Abstract. Every year, from December to April, anthropogenic haze spreads over most of the North Indian Ocean, and South and Southeast Asia. The Indian Ocean Experiment (INDOEX) documented this Indo-Asian haze at scales ranging from individual particles to its contribution to the regional climate forcing. This study integrates the multiplatform observations (satellites, aircraft, ships, surface stations, and balloons) with one-and fourdimensional models to derive the regional aerosol forcing resulting from the direct, the semidirect and the two indirect effects. The haze particles consisted of several inorganic and carbonaceous species, including absorbing black carbon clusters, fly ash, and mineral dust. The most striking result was the large loading of aerosols over most of the South Asian region and the North Indian Ocean. The January to March 1999 visible optical depths were about 0.5 over most of the continent and reached values as large as 0.2 over the equatorial Indian ocean due to long-range transport. The aerosol layer extended as high as 3 km. Black carbon contributed about 14% to the fine particle mass and 11% to the visible optical depth. The single-scattering albedo estimated by several independent methods was consistently around 0.9 both inland and over the open ocean. Anthropogenic sources contributed as much as 80% (_+10%) to the aerosol loading and the optical depth. The in situ data, which clearly support the existence of the first indirect effect (increased aerosol concentration producing more cloud drops with smaller effective radii), are used to develop a composite indirect effect scheme. The Indo-Asian aerosols impact the radiative forcing through a complex set of heating (positive forcing) and cooling (negative forcing) processes. Clouds and black carbon emerge as the major players. The dominant factor, however, is the large negative forcing (-20 +_ 4 W m -t) at the surface and the comparably large atmospheric heating. Regionally, the absorbing haze decreased the surface solar radiation by an amount comparable to 50% of the total ocean heat flux and nearly doubled the lower tropospheric solar heating. We demonstrate with a general circulation model how this additional heating significantly perturbs the tropical rainfall patterns and the hydrological cycle with implications to global climate.

[1] During the Department of Energy's Atmospheric Radiation Measurement Program's Mixed-Phase Arctic Cloud Experiment (M-PACE) in fall 2004, the University of North Dakota Citation measured 53 profiles within single-layer stratus clouds by executing spiral ascents and descents over Barrow and Oliktok Point, Alaska, and by flying ramped ascents and descents between. Cloud phase was identified from an algorithm that uses voltage change from the Rosemount ice detector, the size distribution (SD) shape measured by the Forward Scattering Spectrometer Probe (FSSP), and manual identification of particles imaged by the Cloud Particle Imager, the two-dimensional cloud probe (2DC) and the high-volume precipitation sampler (HVPS). Size and mass distribution functions were derived using data from the FSSP, one-dimensional cloud probe, 2DC and HVPS in conjunction with total water content (TWC) measured by the Counterflow Virtual Impactor. With clouds defined as locations where TWC > 0.001 g m À3 , there were a total of 513 30-s averaged SDs in single-layer clouds, of which 71% were in mixed-phase parcels, 23% in ice-phase and 6% in liquid-phase. The mixed-phase parcels were dominated by contributions from liquid drops, with the liquid mass fraction f l having averages and standard deviations of 0.89 ± 0.18 with 75% of cases having f l > 0.9. For these single-layer clouds, f l increased with normalized cloud altitude z n , defined as linearly increasing from 0 at cloud base to 1 at cloud top with f l averaging 0.96 ± 0.13 near z n = 1 and 0.70 ± 0.30 near z n = 0. The effective radius of water droplets r ew increased with z n , from an average of 6.9 ± 1.8 mm near z n = 0 to 11.4 ± 2.4 mm near z n = 1, whereas the effective radius of ice crystals r ei (25.2 ± 3.9 mm) was nearly independent of z n . The averaged cloud droplet number concentration and concentrations of ice crystals with maximum dimensions greater than 53 mm were 43.6 ± 30.5 Â 10 3 L À1 and 2.8 ± 6.9 L À1 , respectively, and nearly independent of z n . In contrast to past measurements in mixedphase clouds combined from many geographical locations where f l increased with temperature, f l decreased from À12°to À3°C as clouds typically consisted of a liquid topped layer with precipitating ice below.

ABSTRACT:Results are presented from an intercomparison of single-column and cloud-resolving model simulations of a cold-air outbreak mixed-phase stratocumulus cloud observed during the Atmospheric Radiation Measurement (ARM) programme's Mixed-Phase Arctic Cloud Experiment. The observed cloud occurred in a well-mixed boundary layer with a cloud-top temperature of −15 • C. The average liquid water path of around 160 g m −2 was about two-thirds of the adiabatic value and far greater than the average mass of ice which when integrated from the surface to cloud top was around 15 g m −2 .Simulations of 17 single-column models (SCMs) and 9 cloud-resolving models (CRMs) are compared. While the simulated ice water path is generally consistent with observed values, the median SCM and CRM liquid water path is a factor-of-three smaller than observed. Results from a sensitivity study in which models removed ice microphysics suggest that in many models the interaction between liquid and ice-phase microphysics is responsible for the large model underestimate of liquid water path.Despite this underestimate, the simulated liquid and ice water paths of several models are consistent with observed values. Furthermore, models with more sophisticated microphysics simulate liquid and ice water paths that are in better agreement with the observed values, although considerable scatter exists. Although no single factor guarantees a good simulation, these results emphasize the need for improvement in the model representation of mixed-phase microphysics.

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Measurements from the US Department of Energy Atmospheric Radiation Measurement Program's 2004 Mixed‐Phase Arctic Cloud Experiment (M‐PACE) provide a unique opportunity to study poorly understood ice formation processes in mixed‐phase stratocumulus. Using meteorological, aerosol, and ice nucleus measurements to initialize large‐eddy simulations with size‐resolved microphysics, we compare predicted liquid and ice mass, number, and size distribution with observations from a typical flight. We find that ambient ice nuclei appear insufficient by a few orders of magnitude to explain observed ice, consistent with past literature. We also find that two processes previously hypothesized to explain the discrepancy, shatter of freezing drops and fragmentation during ice‐ice collisions, were not significant sources of ice based on parameterizations from existing studies. After surveying other mechanisms that have been hypothesized to explain ice formation in mixed‐phase clouds generally, we find two that may be strong enough: (1) formation of ice nuclei from drop evaporation residuals, a process suggested by sparse and limited measurements to date, and (2) drop freezing during evaporation, a process suggested only by inference at this time. The first mechanism can better explain the persistence of mixed‐phase conditions in simulations of less vigorous stratus observed during the Beaufort Arctic Storms Experiment (BASE). We consider conditions under which emission of nuclei from the ocean surface or activation through cloud‐phase chemistry could provide alternative explanations for M‐PACE observations. Additional process‐oriented measurements are suggested to distinguish among ice formation mechanisms in future field studies.