Photo courtesy of Paul Lawson/J.H. Bain An Overview of Cirrus Cloud Thinning and Determining Its Scientific Feasibility David L. Mitchell Desert Research Institute, Reno, Nevada, USA

Role of Cirrus Clouds in Earths Radiation Budget

Homogeneous Freezing Nucleation: T 145%. Higher nucleation rates, smaller crystals, lower fall speeds, greater IWP & cloud coverage. Coldest = strongest greenhouse effect T = -40°C Frozen haze droplet Heterogeneous Nucleation Processes: T > - 40°C, 100% < RH i < 145% Lower nucleation rates, larger ice crystals, higher fall speeds, lower IWP & cloud coverage, more OLR. Frozen droplet

KEY POINTS ● Cirrus CE is not SRM, but can be called RM or TRM ● Assumes homogeneous nucleation. Since ice crystals are produced at a lower rate with heterogeneous nucleation, introducing ice nuclei will result in larger ice crystals with higher fall speeds. ● Higher ice fall speeds reduce cirrus cloud coverage and ice content, which allows more outgoing longwave radiation (OLR) to escape to space. ● These higher ice fall speeds also remove ice from the upper troposphere (UT) at a higher rate, dehydrating the UT and thus allowing additional OLR to escape to space. ● Global climate model simulations show that this process can cool the planet by about 2 W m -2 on average, or a mean global surface cooling of 1.4°K.

Cirrus CE will only work if homogeneous ice nucleation dominates ice production in non-anvil cirrus clouds. 1.Evidence from field campaigns show that either homo- or heterogeneous ice nucleation may dominate, and this may depend on mineral dust concentration that depends on latitude and season. Recent findings suggest ~ 90% of cirrus are formed through heterogeneous nucleation. But in what locations were the field campaigns? 2.Storelvmo and Herger (JGR, 2014) show that seeding 15% of the globe having the highest solar noon zenith angles at any given time yields the same global mean cooling as a seeding strategy that involves 45% of the globe, cooling the planet by 1.4°C on average. Field campaigns were generally not conducted at these high latitudes where mineral dust concentrations should be minimal. 3.To determine the scientific feasibility of cirrus CE, we need to know whether homogeneous nucleation dominates at high latitudes (e.g. the Polar Regions), and if so, is there a seasonal dependence? The most practical way to answer these questions may be to use satellite remote sensing, but that requires a means of evaluating the mode of ice nucleation (i.e. homo- or heterogeneous nucleation).

Evidence of homogeneous freezing nucleation? Results from SPARTICUS over the central USA. PSD were averaged over 5°C temperature intervals. Homogeneous nucleation is possible when T < -40°C, and is associated with higher concentrations of small ice crystals.

A Proposed Satellite Retrieval for Ice Nucleation Mode Assessment The effective absorption optical depth ratio, β eff : Sensitive to small ice crystals β eff = τ abs (12 μm)/ τ abs (11 μm) or τ abs (12 μm)/ τ abs (10.5 μm) adjusted for scattering Channels for CALIPSO IIR microphysical parameter PSD example from Cotton et al. (2012, QJRMS) with T = -38°C & sampled 2.75 km below cloud top in synoptic cirrus. PSD is composite from 5 probes, sampled over northern Scotland. Ice crystal size dependence of the wave resonance contribution (i.e. tunneling) to absorption efficiency (which determines β eff ). Wavelength dependence of Q abs for 2 PSD with and without small crystals. The difference in Q abs between PSD at the indicated IIR channels is due to wave resonance. Note that β eff is also defined as Q abs (12 μm)/Q abs (11 μm), adjusted for scattering effects.

For T < -60°C, N for continental anvils appears consistent with homogeneous nucleation, but not N for maritime anvils. Scatter is reduced by using N/IWC. Near cloud top N/IWC may depend mostly on the ice nucleation rate, while further below it depends on both ice nucleation and ice particle mass growth.

Krämer et al. (AMS Cloud Phys. Mtg.) found the ice nucleation mode can be discriminated based on IWC and RH, with higher IWC at a given T associated with homogeneous nucleation. Heterogeneous nucleation? Homogeneous nucleation? Heterogeneous nucleation? A surprisingly tight relationship was found between β eff & N/IWC that can be used to retrieve N/IWC. A tentative condition for homogeneous nucleation might be β eff > Another tentative condition for homogeneous nucleation might be IWC > ~ 5×10 -4 g m -3 for T < -55°C, following recent results from Krämer et al.

homogeneous heterogeneous Based on these criteria, synoptic cirrus sampled during SPARTICUS appear to be formed through heterogeneous or a mixture of homo- and heterogeneous nucleation near cloud top. Temperature dependence of the ice particle number concentration N for synoptic mean PSD (left). PSD have been averaged over 5°C intervals. Right panel shows the same N after normalizing it by the mean PSD IWC. Vertical bars are standard deviations; points without bars are based on less than 3 PSD.

Global views of β eff (along with cloud emissivity and brightness temperature) in mid-June from GOES-11; courtesy of Andy Heidinger, Univ. Wisconsin, USA

A Possible Satellite Retrieval Method 1.Using the Imaging Infrared Radiometer (IIR) and CALIOP lidar aboard CALIPSO, retrieve β eff and the IWC profile in the upper cloud. 2.Based on β eff and the IWC profile, estimate whether homo- or heterogeneous nucleation is dominating. 3.Determine whether this discrimination can be facilitated using the lidar depolarization ratio (an ice particle shape index). 4.Regardless of how this affects climate engineering, this knowledge is needed for accurate assessments of the cirrus cloud climate-radiative feedback.

SUMMARY & END OF TALK

From Storelvmo & Herger, JGR, MAM JJA SON DJF HOMHET_5% (5% of dust particles produce IN) HOMHET_50% (50% of dust particles produce IN) HOMHET_100% (100% of dust particles produce IN)

MODIS retrievals over ocean near Costra Rica during the TC4 campaign, based on the 11 & 12 μm channels. β eff calculated from in situ (ice PSD) measurements in maritime (TC4) and continental (SPARTICUS) cirrus. Retrieved and in situ TC4 β eff values are consistent for T > -55°C. Retrieved β eff might not be sensitive to uppermost cloud if IWC is very low.