Satellite measurements of CCN and cloud properties at the cloudy boundary layer: The Holy Grail is it achievable?

Transcription

1 Satellite measurements of CCN and cloud properties at the cloudy boundary layer: The Holy Grail is it achievable? C 1 Daniel Rosenfeld, The Hebrew University of Jerusalem

2 E -85 D C E -85 D C A B A B -104

3 1 3 Red: Visible reflectance Green: 3.7 µm reflectance Blue: 11 µm temperature tre_out Cloud temperature o C Cloud drop effective radius, µm Houston area NPP/VIIRS :22 UT

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7 1 3 Red: Visible reflectance Green: 3.7 µm reflectance Blue: 11 µm temperature tre_out Cloud temperature o C Cloud drop effective radius, µm W i n d Houston area NPP/VIIRS :22 UT

8 8000 India Texas CDP_Re[µm] Adiabatic Re 6000 CDP Re [µm] Adiabatic Re Altitude (m) Altitude (m) Cloud Droplet Effective Radius [µm] Cloud Droplet Effective Radius [µm] r e = 1.08R V The cloud is dominated by inhomogeneous mixing. Therefore cloud drop size behaves almost as adiabatic cloud. This explains the tight relations between Re and height or D. Therefore, a well defined depth for rain initiation should exist, which depends on the nucleated cloud drop concentration. 9 Giant CCN can initiate raindrops at smaller r e.

9 1 0.1 A _DSD LWC [g m -3 µm -1 ] Droplet Diameter [µm] 10 r e = 1.08 r v The self similarity of convective DSDs causes a similarity and a fixed relation between r v and r e.

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. 12 Results: Drop concentration decreased Drop size conserved

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. 13 Results: Drop concentration preserved Drop size decreased

12 -actual 14 Cloud drop effective radius, r e [µm] Freud and Rosenfeld, submitted.

13 The number of activated CCN at cloud base will be obtained from the microphysical vertical profiles of the convective clouds (T-r e relations) 15

14 q La Where r v = (3q L /4πρ w N) 1/3 q L is the cloud water The reduction in N a with respect to adiabatic depends ρ w is the water density on the relative humidity N of is the air cloud that drop is concentration mixed with the cloud. This can be retrieved by the thermal T and NIR water vapor channels, with reference to the cloud surfaces. 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 16 correct the aircraft measured r e to adiabatic r ea according to the scheme of Freud and Rosenfeld, ACP On average N a ~ 0.77N a adiabatic.

15 Retrieving CCN from satellite-measured T- r e relations of convective clouds Satellite: T, r e, D, D* Depth above cloud base Temperature Tmin Dmax -40 C Daniel Rosenfeld Calculating N a from observed D- or T-r e N a = N a = Tglaciation N a = 0 C D* 20 C Tbase D=0 Tsurface N a α dr e3 /dd Retrieved CCN(S) Surface CCN(S) Measurements Wb Rain Radar gauges & Lidar: Rain, Hail, Snow, Cloud, W, Wb, Aerosols Satellite Retrieved CCN, cm -3 Validation of the retrieved CCN over SGP Surface measured CCN, cm -3

16 Bull. Amer. Met. Soc.

17 Solar radiation Coarse Satellite Spatial resolution of ~100 m is required for same vertical resolution from reflectance from vertical sides of clouds. Lower resolution misses all but largest and deepest clouds. The solar reflectance depends on cloud top topography and orientation. Therefore, it is necessary to resolve the 3D radiative effects and dynamic structures by stereoscopic measurements. Fine R1 R2 19 Measurement concept for T-r e based CCN retrievals

18 CHASER The Scientific Basis Daniel Rosenfeld The Hebrew University of Jerusalem 20

19 Nilton O. Rennó, Earle Williams, Daniel Rosenfeld et al., 2013: CHASER: An Innovative Satellite Mission Concept to Measure the Effects of Aerosols on Clouds and Climate. BAMS, May 2013.

20 Height Updraft Wb 2 i N = iw Wi N W i i > 0 Thermals in the well mixed boundary layer, as seen by the vertically pointing Doppler radar at the SGP site. The retrieved cloud base updraft speeds are denoted. The vertical line denotes the overpass time.

21 Satellite Retrieved CCN, cm Surface measured CCN, cm -3 Validation of the satellite-retrieved CCN by surface measurements at the SGP for the 9, 13, 19, 24 and 25 of July The median r e for a given T was used.

22 Yes, it might be possible C