(some) Observed properties ofmoist convection on earth
Adam Sobel
FDEPS Kyoto

Outline
1. Marine boundary layer – stratocumulus totrade cumulus transition
2. Deep convection
3. Climatology of precipitation and relatedquantities

Moist boundary layer over ocean
Adam Sobel
FDEPS Kyoto

Klein et al. (1995, J. Climate) – satellite image of Scu-Cu transition – visible
reflectance from satellite

35N

30N, downstream

Klein et al. (1995, J. Climate) – satellite image of Scu-Cu transition – visible
reflectance from satellite

2500 kmSouthAmerica3000 kmKlein et al. (1995, J. Climate) – climatological frequencies of stratocumulus
and cumulus

Schematic of stratocumulus-topped marine boundary layer (Stevens,2006, Theor. Comp. Fluid Dyn.) – layer is well-mixed, turbulence is driven
largely by radiative cooling at cloud top

Composite shallow Cu boundary layers, from Albrecht et al. (1995, JGR)

FIRE – SCu
ASTEX – SCu/Cu transition
TIWE – Trade Cu

Composite soundings normalized by inversion height, zi

Albrecht et al. (1995, JGR)Relative humidity

Albrecht et al. (1995, JGR)µe, µe*

Note shallow layerof conditional
instabilitydθes/dz<0, andsfc θe larger thanUpper θesAlbrecht et al. (1995, JGR)

Scu – Trade Cu transition• Stratocumulus is well mixed in both µv and q –somewhat closer to dry convection, just withsaturated layer at top (since q* drops withheight while q~const)• As surface warms and buoyancy increases,
cumuli start to punch deeper; we see
increasingly stable and dry layer form
• Entrainment of dry air from above eventuallydries out SCu layer
E.g., Wyant et al. 1997, J. Atmos. Sci. 54, 168

1997

Wyant et al. (1999, J. Atmos. Sci.): Lagrangian LES simulations of column
moving over increasingly warm waterµv

qtql

Wyant et al. (1997, J. Atmos. Sci.): Lagrangian LES simulations of column
moving over increasingly warm waterµv

Increasing stability in trade cu layer(but less than moist adiabatic)qtql

Wyant et al. (1997, J. Atmos. Sci.): Lagrangian LES simulations of column
moving over increasingly warm waterµv
stable layer below cloud baseqt

qlStep 1 (Wyant et al. 1997):“As SST warms and the MBL deepens, upward latent heat fluxes in the boundary
layer increase dramatically. This increases the buoyancy fluxes and turbulence levels
within the cloud, creating more entrainment per unit of cloud radiative cooling. The
increased entrainment leads to increasingly negative buoyancy fluxes below
cloud base associated with a downward flux of warm entrained air. This disrupts
the mixed layer and creates a weak stable layer … below cloud base.
The stable layer acts as a valve that allows only the most powerfulsubcloud-layer updrafts to penetrate up to the main stratocumulus cloud base.
As the decoupling becomes more pronounced, the updrafts resemble small cumuli.”Wyant et al. (1997, J. Atmos. Sci.)

Step 2 (Wyant et al. 1997):“While the cumulus clouds sustain the stratocumulus by detrainment ofliquid water near the inversion, we suggest that they also cause its ultimate
demise. Within the cumulus layer, the stratification is much weaker thanmoist adiabatic, so conditional available potential energy (CAPE) buildsup rapidly as the cumulus layer deepens. … penetrative entrainment ofdry free tropospheric air by increasingly vigorous cumulus clouds
evaporates most of the liquid water in the updraft before it is detrained,leaving smaller and smaller stratocumulus cloud patches around thecumulus. The ratio of penetratively entrained mass flux of dry air to upward
cumulus mass flux of moist surface-layer air increases, drying the cloudlayer.”Wyant et al. (1997, J. Atmos. Sci.)

