Number of activated CCN as a key property in cloud-aerosol interactions. Or, More on simplicity in complex systems

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1 Number of activated CCN as a key property in cloud-aerosol interactions Or, More on simplicity in complex systems 1 Daniel Rosenfeld and Eyal Freud The Hebrew University of Jerusalem, Israel

2 Uncertainties in aerosol cloud-mediated radiative forcing: Two large and highly uncertain opposite effects from shallow and deep clouds 2 Daniel Rosenfeld The Hebrew University of Jerusalem, Israel WCRP First Open Science Conference, Denver, October 2011

deep clouds 2 Daniel Rosenfeld The Hebrew University of

3 3 Heavy Drizzle Threshold

4 Rosenfeld et al., Science, 2008 Detrained vapor add GHG aloft Colder and wider anvils radiate less IR energy to space positive RF 4 Growing Mature Hail Dissipating

5 Aircraft measurements in convective clouds show tight D-r e relations, almost as if the cloud was adiabatic parcel. 6 Cloud drop effective radius, r e [ m]

6 Aircraft measurements in convective clouds show tight height Height-r e relations, almost as if the cloud was adiabatic parcel. 7 Cloud drop effective radius, r e [ m]

7 Combined Why are the H-r e relationships so tight, and how can we use them to retrieve N a, the concentrations of activated CCN into drops at cloud base? 8 Cloud drop effective radius, r e [ m]

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9 10

10 Extreme inhomogeneous mixing 1. Dry air penetrates Saturated cloudy air parcel Dry entrained air parcel 2. Drops at the border of the dry parcel completely evaporate until saturating the mixed parcel. Cloud drop 3. The saturated parcel mixes and dilutes the drop concentration without further evaporating them. 11 Results: Drop concentration decreased Drop size conserved

Drops at the border of the dry parcel completely evaporate until saturating the mixed parcel.

11 Extreme _homogeneous mixing 1. Original unmixed cloud Saturated cloudy air parcel Sub-saturated mix of cloud with entrained air Cloud drop 2. The cloud mixes homogeneously with dry air and becomes sub saturated, before cloud drops had time to evaporate. 3. The cloud drops evaporate partially and reduce their size until the cloudy air saturates again. Results: Drop concentration slightly 12 decreased Droplet size decreased

12 -actual 13 Cloud drop effective radius, r e [ m] Freud and Rosenfeld, JGR.

13 RH=100% RH=90% RH=0% Mixing diagram: the relationship between droplet mean volume radius (r v ) and the adiabatic fraction (AF) for fully homogeneous and extreme inhomogeneous mixing events between an adiabatic cloud parcel and entrained non-cloudy air with varying relative humidity (RH) at the mixing level of 2200 m above cloud base, where the temperature is 10ºC and the adiabatic mixing ratio is 5 g kg 1. Cloud base is at 850 hpa and 20ºC. Concentration of activated CCN (N a ) is 500 mg 1. It can be seen that entrained air with higher RH results in smaller dependence of r v on AF, especially for 14 AF>0.2. When entrained air is saturated (RH=100%), or when mixing is extremely inhomogeneous, r v is constant. Freud et al. ACPD 2011

and the adiabatic mixing ratio is 5 g kg 1. Cloud base is at 850 hpa and 20ºC. Concentration of activated CCN (N a ) is 500 mg 1.

14 Adiabatic r v Best RH % The mixing lines that best fit the measurements are equivalent to homogeneous mixing with ambient air of >95%, and denoted in the legend. This means that the mixing is very close to extreme inhomogeneous. Such mixing preserves r e as in an adiabatic cloud. 15 Freud et al. ACPD 2011

15 q La Na init =1101 mg -1 N a =895 mg -1 r v = (3q L /4 w N) 1/3 Where q L is the cloud water w is the water density N is the cloud drop concentration r v is the drop radius for a mono-dispersed DSD having the same cloud water q L r e = 1.08 r v If T-r e is so tight, it means that mixing with ambient dry air does not change much r e, and it is similar to r e of an unmixed (adiabatic) cloud. If r e is adiabatic, we can calculate N adiabatic, or N a, the number of activated drops at cloud base, N a, as shown above. We can correct the measured r e to adiabatic r ea using the best fit RH. 16 Freud et al. ACPD 2011

08 r v If T-r e is so tight, it means that mixing with ambient dry air does not change much r e, and it is similar to r e of an unmixed (adiabatic) cloud.

