Radiative Influences on Glaciation Time-Scales in Mixed-Phase

CloudsZachary Lebo, Nathanial Johnson, and

Jerry HarringtonPenn State University

Acknowledgements: DOE-ARM and Dennis Lamb for many useful

discussions.

• Cloud tops can maintain narrow liquid layers if ice crystals remain small and updrafts are sufficiently strong (Rauber and Tokay, 1991)

• It is possible to maintain a mix of liquid and ice during ascent (Tremblay et al., 1996)

• Liquid topped arctic clouds that precipitate ice are possible if ice nuclei concentrations are small (Pinto, 1998, Harrington et al., 1999) and if ice nuclei are removed through sedimentation (Harrington and Olsson, 2001, Morrison et al., 2005).

Why Can Liquid and Ice Persist in Mixed-Phase

Clouds?Previous work has shown:

Glaciation Time-Scales

• Hence, time-scale for complete glaciation of mixed-phase clouds is important and depends on (at least):– Ice concentration

and updraft velocity

• Since radiation affects the growth of drops, is there a similar influence on the Bergeron process? From Korolev and Isaac (2003)

1.6 min

16 min

160 min ~ 2.6 hrs

Radiatively Modified Ice Growth

• Method is that of Korolev and Isaac (2003) but add the radiative term for the growth of ice:

Radiative Effect = Ed

• Start with a simple box model– Integrate above equation numerically until a fixed

amount of cloud liquid water content (LWC) is depleted.

Computing Radiative Influence

• Use simple, static adiabatic model of stratiform arctic cloud.

• Solar (SW) and infrared (LW) radiative heating computed via two-stream model (Harrington and Olsson, 2001)

Radiative Heating/Cooling of Crystals

• Ed easily computed at each vertical level within the idealized cloud.

• LW Cooling: Increases rapidly while SW Heating increases more slowly with size.

• Net Effect: LW dominates at small sizes with cross-over to net heating at large sizes

Ed At Cloud TopPlate Crystals

Radiative Influences on Ice Supersaturation

• Cloud Top: Radiative cooling dominates, sui increases to over 30% from ~ 15%

• Mid Cloud: SW heating dominates decreasing sui to less than 15%.– When SW Heating

becomes large enough Crystals will actually sublimate

CrystalGrowth

CrystalSublimation

Plate CrystalsNi = 1 L-1

Ttop = -15 C0 = 450

Radiative Influence on Glaciation Time-Scale

• No Radiation: Results similar to Korolev and Issac.

• LW Cooling: Drastic decrease in glaciation time– Positive feedback:

Larger crystals, more cooling, etc.

• SW Heating: Reduces LW effect at cloud top.

Initial LWC: 0.1 g m-3

Ni = 1 L-1

Plate Crystals

Radiative Influence on Glaciation Time-Scale

• LW Cooling drops off rapidly. – 100 m below cloud

top glaciation time-scales not at strongly impacted.

• Mid-Cloud: Since SW heating dominates, glaciation does not occur.– Crystals grow to

radiatively limited sizes.

Initial LWC: 0.1 g m-3

Ni = 1 L-1

Plate Crystals

Glaciation Time-Scale: Fixed Rates

• Crystals grow too large in box model– Fix ice growth rates at

a particular size

• Small crystals, glaciation time is long radiative influences don’t matter

• Larger crystals, glaciation times shorter (< 100min) so radiative influences quite important.

No Radiation

Concluding Remarks

• Simple box model calculations suggest that radiative heating and cooling may substantially influence glaciation times.– LW cooling at cloud top may enhance

crystal growth– SW heating (even when weak) may

substantially increase mixed-phase cloud lifetimes (as long as > 750)

• Computations with bin microphysical model tend to corroborate these results.

• Next plan to incorporate into parcel models, and LES, to test radiative influences on more realistically simulated clouds.

Stratiform Arctic Mixed-Phase Persistence

• In the Arctic: Mixed-phase clouds occur throughout the year.

• Ice nuclei ice concentration (& size) Important for mixed-phase longevity (Pinto, 1998; Harrington et al., 1999; Morrison et al., 2005).

LES-Derived Water Paths

M-PACEObservations

Radiative Influence on Glaciation Time-Scale

• LW Cooling and SW Heating using spheres: Results similar to those for plates.

SpheresInitial LWC: 0.1 g m-3

Ni = 1 L-1

Cloud Top