Deep Convection
Adam Sobel
FDEPS Kyoto

When sea surface is warm enough (relative to atmosphere) deep, strongly
precipitating convection occurs

Seasonal cycle of precip follows the SST, which (very broadly) follows the sun,with a lag of a couple months
January

July

AprilOctober

http://iridl.ldeo.columbia.edu/maproom/.IFRC/.Forecasts/

The deep convection is there because of conditional instability

kSobel et al. 2004, Mon. Wea. Rev
KWAJEX field expt., Marshall Islands

July precip climatology

Relative humidity doesn’t show strong drop at PBL top. PBL not even thatwell defined (no inversion) though there is still a shallow mixed layer where
RH increases with height (q ~ const)

Sobel et al. 2004, Mon. Wea. Rev

Day-to-day variability in temperature is small, in humidity is larger

Sobel et al. 2004, Mon. Wea. Rev

Much of the variability is related to coherent propagating large-scale disturbances

Time ->

Black-body temperature (cold = highclouds = deep convection)

Radar rain rate
From a ~200 km
Radius aroundKwajalein (whiteLine)Longitude ->Sobel et al. 2004, Mon. Wea. Rev

Many different morphologies and degrees and kinds of space-time organization

Webster and Lukas, BAMS 1992 (TOGA COARE)The really strong precipitation and heating often comes from convective cloudsystems that are large in horizontal extent, and have two distinct regions

Trailing stratiform anvil
Leading line, or convective
regionRadar reflectivity, Kansas squall line, “PRE-STORM
Experiment (Smull and Houze 1987)

Along-line cross-sections for same storm“bright band”

Radar reflectivity(dBz)

Vertical velocity(m/s)(Smull and Houze 1987)

Bright band in radar image from DYNAMO,Maldives, November 27 2011

In convective region updraft vertical velocities > terminal velocity of hydrometeors;
In stratiform region converse is true.

Houze, R. A., Jr, 1989: Quart. J. Roy. Met. Soc., 115, 425-461

Next few slides on mesoscaleconvective systems from Sandra Yuter,
NC Statestratiform

Particle
growthmechanisms

Accretional growth =riming andcollision/coalescenceconvectiveShading shows higher intensities of radar echo, with hatching indicating the strongest echo. In(b) cloud is shown at a succession of times t0, ,,,,tn. Growing precipitation particle is carried
upward by strong updrafts until t2 and then falls relative to the ground, reaching the surface justafter t5. After t5 the cloud may die or continue for a considerable time in a steady state before
dissipation sets in at tn-1 and tn,. The dashed boundary indicates evaporating cloud.

conv

stratiform

Particle
growthmechanisms

Accretional growth =riming andcollision/coalescenceconvective
Shading shows higher intensities of radar echo, with hatching indicating the strongest echo. In(b) cloud is shown at a succession of times t0, ,,,,tn. Growing precipitation particle is carried
upward by strong updrafts until t2 and then falls relative to the ground, reaching the surface justafter t5. After t5 the cloud may die or continue for a considerable time in a steady state before
dissipation sets in at tn-1 and tn,. The dashed boundary indicates evaporating cloud.

stratiform conv

Particle
growthmechanisms

Accretional growth =riming andcollision/coalescenceconvective
Shading shows higher intensities of radar echo, with hatching indicating the strongest echo. In(b) cloud is shown at a succession of times t0, ,,,,tn. Growing precipitation particle is carried
upward by strong updrafts until t2 and then falls relative to the ground, reaching the surface justafter t5. After t5 the cloud may die or continue for a considerable time in a steady state before
dissipation sets in at tn-1 and tn,. The dashed boundary indicates evaporating cloud.

