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an atmospheric “ mesoscale ”: where convection meets waves (rotation optional)

an atmospheric “ mesoscale ”: where convection meets waves (rotation optional). Brian Mapes University of Miami. for oceanographers. In a moist convecting atmosphere, small scale vertical motions don’t just carry fluxes, they cause latent heating

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an atmospheric “ mesoscale ”: where convection meets waves (rotation optional)

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  1. an atmospheric “mesoscale”:where convection meets waves(rotation optional) Brian Mapes University of Miami

  2. for oceanographers • In a moist convecting atmosphere, small scale vertical motions don’t just carry fluxes, they cause latent heating • OK, you can view it as a vertical flux of water substance upward past the condensation level. • Spectral space: energy injection across scales • Physical space: feedback small updrafts • UV catastrophe of conditional instability (Lilly 1961) • Smallest updrafts, broadest subsidence (Bjerknes 1938)

  3. mesoscale convection • “Mesoscale” convection events (meso = middle, in between L~H “convective” scale and N/fH “Rossby radius”) are less theoretically tidy than parcel or exp(ikx) UV catastrophe, but profound & real • convectively coupled internal waves

  4. 5 decades 3 decades Convectively coupled gravity waves in 2D CRMNo preferred hor. scale } Eureka! 5/3! Stefan Tulich (Mapes et al. 2009 JMSJ)

  5. ...pas? Scale Interactions cascade...

  6. mesoscale convection • Are these things coherent aspects (the spectral tail) of the large-scale flow, or an emergent metaphenomenon bubbling up from convection? • Implication: is it better to spend computing DOFs to resolve the mesoscale? Or rather on little hi-res but periodic “sample” patches of convective-scale flow, coupled across an enforced scale separation? • (MMF or “super-parameterization”)

  7. 3D global simulations • GEOS-5 global AGCM at 5km mesh size • by Bill Putman, Max Suarez, others at NASA GSFC • 20-day run analyzed here • Cubed sphere grid, nonhydrostatic • GCM physics left on – mostly • subgridscale plumes hobbled by entrainment • disabled subgridorographic gravity wave drag

  8. comparison to satellite imagery http://earthobservatory.nasa.gov/IOTD/view.php?id=44246&src=eoa-iotd predicted cloud features for February 6, 2010 2 weeks into simulation

  9. 5kmGCM: detailed examinations • 1. Tropical mesoscale rain events: case studies • One scale selected in to analysis: 250km events • Rebinrainrate to 2.5deg, find 10 largest maxima • in ~20 day simulation period (Jan-Feb 2010) • in 15N-15S, to minimize cyclone dominated cases • Extract space-time cubes around these events • (+/-18h, +/- 3 degrees) • 10 wettest cases, plus composite mean case • 2. Vertical flux [wq], partitioned by scale • through simple coarse-graining (rebinning)

  10. http://www.rsmas.miami.edu/personal/ssong/research/HR_250kmevents.htmhttp://www.rsmas.miami.edu/personal/ssong/research/HR_250kmevents.htm

  11. animation

  12. Tropical cyclone: 1 case in top 10 (in 15N-15S belt, 250km scale)

  13. http://www.rsmas.miami.edu/personal/ssong/research/HR_250kmevents.htmhttp://www.rsmas.miami.edu/personal/ssong/research/HR_250kmevents.htm

  14. Anim: composite of 10 cases

  15. 99% is from resolved condensation process: good • m composite basis HOURS RELATIVE TO MAX 250km RAIN

  16. Tropical radar observations (EPIC 2001)Time scale is hours even for small space scalesMesoscale is real (if broadband) 96km radius cell: <1h 8km radius MCS: 10h Mapes and Lin 2006 MWR

  17. m HOURS RELATIVE TO MAX 250km RAIN

  18. T’(t,p): 250km area mean p (hPa) leading nose HOURS RELATIVE TO MAX 250km RAIN

  19. 250km water vapor mixing ratio (t,p) W

  20. Low-level “valve” on convection

  21. RH(t,p) p (hPa) W W HOURS RELATIVE TO MAX 250km RAIN

  22. trimodal: shallow, medium, deep similar to obs (if a bit off in exact heights) Mapes et al. 2006 DAO

