Final paper 411

Final concentration: Correcting LWP and IWP biases for Arctic mixed-phase clouds in CAM5

Introduction:

Observational background

physics of AMPCs

  • Mixed phase clouds at warm temperatures (which??) are microphysically unstable. [RESILIENCE]
    • WBF can glaciate in hours
    • But observations of mixed phase clouds show they can persist for days or weeks.
    • Arctic MPCs different (HOW?)

Replenishing moisture

  • Turbulence and cloud-scale upward air motion seem to be critical in maintaining mixed-phase clouds under weak synoptic-scale updrafts [RESILIENCE]
  • Strong enough updrafts -> supersaturated w.r.t. ice and water (both grow simultaneously) [RESILIENCE]
  • supercooled droplets lead to strong longwave radiative cooling (60K per day near cloud top whoa)
  • Coupling to synoptic conditions: Large scale advection creates frequent moisture inversions near cloud top
  • Entrainment of above-cloud air actually replenishes cloud water lost to (ice) precip
  • Unlike lower latitudes, Special: AMPCs can persist and be replenished even if not dynamically coupled to surface fluxes
  • "Weak solar heating, coupled with strong inversions and and a combination of sea ice and ocean at the lower boundary procude clouds with stable temperature profiles" [MPACE]

Surface coupling:

  • Radiative cooling of SC water warms surface, cools atmosphere
  • Source of static instability: sensible heat flux and moisture drawn upwards into cloud
  • Ice often only forms after SC water is present (even if there is substantial ice-supersaturation before LW is present)
  • Modeling of these clouds is tremendously difficult [RESILIENCE] last paragraph

These clouds are complex to model

Measuring AMPCs

  • Atmospheric Radiation Measurement program set up in alaska:

    • Barrow (another name) -> oliktok point 0> toolik lake 0-> atqasuk
    • Approximates grid cell of GCM, for use in the Single Column Modeling methodology

  • Barrow site:

    • High Spectral Resolution Lidar
    • depolarization lidar
    • Atmospheric Emitted Radiance Interferometer
    • Instrumented aircrafts
    • Citation
    • High Volume Precipitation Sampler
    • Counterflow Virtaul Impactor
    • Continuous flow ice thermal diffusion chamber.
    • Above-clouddeck flights combined with in-situ measurements constrain both radiative budget at cloud boundary as well as concentration of species in cloud

  • 27 sept 2004 to 22 oct 2004

  • Three separate exemplary clouds.

  • Main event for us: Single layer stratoform mixed phase

    • High pressure over ice pack
    • transient low draws moisture to NSA site.

  • 9-11 of october: single layer stratus cloud

  • Low level northeasterly flow off the ipack ice and over ocean. Persistent low-level clouds under a sharp inversion for entire period

  • Include: Figure 6 is ground truth.

    • Ice precipitation indicated persistently underneath cloud base
    • Cloud base at 900m.

  • Cloud top values for temperature, liquid water content, mean diameter were -16.9C, 0.36g/m^3, 25 um.

    • Number concentration 25 #/cm^3 throughout

  • Figure 7: explains background state given above

  • Narrow cloud drop distribution. Heterogeneous ice crystals through entire cloud body.

  • Mixed phase present even with cloud-top temperatures as low as -30C

  • Fig 1 [MPACE_MIP]: aerial picture of clouds

Modeling Background

  • Retrievals from this stratiform cloud case were used to create cloud physical and dynamical fluxes
  • mm wavelength cloud radar
  • lidar, microwave radiometer.
  • Fields:
    • cloud top/base
    • liquid water/ice water content
    • effective particle sizes of liquid/ice
  • Uncertainties:
    • 20g/m^-2 for liquid water path
    • ice water content: 0.002 g/m^-3 based on minimal detectable radar signal (dubious)
  • well mixed and capped by inversion
  • cloud-topped boundary layer
  • Cloud top initially pure liquid
  • Temperature and water mixing ration advection calculated from ECMWF reanalysis data
  • No in-situ observations of surface fluxes. Taken from ECMWF data
  • Focus on single-vs-double moment schemes:
    • Mixed-phase processes such as the Wegener Bergeron Findeisen process rely on ice crystal concentration number
    • Single moment schemes have only mass, not number concentration
    • 10 SCMs with single moment and 10 with double-moment schemes in this paper
    • Discussion of bin microphysics schemes beyond scope of this paper
  • Table IV:
  • 160 pm 60 g/m^2 for liquid water and 7-30 g /m^2 of ice water path.
    • Aircraft IWP much higher than remote sensing IWP
  • Single model with T-dep ice/liquid partitioning: 21.2 g/m^2
  • Single moment but separate ice/water microphysics: 72.8 g/m^2
  • double moment: 100 g/m^2
  • Ice water path: 33.8, 31.8, 19.9
    • That is, models with higher-complexity treatment of ice microphysics are closer to observed values.
    • Figure 8: water content vs aircraft retrievals, ice water vs retrievals
    • LWC
    • Figure 10: ICNC vs liquid water path: absurd scatter across models (should include this)
    • No agreement about ICNC

High degree of heterogeneity among models. Even those that share, e.g., a double moment scheme.