Mesoscale convective system = a cumulonimbus cloud system that produces a contiguous area
~100 km or more in at least one direction

IR image

36

Slide by Sandra YuterMCS

Idealized horizontal map of radarreflectivityConvective and stratiform
precipitation regions

37

Slide by Sandra Yuter

Definitions of Tropical Precipitation Typesfrom Houze (1997) and applied in Steiner et al. (1995) and TRMM algorithms (1997)

1) Convective2) Stratiform

Young, vigorous
convection

Old convection

Cellular, verticallyoriented reflectivity
maximaRelatively
homogenous in thehorizontal, layeredreflectivity structure

Dominant accretional Dominant vapor
growth of precipitation deposition growth of
precipitation“Convective” latentheating profile“Stratiform” latentheating profile

Slide by Sandra Yuter

Definitions of Tropical Precipitation Types

Heightfrom Houze (1997) and applied in Steiner et al. (1995) and TRMM algorithms (1997)

1) Convective2) Stratiform

Young, vigorous
convection

Old convection

Cellular, verticallyoriented reflectivity
maximaRelatively
homogenous in thehorizontal, layeredreflectivity structure

Dominant accretional Dominant vapor
growth of precipitation deposition growth ofprecipitation–+“Convective” latentheating profile“Stratiform” latentheating profile

Heating Rate

Slide by Sandra Yuter

Definitions of Tropical Precipitation Types

Heightfrom Houze (1997) and applied in Steiner et al. (1995) and TRMM algorithms (1997)

1) Convective2) Stratiform

Young, vigorous
convection

Old convection

Cellular, verticallyoriented reflectivity
maximaRelatively
homogenous in thehorizontal, layeredreflectivity structure

Dominant accretional Dominant vapor
growth of precipitation deposition growth of
precipitation–+“Convective” latentheating profile“Stratiform” latentheating profile

Heating Rate

Slide by Sandra Yuter

Definitions of Tropical Precipitation Types
1) Convective2) Stratiform

Young, vigorous
convection

Old convection

Cellular, verticallyoriented reflectivity
maximaRelatively
homogenous in thehorizontal, layeredreflectivity structure

Dominant accretional Dominant vapor
growth of precipitation deposition growth of
precipitation–+

Heating Rate

“Convective” latentheating profile“Stratiform” latentheating profile

HeightHeightfrom Houze (1997) and applied in Steiner et al. (1995) and TRMM algorithms (1997)–+

Heating Rate

Slide by Sandra Yuter

Definitions of Tropical Precipitation Types
1) Convective2) Stratiform

Young, vigorous
convection

Old convection

Cellular, verticallyoriented reflectivity
maximaRelatively
homogenous in thehorizontal, layeredreflectivity structureDominant accretional Dominant vapor
growth of precipitation deposition growth ofprecipitation–+

Heating Rate

“Convective” latentheating profile“Stratiform” latentheating profile

HeightHeightfrom Houze (1997) and applied in Steiner et al. (1995) and TRMM algorithms (1997)–+

Heating Rate

Isolated weak echoes could be either in the initialstages of a developing cell or final stages of mature cell
Slide by Sandra Yuter

Idealized Mesoscale Convective System
Storm Structure

Houze et al. 1989

Slide by Sandra Yuter

Idealized Mesoscale Convective System
Storm Structure
Radar echo boundary

0°C level

Houze et al. 1989

Cloud boundary

Surface precipitation

Stratiform

Convective
Slide by Sandra Yuterconvective & stratiform regions have distinctly different vertical structures forthe apparent convective heating, though what we observe on large scales is thecombination

convective

Houze 1989

stratiform

Deep convection over land has a much stronger diurnal cycle and can attaingreater intensity

Their frequency of occurrence
as fn. Of time of day

Frequency of most intense convective systems

Zipser et al. (2006, BAMS)Much more lightning, and deeper convection over land, for same rain amount(Takayabu 2006, Geophys. Res. Lett.)

Example of very severe, long-lived squall line:
2012 derecho in USA
http://www.crh.noaa.gov/iwx/?n=june_29_derecho

Strong low-level vertical wind shear is favorable for strong, long-lived squalllines, due to interaction with cold pools (Rotunno, Klemp & Weisman 1988)

Climatology of precipitation and related
fields from ERA40 Atlas

Precipitation – DJF & JJA

Precipitation – MAM & SON

500 hPa omega & Precipitation, annual mean

500 hPa omega, DJF & JJA

500 hPa omega, MAM & SON

Column-integrated “diabatic heating” and precipitation, annual mean

Heating, DJF & JJA

Heating, MAM & SON

Zonal mean heating and mean meridional
stream function, annual mean

Zonal mean heating and mean meridional
stream function, DJF

Zonal mean heating and mean meridional
stream function, JJATOA thermal radiation (OLR) and precip, annual mean

