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Chapter 1: What is the Mesoscale?

Chapter 1: What is the Mesoscale?. Mesoscale energy sources. (1) Scales of atmospheric motion. k = 1/ l. Note two spectral extremes: (a) A maximum at about 2000 km (b) A minimum at about 500 km. [shifted x10 to right]. inertial subrange ( Kolmogorov 1941). power spectrum

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Chapter 1: What is the Mesoscale?

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

  2. (1) Scales of atmospheric motion k = 1/l Note two spectral extremes: (a) A maximum at about 2000 km (b) A minimum at about 500 km [shifted x10 to right] inertial subrange (Kolmogorov 1941) power spectrum units: m2 s-2 per wavenumber (m-1) bin 1 100 10 1000 wavelength [km] Gage and Nastrom (1985)

  3. energy cascade Big whirls have little whirlsthat feed on their velocity;and little whirls have lesser whirls,and so on to viscosity.                  -Lewis Fry Richardson 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) mesoscale

  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 vortadv > relative vortadv • hydrostatic balance applies • synoptic scale: cyclonic storms and planetary-wave features: baroclinic instability (~3000 km) – deep stratiform clouds • size controlled by b=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 2p/N and 2p/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: 2p/N ~ 2p/10-2 ~ 10 minutes inertial: 2p/f = 12 hours/sin(latitude) = 12 hrs at 90°, 24 hrs at 30°

  5. Fig. 1.1

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

  7. L geostrophic adjustment: principle

  8. Will a feature last or dissipate? Estimate its LR

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

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

  11. 1.2 Mesoscale vs. synoptic scale Ro≥1 for mesoscale flow The aspect ratio (D/L) determines whether hydrostatic balance applies 1.2.1 gradient wind balance 1.2.2 hydrostatic balance on chalkboard  key results: Fig. 1.4

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