16 1 0.1 A _DSD LWC [g m -3 µm -1 ] Droplet Diameter [µm] r e = 1.08 r v The similar shapes of convective DSDs causes a fixed relation between r v and r e. 17 Freud et al. ACPD 2011

2480 0.0001 10-5 0 10 20 30 40 50 Droplet Diameter [µm] r e = 1.

17 18 Freud and Rosenfeld, ACPD

18 Presently available AVHRR MODIS satellite AQUA 0801 measurements can sat=6, reach sat=16, sol=32,rel=27 sol=30, such results rel=22 only in special cases, CDP_Re[um] FSSP_Re[um] and only for large r15 convective clouds. r50 r85 Even so, we often miss the lower parts of the clouds. -10 Aircraft Satellite T [C] Cloud drop effective Re [ m] radius, r e [ m] Satellite retrievals validated by aircraft measurements

and only for large r15 convective clouds. r50 r85 Even so, we often miss the lower parts of the clouds.

19 Theoretical D c and N a relationships q La D cb ; b 1 q La r 3 Va N a D c (r Va 3 N a ) 1/b Adiabatic water increase linearly with depth Cloud water = drop volume * drop numbers Hence depth for a given r Va is linear with N a T base =11.5 C P base =850hPa r e =13 m 20 Na [mg -1 ] Na [mg -1 ] Freud and Rosenfeld, JGR

volume * drop numbers Hence depth for a given r Va is linear with N a T base

20 Does a critical r e exist? Yes! At least when no Giant CCN (GCCN) are present. r ec m R is rain water content in drops with diameters of mm, as measured by the Cloud Imaging Probe (CIP). R> 0.01 g/m 3 raises the precipitation flag. R> 0.01 g/m 3 21 Freud and Rosenfeld, JGR.

r ec 10 12 m R is rain water content in drops with diameters of 0.1-0.

21 How sensitive is the threshold value? Rain initiates at m, but accelerates strongly above 14 m. re c 14 m R is rain water content in drops with diameters of mm, as measured by the Cloud Imaging Probe (CIP). R> g/m 3 R> g/m 3 raises the precipitation flag. 22 Freud and Rosenfeld, submitted.

22 Increasing condensed cloud water 23 Found for Marine Stratocumulus, Valid also for deep clouds 600 Drizzle formed 1.9 by coalescence Cloud Depth Cloud thickness above base [m] (m) Drizzle formed by diffusional growth Heavy Drizzle Re=16 m Re=14 m Light Drizzle Cloud drop concentration [cm -3 ] 0.76 No drizzle No Drizzle Drizzle in Marine Sc. (After Gerber, 1996; VanZanten et al., 2005) D. Rosenfeld et al.: Aerosols closing open Benard cells, Atmos. Chem. Phys., 6, , 2006

23 Heavy Drizzle Threshold 24 Effective radius [ m] Rosenfeld et al.: Atmos. Chem. Phys., 6, , 2006

24 Rosenfeld et al., Science, Growing Mature Hail Dissipating

25 Conclusions Vertical profiles of r v in convective clouds behave nearly as in adiabatic parcel. r v and r e are very highly linearly correlated, owing to a nearly fixed shape of DSDs with height, as long as no significant rain has developed. This allows using T-r e relations for retrieving the number of aerosols activated into drops at cloud base, N a. This can be done from space. Height to reach r e for initiation of precipitation, r ep, is linear with N a. Increasing N a by 100 cm -3 leads to increasing r ep by ~250 m. This potentially explains when closed MSC opens into POCs and respective large negative radiative forcing. This also explains the transition in deep convection from a regime of warm rain to mixed phase precipitation, and 26 the respective changes in cloud dynamics and vertical heating profiles and large positive radiative forcing.

26 This allows to assess aerosol impacts on: On cloud reflectance, cover, depth and hence cloud radiative forcing, for clouds ranging from marine stratocumulus (cooling) to deep tropical convective clouds (mainly warming). On moistening the UTLS and the respective positive radiative forcing. On vertical distribution of latent heating and impacts on circulation systems. On modulating the intensity of tropical cyclones. On convective severe local storms and their propensity to produce hail and tornadoes. On parameterization of cloud-aerosol-precipitationclimate processes, for a better understanding of the climate system and climate prediction, where our understanding is most lacking. 27

This chapter discusses: 1. Definitions and causes of stable and unstable atmospheric air. 2. Processes that cause instability and cloud development

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