OLR, DJF & JJA

OLR, MAM & SON

Surface latent heat flux and precipitation, annual meanSurface LH flux, DJF & JJA

Surface LH flux, MAM & SON

Column-integrated moisture flux and convergence, DJF & JJA

Column-integrated moisture flux and convergence, MAM & SON

column integrated water vapor & Precipitation, annual meanColumn water vapor, DJF & JJA

Column water vapor, MAM & SON

Total surface heat flux (latent+sensible+radiative)
annual mean

Total surface heat flux (latent+sensible+radiative)
DJF & JJA

Summary• Precipitation and vertical motion are tightly
linked• Surface evaporation is much more evenly
distributed than precipitation• The circulation transports water vapor toconcentrated precipitation zones• These are really just statements about thebudgets of moisture and dry static energy (orpotential temperature)Summary• Total surface heat flux is negligibly small over land
• Column-integrated water vapor and precipitation
are also tightly associated (not a simple budgetstatement)• Sea surface temperature and precipitation are
also tightly associated (not a simple budgetstatement)• Free tropospheric temperature has very weakgradientsReferences 1• Albrecht, B. A., Jensen, M. P., Syrett, W. J. , 1995: Marine boundary layer
structure and fractional cloudiness. J. Geophys. Res., 100, 14209–14222.
• Houze, R. A., Jr., 1989: Observed structure of mesoscale convective
systems and implications for large-scale heating. Quart. J. Roy. Met. Soc.,
115, 425-461.
• Houze, R. A., Jr., Rutledge, S. A., Biggerstaff, M. I., Smull, B. F., 1989:
Interpretation of Doppler weather radar displays in midlatitude mesoscaleconvective systems. Bull. Amer. Meteor. Soc., 70, 608-619.
• Klein, S. A., Hartmann, D. L., Norris, J. R., 1995: On the Relationships
among Low-Cloud Structure, Sea Surface Temperature, and AtmosphericCirculation in the Summertime Northeast Pacific. J. Climate, 8, 1140–1155.
• Rotunno, R., Klemp J. B., Weisman, M. L., 1988: A Theory for Strong, LongLived Squall Lines. J. Atmos. Sci., 45, 463–485.
• Smull, B. F., Houze, B. F., 1987: Rear Inflow in Squall Lines with Trailing
Stratiform Precipitation. Mon. Wea. Rev., 115, 2869–2889.

References 2• Sobel, A. H., Yuter, S. E., Christopher S. Bretherton, George N. Kiladis,
2004: Large-Scale Meteorology and Deep Convection during TRMMKWAJEX*. Mon. Wea. Rev., 132, 422–444.
• Stevens, B., 2006: Bulk boundary-layer concepts for simplified models oftropical dynamics. Theor. Comp. Fluid Dyn. 20:5-6, 279-304.
• Takayabu, Y. N., 2006: Rain-yield per flash calculated from TRMM PR andLIS data and its relationship to the contribution of tall convective rain.
Geophys. Res. Lett., 33, L18705.
• Webster, P. J., Lukas, R., 1992: TOGA COARE: The Coupled Ocean—Atmosphere Response Experiment. Bull. Amer. Meteor. Soc., 73, 1377–1416.
• Wyant, M. C., Bretherton, C. S., Rand, H. A., Stevens, D. E., 1997:
Numerical Simulations and a Conceptual Model of the Stratocumulus to
Trade Cumulus Transition. J. Atmos. Sci., 54, 168–192.
• Zipser, E. J., Liu, C., Cecil, D. J., Nesbitt, S. W., Yorty, D. P., 2006: WHERE ARE
THE MOST INTENSE THUNDERSTORMS ON EARTH?. Bull. Amer. Meteor.
Soc., 87, 1057–1071.