Modeling Interventions

Summary of CAM5

  • CAM 3 had single-moment microphysics.

  • But CAM 5 has double moment microphysics

    • Microphysics predicts both number and mass concentration for ice and water
    • precipitation in ice and snow is diagnosed (from what?)
    • Particle distributions treated as gamma functions

  • Double moment schemes allow cloud properties to be physically coupled to aerosols

  • Modal aerosol module:

    • MAM activates aerosols with appropriate properties to be CCN/INP and generates droplets and ice crystals
    • Interactive aerosol effects on both warm and cold clouds

  • Separate WBF for ice crystals/snow (four fields)

  • CAM 5 with FV dycore at f19 resolution,

  • Initialized with MERRA data beginning of each day for M-PACE

  • Temperature of homogeneous freezing (but only for rain!) increased form -40C to -5C in released cam 5 to tune arctic surface flux

  • Ice nucleation linked with aerosol properties in this model

    • Homogeneous nucleation of sulfate competing with heterogeneous nucleation in mineral dust for ice clouds.
    • Mixed phase clouds: deposition/condensation nucleation drawn from Meyers 1992
    • Constant IN concentration for T<-20C
    • Contact freezing by mineral dust, and Hallet-Mossop SIP are included.
    • Immersion of cloud freezing is included.

  • Limitations of Meyers parameterization:

    • Constant amount of IN means that particles are implicitly replenished when ice crystals scavenge INPs.
    • Shown in Prenni '07 that Meyers produces too many IN for MPACE case (might be worth a note)

  • Purpose of this study:

    • What happens when Meyers is changed out for Phillips IN scheme
    • Empirically derived relationship between mineral dust, black carbon, and hydrophobic organics with temperature

Results:

  • CAM5 underestimates LWC by 70% and overestimates IWC by factor of 2
  • Figure 6 (qualitative over/under estimates)
  • Figure 7: cloud fraction is good, but persistent biases in LWC, IWC
  • From 5-12th october, cam5 undreestimates downward LW radiative fluxes by 20-40 W/m^2
    • Largely due to underestimate of cloud liquid mixing ratio
    • Overestimates OLWR by 10 W m^2
  • Two processes for turning cloud liquid into snow in CAM5:
    • Collection of water by snow
    • Evaporation/deposition by WBF
  • Figure 10: Ice/Water Budgets
    • Fixing instantaneous freezing fixes budget substantially
  • Cam5 significantly underestimates aerosol optical depth (is this reanalysis artifact?)
  • Tuning parameter for autoconversion from

WBF

  • Mixed-phase clouds are likely spatially heterogeneous (pockets of ice/water) on the scale of 10^2 m
    • Separation of ice from water also slows WBF
  • In this study: replace Meyers IN parameterization with calssical nucleation theory
    • Nucleation is stochastic, depends on number and size of aerosol particles
    • Note: different from above. Same goal
  • New WBF:
    • WBF process largely depends on contact volume between supercooled liquid droplets and ice crystals
    • Typical contact volume in homogeneous gridcell:10^310^510^5
    • in heterogenous grid cell: 1010^310^3 if 100 m pockets of liquid and ice butt against each other.
    • Relaxation timescale is inversely proportional to contact volume.
  • Mass weighted water vapor:
    • Certain work indicates RH in mixed phase clouds indicate RH should be close to 100%.
    • Heterogeneity could explain observed deviance from the SVP expected if ice and snow are homogeneously mixed.
  • Figure 4: comparable to figures from other paper
    • Most aggressive slowdown in WBF still underestimates LWC by factor of two.
  • Figure 6: Breakdown by microphysical process
    • liquid water detrailment from shallow convection is liquid source in ctrl
    • WBF process major sink in CTL
  • Figure 8:
    • If you also decrease accretion rate of liquid rain by snow by same amount as WBF, then you match LWC
    • High enough vertical resolution is crucial for maintenance of cloud liquid layers!
  • Recommendation:
    • Physically based representation of heterogeneous structure rather than tuning parameter.

Integrated sensitivity

Conclusions

  • Bin microphysics and SIP

Lonely science: can it be studied in isolation?

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