  23. GCRM detailed examinations • 1. Tropical mesoscale rain events: case studies • One scale built in to analysis: 250km • Rebinrainrate to 2.5deg, find 10 largest maxima • in ~20 day simulation period (Jan-Feb 2010) • in 15N-15S, to minimize cyclone dominated cases • Extract space-time cubes around these • (+/-18h, +/- 3 degrees) • 10 cases, and composite mean case • 2. Vertical enthalpy flux, partitioned by scale • through simple coarse-graining (rebinning)

  24. 2. Enthalpy flux • Enthalpy = sensible heat + latent heat • CpT + Lqv • Flux thru 500mb level balances ~23 Wm-2 radiativecooling above that level • sensible heat flux Cp [wT] ~ 7 Wm-2 • latent heat flux L [wq]: ~ 16 Wm-2 • destined to condense up there

  25. Latent flux across 500mb snapshot by scales resolved in 80kmrebinning sub-80km = total flux minus the above

  26. Latent flux snapshot by scales resolved in 250kmrebinning sub-250km = total explicit flux minus above

  27. Latent flux snapshot by scales resolved in 500 km rebinning sub-500km = total explicit flux minus above

  28. sub-80km and super-80km scales conspire to carry flux: convection occurs in mesoscale clusters

  29. Flux partitoned by scales • Vapor flux by convective (5-80km) scales is colocated with flux in >80km scale mesoscale updrafts. • Small scales mainly just add a bit (10 - 40%) to the flux by mesoscale mean updrafts • Might this be true at still-finer scales? • Borrowed slides (with permission, and email discussion last 2 days) from Chin-Hoh Moeng (NCAR)

  30. Marat Khairoutdinov (Stony Brook) ran “Giga-LES” • Moeng et al. 2009, 2010 JAMES

  31. Split the LES flow into: “resolvable” grid-scale (GS) & “unresolved” scale (SGS) apply “smoothing” Giga-LES CRM resolvable SGS is the difference. Moeng et al. 2010 JAMES

  32. GS: CRM-grid scales SGS: CRM-SGS SGS(w-var) Apply “smoothing” with a width of 4 km GS most of w-kinetic energy in SGS SGS(q-var) GS SGS (wq-cov) ~ half of moisture flux in SGS GS large scales small scales Moeng et al. 2010 JAMES

  33. SGS flux Moeng et al. 2010- JAMES

  34. SGS flux is in clouds • condensedwater path (vertical integral) Moeng et al. 2009 JAMES

  35. Flux partitoned by scales • Vapor flux by convective (5-80km) scales is colocated with flux in >80km scale mesoscale cloud system updrafts. • Small scales mainly just add a bit (up to 40%) to the flux by mesoscale mean updrafts • Vapor flux by sub-convective (0.1-4km) scales is colocated with >4km scale convective cloud updrafts. • Small scales mainly just add a bit (~40%) to the flux by convective mean updrafts

  36. Flux part summary • Mesoscale updrafts are moist, fluxing q up • Convective updrafts are inside, adding to it • Sub-drafts inside the convective drafts: ditto • Q: How might poorly-resolved convection be distorted by having to carry the flux of missing sub-scales? (and can param’z’n fix it?) • Q2: Is subgridparam’z’n a flux amplifier? Is that safe numerically?

  37. Summary • Deep convection – gravity wave interactions are common: a “mesoscale” • Broadband (meso synoptic, in tropics) • -5/3, but NOT a swirls-advecting-vorticity cascade • has a velocity scale, not a length scale • multicellular: hours, not minutes (not just H/w) • “Mesoscale convection”, convective cells, and sub-cellular drafts all conspire to carry geophysically (radiatively) demanded vertical energy flux • Do we need to resolve them all? Or might truncation + parameterization suffice?

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