an atmospheric “mesoscale”: where convection meets waves (rotation optional) brian mapes...
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an atmospheric “mesoscale”:where convection meets waves
(rotation optional)
Brian MapesUniversity of Miami
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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)
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mesoscale convection• “Mesoscale” convection events (meso =
middle, in between L~H “convective” scale and N/f H “Rossby radius”) are less theoretically tidy than parcel or exp(ikx) UV catastrophe, but profound & real
• convectively coupled internal waves
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Convectively coupled gravity waves in 2D CRMNo preferred hor. scale
Stefan Tulich (Mapes et al. 2009 JMSJ)
3 decades
5 decades
}
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Scale Interactionscascade... ...pas?
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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”)
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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 subgrid orographic gravity wave drag
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comparison to satellite imageryhttp://earthobservatory.nasa.gov/
IOTD/view.php?id=44246&src=eoa-iotd
predicted cloud features for February 6, 20102 weeks into simulation
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5km GCM: detailed examinations• 1. Tropical mesoscale rain events: case studies
– One scale selected in to analysis: 250km events• Rebin rainrate 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)
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http://www.rsmas.miami.edu/personal/ssong/research/HR_250kmevents.htm
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animation
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Tropical cyclone: 1 case in top 10 (in 15N-15S belt, 250km scale)
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http://www.rsmas.miami.edu/personal/ssong/research/HR_250kmevents.htm
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Anim: composite of 10 cases
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• m
composite basis
HOURS RELATIVE TO MAX 250km RAIN
99% is from resolved condensation process: good
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Tropical radar observations (EPIC 2001)Time scale is hours even for small space scales
Mesoscale is real (if broadband)
cell: <1h
MCS: 10h8km radius
96km radius
Mapes and Lin 2006 MWR
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• m
HOURS RELATIVE TO MAX 250km RAIN
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T’(t,p): 250km area mean
leading nose
p (hPa)
HOURS RELATIVE TO MAX 250km RAIN
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250km water vapor mixing ratio (t,p)
W
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Low-level “valve” on convection
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RH(t,p)
HOURS RELATIVE TO MAX 250km RAIN
p (hPa)
W
W
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trimodal: shallow, medium, deep similar to obs (if a bit off in exact heights)
Mapes et al. 2006 DAO
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GCRM detailed examinations• 1. Tropical mesoscale rain events: case studies
– One scale built in to analysis: 250km– Rebin rainrate 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)
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2. Enthalpy flux• Enthalpy = sensible heat + latent heat
– CpT + Lqv
• Flux thru 500mb level balances ~23 Wm-2 radiative cooling above that level
– sensible heat flux Cp [wT] ~ 7 Wm-2– latent heat flux L [wq]: ~ 16 Wm-2
• destined to condense up there
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Latent flux across 500mbsnapshot
by scales resolved in 80km rebinning
sub-80km = total flux minus the above
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Latent fluxsnapshot
by scales resolved in 250km rebinning
sub-250km = total explicit flux minus above
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Latent fluxsnapshot
by scales resolved in 500 km rebinning
sub-500km = total explicit flux minus above
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sub-80km and super-80km scales conspire to carry flux: convection occurs in mesoscale clusters
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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)
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• Marat Khairoutdinov (Stony Brook) ran “Giga-LES”
• Moeng et al. 2009, 2010 JAMES
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Split the LES flow into: “resolvable” grid-scale (GS) & “unresolved” scale (SGS)
)()(~
)( xfxfxf SGS is the difference.
apply “smoothing”
CRM resolvableGiga-LES
Moeng et al. 2010 JAMES
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Apply “smoothing” with a width of 4 km
GS: CRM-grid scales
GSSGS(w-var)
SGS: CRM-SGS
SGS (wq-cov)
GS
GS
SGS(q-var)most of w-kinetic energy in SGS
~ half of moisture flux in SGS
large scales small scalesMoeng et al. 2010 JAMES
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• SGS flux
Moeng et al. 2010- JAMES
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Moeng et al. 2009 JAMES
• SGS flux is in clouds
• condensedwater path (vertical integral)
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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
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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 subgrid param’z’n a flux amplifier? Is that safe numerically?
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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?