1 Detailed Microphysical Model Simulations of Freezing Drizzle Formation Istvan Geresdi Roy Rasmussen University of Pecs, Hungary NCAR Research funded by the Aviation Weather Research Program of the FAA Freezing drizzle accumulation on the wing of the NASA- Glenn Twin Otter Photo courtesy of NASA-Glenn and Ben Bernstein

2 Why use a detailed microphysical model? 1) Provides another basis besides observations to improve bulk microphysics improvements. 2)Gain insight into the processes forming freezing drizzle and check different hypotheses (e.g. longwave radiative cooling, giant nuclei).

3 Detailed microphysical model The detailed microphysical model of was implemented into the MM5 mesoscale model to conduct two-dimensional simulations of drizzle formation in a stably stratified cloud (Rasmussen et al. 2002, JAS): -Simple bell-shaped mountain used to generate 6 – 10 cm/s uplift over 100 km horizontal scale as typically observed during freezing drizzle cases (Rasmussen et al. 1995, Cober et al. 1996, Murakami 1992, Politovich 1989, Song and Marwitz 1989). -Cloud top temperature of -12 C simulated, also typical of freezing drizzle cases.

Detailed Microphysical Model simulations

5 Detailed Microphysical Model Five different hydrometeor species simulated with 36 size bins for each species: -Hydrometeor species: Water drops, pristine ice crystals, rimed ice crystals, snowflake aggregates, and graupel -Moment conserving technique was implemented to prevent artificial broadening of the hydrometeor distributions by numerical diffusion. -Interactions allowed between the various hydrometeor types

6 Hydrometeor processes 1. Diffusional growth of different types of particles. 2. Freezing of supercooled drops. If the diameters of the drops are < 50  m, pristine ice crystals are formed. Larger drops become graupel. 3. Melting of ice particles. 4. Collision and coalescence of water droplets. 5. Pristine ice crystal and water drop collisions to form rimed ice. If the mass of the water drop is greater than the pristine ice crystal, then graupel forms.

7 Hydrometeor processes (cont.) 6. Rimed ice crystal and water drop collisions resulting in the growth of rimed ice. If the mass of the water drop is greater than the rimed ice crystal, graupel forms. 7. Self collisions between pristine ice crystals and rimed ice crystals to form snowflake aggregates. 8. Riming of aggregates results in graupel. 9. Riming of graupel increases the mass of the graupel.

8 Detailed Microphysical Model (cont.) Cloud droplets initiated from a CCN spectra as a function of supersaturation typical of continental and maritime clouds

9 Detailed Microphysical Model (cont.) Ice initiation via deposition or condensation freezing

Cloud water mixing ratio (g/kg) 0.35 g/kg Maritime case, Cooper ice initiation Height (km)

Rain water mixing ratio (g/kg) Height (km) 0.08 g/kg

Rimed ice mixing ratio (g/kg) Height (km) g/kg

Aggregated ice crystals mixing ratio (g/kg) Height (km) g/kg

Graupel mixing ratio (g/kg) Height (km) g/kg

16 Summary of Detailed Microphysics runs: 1.Low CCN concentrations (~ 50 cm -3 cloud droplet conc.) lead to relatively rapid formation of freezing drizzle (within 1-2 hours) in clouds as thin as 200 m. 2.Clouds with high CCN (~ 300 cm -3 cloud droplet conc.) can produce freezing drizzle given sufficent cloud depth and time (> 800 m deep and more than 4 hours cloud lifetime).

17 Summary of Detailed Microphysics Results (cont.): 3.Low ice crystal concentrations (< 0.08 L -1 ) in the region of freezing drizzle is a necessary condition for drizzle formation (from both the model and observations). 4.Ice nuclei depletion a necessary condition for drizzle formation. Without depletion too many ice crystals are formed which deplete the SWL, preventing the onset of drizzle. 5.Radiative cooling at the cloud top has no effect on the drizzle formation.

18 Summary of Results (cont.): 6.The maximum cloud water mixing ratio and threshold amount for the onset of drizzle in stably stratified clouds was shown to strongly depend on the CCN concentration. - Low CCN: 0.35 g/kg cloud water threshold for drizzle onset - High CCN: 0.75 g/kg cloud water threshold for drizzle onset

20 Giant nuclei and solubility study with detailed model Part II paper submitted to JAS Part II study investigated the role of giant nuclei, solubility and shape of aerosol size distribution.

21 Giant nuclei and solubility study with detailed model Part II paper submitted to JAS

22 Initial sounding

23 Simulations Performed Summary of the simulations. Type of the size distribution Soluble fraction (  ) Giant nuclei Ice physics M01GOFFMARITIME0.1no M10GOFFMARITIME1.0no M01GONMARITIME0.1yesno M10GONMARITIME1.0yesno CA01GOFFCONT-A0.1no CA10GOFFCONT-A1.0no CA01GONCONT-A0.1yesno CA10GONCONT-A1.0yesno CB10GONCONT-B1.0yesno CC10GONCONT-C1.0yesno M01ICEONMARITIME0.1yes C10ICEONCONT-A1.0yes P1MICEOFF * maritime--no P1MICEON * maritime--yes P1CICEOFF * continental--no P1CICEON * continental--yes * P1 means that the cases were presented in Rasmussen at al, In these cases CCN concentration as a function of the supersaturation is given instead of the size distribution type. The shape of the CCN function depends on the type of the air mass.

24 Time evolution of the size distributions at different gridpoints in the case of CA01GOFF

25 Time evolution of the domain maximum water and drizzle nixing ratios.

26 Time required for onset of drizzle formation

27 Comparison to part I paper

28 Giant nuclei and solubility study with detailed model 1. Stably stratified clouds with weak updraft (< 10 cm/s) can form drizzle relatively rapidly for the maritime size distributions with any aerosol particle solubility, and for continental size distributions with highly insoluble particles due to the low number of activated CCN (< 100 cm -3 ) as a result of low supersaturation achieved in these types of clouds (< 0.1 % typically).

29 Giant nuclei and solubility study with detailed model 2. Drizzle is suppressed in stably stratified clouds with weak updraft (< 10 cm/s) for highly soluble urban and extreme urban size distributions. These distributions had sufficiently increased numbers of water drops to mitigate drizzle formation and are characterized by significant increases of the aerosol concentration for particles larger than 0.1  m. Thus, drizzle formation may be suppressed near urban areas.

30 Giant nuclei and solubility study with detailed model 3. The presence of giant nuclei has only a minor effect on drizzle formation.