Tropical anvil characteristics and water vapor of the tropical tropopause layer: Impact of heterogeneous and homogeneous freezing parameterizations

Abstract

[1] Two isolated deep convective clouds (DCCs) that developed in clean-humid and polluted-dry air masses, observed during the Tropical Pacific Warm Pool International Cloud Experiment (TWP-ICE) and U.K. Aerosol and Chemical Transport in Tropical Convection (ACTIVE) campaigns, respectively, are simulated using a three-dimensional cloud-resolving model with size-resolved aerosol and cloud microphysics. We examine the impacts of different homogeneous and immersion freezing parameterizations on the anvil characteristics and the water vapor content (WVC) in the tropical tropopause layer (TTL) for the two DCCs that developed in contrasting environments. The modeled cloud properties such as liquid/ice water path and precipitation generally agree with the available radar and satellite retrievals and in situ aircraft measurements. We find that anvil microphysical properties such as ice number concentration and ice effective radius (r ei ) are sensitive to the homogeneous freezing parameterizations (HomFPs) under both the clean-humid and polluted-dry conditions, while upper level convection and WVC in the TTL air are only sensitive to HomFPs under the polluted-dry condition. Specifically, the cloud anvil with the Koop et al. (2000) relative humidity dependent scheme has up to 50% and 70% lower ice number than those with other schemes (temperature dependent) for the clean-humid and polluted-dry cases, respectively. Consequently, the r ei is increased by 20-30 mm in both cases. As a result, extinction coefficient of cloud anvils is reduced by over 30%. Anvil size and evolution are also much affected by HomFPs. Higher immersion-freezing rate (Bigg, 1953) leads to a stronger convective cloud due to larger latent heat release resulted from much higher freezing rates, with larger ice water path but less precipitation in both humid and dry conditions. Consequently, the domain-averaged homogeneous freezing rates are enhanced by over 15%. Also, the higher immersion-freezing rate results in over 1.5 times higher ice number concentrations, much reduced r ei in the cloud anvil, and larger anvil size. The moistening effect of deep convection on the TTL clear air is very significant, with increases of a few times relative to the WVC before convection under both humid and dry conditions. Different HomFPs does not make much difference in WVC and upper level convection in the clean-humid case, but in the polluted-dry case, the HomFPs with lower nucleation rates predict about 25% lower WVC relative to the HomFPs with higher nucleation rates. Under both humid and dry conditions, the Bigg (1953) immersion freezing predicts about 25% higher WVC relative to the Vali (1975) parameterization, due to stronger transport and the larger anvil area in the domain.

Evaluation of Hydrometeor Phase and Ice Properties in Cloud-Resolving Model Simulations of Tropical Deep Convection Using Radiance and Polarization Measurements

“…However, some important diagnostics related to ice formation processes, such as ice crystal size and glaciation temperature, remain uncertain owing to the lack of in situ measurements in strong updraft regions, commonly poor spatiotemporal coverage of cloud sampling throughout, and issues associated with shattering of large ice crystals on aircraft probe inlets (e.g., Cober et al 2001;Lawson et al 2010;Korolev et al 2011). For instance, previous studies using detailed cloud-resolving model simulations (e.g., Philhps et al 2007;Fan et al 2010) show that the choices between currently proposed homogeneous and heterogeneous freezing parameterizations significantly affect the simulated anvil macrophysical properties, such as anvil areal extent and convective strength, and microphysical properties, such as ice number concentrations and sizes, but that results could not be sufficiently constrained by the available in situ measurements.…”

“…, Ti * adequate parameterization of deep convective cloud systems in climate models (e.g.. Fowler et al 1996;Baker 1997;Wu et al 2009;Heymsfield et al 2009). To better constrain parameterizations of such clouds in climate models, detailed studies of ice formation processes using cloud-resolving model (CRM) simulations and observational constraints are needed (e.g., Moncrieff et al 1997;Bechtold et al 2000;Fridlind et al 2004;Fan et al 2010;Varble et al 2011;Fridlind et al 2012a). Intensive field campaigns have provided a wealth of in situ measurements that can be used to constrain and evaluate cloud-resolving model simulations of tropical convective cloud systems to some extent (e.g.. Brown and Heymsfield 2001;Fridlind et al 2004;Wang et al 2009b;Fan et al 2010).…”

“…To better constrain parameterizations of such clouds in climate models, detailed studies of ice formation processes using cloud-resolving model (CRM) simulations and observational constraints are needed (e.g., Moncrieff et al 1997;Bechtold et al 2000;Fridlind et al 2004;Fan et al 2010;Varble et al 2011;Fridlind et al 2012a). Intensive field campaigns have provided a wealth of in situ measurements that can be used to constrain and evaluate cloud-resolving model simulations of tropical convective cloud systems to some extent (e.g.. Brown and Heymsfield 2001;Fridlind et al 2004;Wang et al 2009b;Fan et al 2010). However, some important diagnostics related to ice formation processes, such as ice crystal size and glaciation temperature, remain uncertain owing to the lack of in situ measurements in strong updraft regions, commonly poor spatiotemporal coverage of cloud sampling throughout, and issues associated with shattering of large ice crystals on aircraft probe inlets (e.g., Cober et al 2001;Lawson et al 2010;Korolev et al 2011).…”

Formation of highly porous aerosol particles by atmospheric freeze-drying in ice clouds

“…As clouds continually form and dissipate, it is likely that aerosol particles undergo cycles of freezing and drying during ice crystal formation and subsequent ice sublimation when exposed to dry, subsaturated conditions outside of the cloud. Such conditions occur in the lower parts of cirrus clouds (10), in the outflow of high convective clouds (11,12), and when clouds are injected into the dry lower stratosphere (13). Here we show that the atmospheric conditions for such aerosol processing are likely to occur readily in deep convective clouds that are common in the tropics and midlatitudes and reach the upper troposphere and tropical tropopause layer.…”

Microphysical effects determine macrophysical response for aerosol impacts on deep convective clouds

“…After the convective core areas were identified, the remaining single-layer clouds with the existence of cloud ice and with the cloud base higher than the melting levels were identified as stratiform/anvil regimes (52,53). We conducted a careful check on snapshot figures to ensure that those criteria were sufficient to identify convective cores and the corresponding stratiform/anvil areas.…”