Effects of aerosols on precipitation and hail formation in midlatitude storms Khain

1. Effects of aerosols on precipitation and hail formation in midlatitude storms • Khain • Department of Atmospheric Science, The Hebrew University of Jerusalem, Israel

2. AEROSOL EFFECTS Most simulations of aerosol on precipitation were carried out for conditions characterizing by high freezing level of ~4 km (tropical convection, Texas clouds), where warm rain is substantial and the increase in aerosol concentration can prevent warm rain. Number of studies where aerosol effects on precipitation from mid-latitude convective clouds are simulated is quite limited. The problem is that in mid-latitude clouds warm rain processes are not efficient independently on aerosol concentration. So, what is aerosol effect on precipitation from mid-latitude convective clouds (or storms)?

3. Hebrew University cloud model (HUCM) with spectral (bin) microphysics (Khain et al 2004, 2008) • Model includes 8 types of particles: • a) water drops, b) plate crystals; c) columnar crystals; • d) dendrites; e) snow; f) graupel; g) hail; h) aerosols • 2) Each type of particles is described by a size distribution function using a mass grid, containing 43 mass bins. The size of hail up to 6 cm in diameter. • 3) It solves an equation system for the size distribution functions.

4. HAIL FORMATION SIMULATION OF HAIL SIZE MECHANISM OF LARGE HAIL FORMATION: Large amount of small supercooled droplets in the deep layer up to 10 km These droplets can be collected by falling hail particles Favorable conditions: large W large amount of small aerosols The main goal of the ANTISTORM project was: to investigate conditions of large hail formation under mid-latitude environmental conditions (over Germany)

5. THE CASE STUDY

6. Max reflectivity: 65 dBZ

10. 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 dBz Observations HUCM, 3000cm-3 HUCM, 100cm-3 65 60 55 50 45 40 35 30 25 20 15 10 5 0 -5 dBz Distance, km Distance, km Figure.8. Left: Maxcappis of radar reflectivity measured at Albis (near Zurich, Switzerland) on 28/06/2006 at 17:25 UTC. Middle: The radar reflectivity field simulated by HUCM at t=9000 s, CCN concentration 3000 cm-3; Right: The radar reflectivity field simulated by HUCM at t=9000 s, CCN concentration 100 cm-3.

11. Vertical velocities: Maximum values ~25m/s (Ts=23C) and ~30 m/s (T=24C), peaks are larger and more frequent in polluted cases

12. ACCUMULATED RAIN 2 2 2 2 1 1 1 1 3 3 1) Surface rain begins first at low AP concentrations. 2) At t> 60 min precipitation in polluted air is dominating. At t=180 min precipitation is maximum at CCN=3000 cm-3 . 3) Accumulated rain in very clean air is significantly lower than in polluted air

13. Accumulated graupel and hail precipitation 3000 cm-3 3000 cm-3 - 3 100 cm • The dependence on AP concentration is non monotonic with maximum at CCN conc= 3000 cm-3 • Precipitation is minimum at CCN conc=100 cm-3

14. Hail and graupel kinetic energy at the surface -3 3000 cm 3000 cm - - 3 3 3000-6000 cm - 3 100 cm - - 3 3 -3 100 cm -3 100 cm Hail and graupel kinetic energy: 1) The kinetic energy is maximum in polluted cases. Size of hail increases with increase in aerosol concentration. 2) Kinetic energies are very small at CCN conc=100 cm-3

15. 100cm-3 3000cm-3 • Microphysical cloud structure: • Cloud water mass is higher in polluted case and reaches higher levels. • Riming of graupel and snow leads to larger hail mass in polluted clouds

16. Why precipitation increases in polluted clouds? In polluted clouds hail contributes significantly to precipitation. The loss of hail mass by sublimation is negligible: hail falls fast and very concentrated in space. In clean clouds ice particles are smaller, more snow forms (because of lack of supercooled droplets). Snow spreads over larger area and loose its mass by sublimation. As a result, precipitation in polluted clouds turns out to be larger than that in clean air

17. Mass and Size distributions of drops at different heights Clean air Polluted air 3000cm-3 100cm-3 Max=1 cm Max=0.4 cm 3000cm-3 100cm-3 • Size distributions: • In clean air the maximum raindrop diameter is 0.2 cm. In polluted air the maximum raindrop diameter is ~ 0.8 cm. • Large raindrops form due to melting of snow, graupel and hail. Collisions of melting ice with water drops increases size of rain drops

18. Mass and Size distributions of hail at different heights 150 min 150 min 150 min 3000cm-3 100cm-3 3000cm-3 100cm-3 Max hail diameter at 10-1 m-3 mm-1 concentration level Collisions in melting layer Typical hail size: 0.25 cm in clean air and 0.8 cm in polluted air, Max hail size in polluted air is 4.8 cm 7 6 5 4 3 2 1 No collisions in melting layer 100 min 3000cm-3 6 7 8 9 10 11 12 13 mm

19. CCN=100 cm-3 CCN=3000 cm-3 Huge hail forms in case of very high supercooled LWC

20. G THANK YOU!

21. snow 100cm-3 3000cm-3 Max=2.4 gm-3 Max=0.45 gm-3 Max=2.4 gm-3 Max=3.5 gm-3 • Microphysical cloud structure: • The lack of supercooled water leads to the formation of larger snow mass in clean air

22. Radar reflectivity Clean air Dirty air Max =62dBZ Max =45dBZ Max =40dBZ Max =45dBZ Max =40dBZ Max =50dBZ Max =35dBZ Max =60dBZ Max =35dBZ Max =60dBZ Max =60 dBZ Max =45dBZ G H SG GH

23. CONCLUSIONS • Simulation of a thunderstorm in the southern Germany shows that: • The model reproduces well the main dynamical and microphysical features: radar reflectivity and the size of graupel and hail. • b) Precipitation starts earlier in clean air. At the same time storms are stronger and produce more precipitation in polluted air. • c) Small aerosols foster the hail formation. This process is closely related to the increase in super cooled water content at upper levels and to intensification of riming. Large hail stones form in highly polluted air. • d) Polarimetric parametrs of the storm are calculated. The main feature of hail storm is high W, high radar reflectivity at upper levels and a significant value of LDR within above the melting layer up to 10 km.

25. Questions for Ryzhkov: • 1. Wet growth of hail: covered by a film of water at T<0. We do not take this into account. • 2. Meaning of parameters. KDP in clean case=0, in polluted case KDP is significant in melting layer. • 3.How to proceed now? How to tune the parameters in formulas? Case studies? Examples of fields of polarimetric parameters to compare the results with observations.

27. The equation for size distribution function fik of a k-th mass bin for cloud particles of type i is: is the fall velocity of cloud particles of type i belonging to the k-th mass bin

28. The stochastic collision equation used for description of water-water collisions is the collision kernel Swept volume Collision efficiency Fluctuations of concentration

29. In HUCM stochastic collision equations are solved for all types of hydometeors Results of collisions of hydrometeors of different type

31. Rimed snow snow drop rimedsnow rimedsnow rimedsnow

32. IMPROVEMENTS (cont) : Improvements (cont.): z graupel Collision kernels increase with height hail graupel drop Wet growth

33. Examples Rimed fraction of snow Snow bulk density

34. Hail+graupel precipitation and kinetic energy (Hail + graupel) kinetic energy: is maximum in polluted cases at CCN conc=3000-6000 cm-3