulerian time scale t e : time for system to pass, assuming no evolution –t e =L/U, where L is size, U is basic wind speed Lagrangian time scale t l : time for particle to travel through system –for tropical cyclone or tornado, –for sea breezes, –for internal gravity waves, Lagrangian Rossby number: intrinsic frequency / Coriolis parameter –Ro l = 1 for inertial oscillations, but Ro l >>1 for buoyancy oscillations Rossby radius of deformation: –see COMET module the balancing act of geostrophic adjustmentthe balancing act of geostrophic adjustment

7 geostrophic adjustment: principle L

8 Will a feature last or dissipate? Estimate its L R

9 1.2 Mesoscale vs. synoptic scale Fig. 1.2 (Fujita 1992)

10 1.2 Mesoscale vs. synoptic scale Storm Predictions Center Meso-analysis page Fig. 1.3 24 hr radar loop

11 1.2 Mesoscale vs. synoptic scale 1.2.1 gradient wind balance 1.2.2 hydrostatic balance on chalkboard key results: Fig. 1.4 Ro1 for mesoscale flow The aspect ratio (D/L) determines whether hydrostatic balance applies

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