Relevance of the Negative Twomey Effect for Cirrus Clouds

The negative Twomey effect (Kärcher and Lohmann 2003, ACP) is a phenomena produced by the interaction of two competing processes of ice nucleation; homogeneous and heterogeneous nucleation. While the Twomey effect increases cloud droplet concentrations and cloud albedo, the negative Twomey effect decreases ice crystal concentrations, cloud albedo and emissivity. It presumes ice crystals in cirrus clouds (T < -38°C) often nucleate homogeneously, and that homogeneous nucleation can be overtaken by heterogeneous nucleation processes when the concentration of ice nuclei is sufficiently high (~ 10 per liter). This occurs since ice nuclei activate to form ice crystals at relative humidities with respect to ice (RHi) lower than the RHi associated with homogeneous nucleation. For example, mineral dust introduced into dust-free cirrus may nucleate ice, lowering the RHi and thus suppressing homogeneous nucleation (which requires higher RHi). Since heterogeneous nucleation rates are lower relative to homogeneous nucleation, the ice crystals are fewer in number and grow to larger sizes with higher fall velocities. The higher fall velocities result in shorter cirrus cloud lifetimes, lower cirrus coverage and lower cirrus ice water path and optical depth. Collectively, these changes can produce significant changes in cirrus cloud radiative properties (e.g. albedo and emissivity).

A recent paper in Science (Cziczo et al. 2013) argues that mineral dust is ubiquitous in the atmosphere and that heterogeneous freezing is the dominant nucleation pathway for the clouds sampled, based on in situ measurements of the composition of the residual particles from sublimated ice crystals. If these findings are universal, then the negative Twomey effect may rarely occur, if ever. This study was based on four field campaigns, CRYSTAL-FACE, TC4, CRAVE, and MACPEX. In the first three, anvil cirrus clouds were sampled, while both anvil and synoptic cirrus clouds were sampled during MACPEX. It is not surprising for mineral dust concentrations to be sufficient for heterogeneous nucleation to dominate in anvil cirrus since deep convection advects boundary layer air to cirrus levels. Moreover, MACPEX was conducted during March and April, which are the two months of the year when East Asian dust storms are most active (dust from these storms are generally lofted much higher than Saharan dust). It thus appears possible that homogeneous nucleation might dominate in synoptic cirrus during periods where dust concentrations at cirrus levels are much lower.

Our study evaluates in situ measurements made during the SPARTICUS field campaign where cirrus clouds were sampled from January to June 2010. Most of the flights in synoptic cirrus were from January and February, when dust levels are typically ~ 1/3 of Spring levels (Yu et al. 2012, Science). Our analysis of the temperature dependence of ice cloud microphysical changes, along with the temperature dependence of in-cloud RHi, suggest homogeneous nucleation was the dominant ice nucleation pathway for synoptic cirrus. This suggests that heterogeneous nucleation prevailing in synoptic cirrus clouds over North America might be episodic in nature, possibly related to east Asian dust events. This also suggests that the negative Twomey effect may occur episodically with associated changes in cirrus cloud radiative forcing (CRF).

Using field observations from the six-month DOE-ASR SPARTICUS campaign, we developed temperature-dependent parameterizations for ice particle effective size De; one for homogeneous and another for heterogeneous nucleation. The ice fall speed was predicted from De. We estimate the potential impact of the negative Twomey effect outside the tropics using the Community Atmosphere Model version 5 (CAM5) and the ECHAM5 GCM. Initial results from CAM5 show a net CRF of about -3 W m-2 (i.e. cooling) in the Northern Hemisphere mid-latitudes and Polar Regions, and a greater net CRF cooling in the Southern Hemisphere.