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Yuh-Lang Lin, Shu-Hua Chen, and Liping Liu

Abstract

A series of idealized numerical experiments and vorticity budget analyses is performed to examine several mechanisms proposed in previous studies to help understand the orographic influence on track deflection over a mesoscale mountain range. When an idealized tropical cyclone (TC) is embedded in a uniform, easterly flow and passes over a mountain with a moderate Froude number, it is deflected to the south upstream, moves over the mountain anticyclonically, and then resumes its westward movement. The vorticity budget analysis indicates that the TC movement can be predicted by the maximum vorticity tendency (VT). The orographic effects on the above TC track deflection are explained by the following: 1) Upstream of the mountain, the easterly basic flow is decelerated as a result of orographic blocking that causes the flow to become subgeostrophic, which advects the TC to the southwest, analogous to the advection of a point vortex embedded in a flow. The VT is primarily dominated by the horizontal vorticity advection. 2) The TC passes over the mountain anticyclonically, mainly steered by the orographically generated high pressure. This makes the TC move southwestward (northwestward) over the upslope (lee slope). The VT is mainly contributed by the horizontal vorticity advection with additional contributions from vorticity stretching and the residual term (which includes friction and subgrid turbulence mixing). 3) Over the lee slope and downstream of the mountain, the northwestward movement is enhanced by asymmetric diabatic heating, making the turning more abrupt. 4) Far downstream of the mountain, the VT is mainly contributed by the horizontal vorticity advection.

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Yan Zhang, Hong-Hai Zhang, Gui-Peng Yang, and Qiu-Lin Liu

Abstract

The total suspended particulate (TSP) samples over the Bohai Sea and the Yellow Sea were collected during two cruises in spring and autumn in 2012. Concentrations of water-soluble ions {Na+, K+, NH4 +, Mg2+, Ca2+, Cl, NO3 , SO4 2−, and CH3SO3 [methanesulfonic acid (MSA)]} and trace metals (Al, Pb, Zn, Cd, Cu, and V) were measured. Mass concentrations of TSP samples ranged from 65.2 to 136 μg m−3 in spring and from 15.9 to 70.3 μg m−3 in autumn, with average values of 100 ± 22.4 and 40.2 ± 17.8 μg m−3, respectively. The aerosol was acidic throughout the sampling periods according to calculation of equivalent concentrations of the cations (NH4 +, nss-Ca2+, and nss-K+) and anions (nss-SO4 2− and NO3 ). Correlation analysis and enrichment factors revealed that the aerosol composition in the coastal marine atmosphere had a feature of a mixture of air masses: that is, crustal, marine, and anthropogenic emissions. Trace metals were enriched by a wide range of 1–103, and enrichment factors for crustal source (EFc) were relatively higher in spring. Species like Cd, Zn, and Pb had an overwhelming contribution from anthropogenic sources. In addition, the concentrations of MSA varied from 0.0075 to 0.17 and from 0.0019 to 0.018 μg m−3 during the spring and autumn cruises, respectively, with means of 0.061 and 0.012 μg m−3, respectively. Based on the observed MSA and nss-SO4 2− concentrations in spring and autumn, the relative biogenic sulfur contributions to nss-SO4 2− were estimated to be 8.0% and 3.5% on average, respectively, implying that anthropogenic sources had a dominant contribution to the sulfur budget over the observational area.

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Yun Lin, Yuan Wang, Bowen Pan, Jiaxi Hu, Yangang Liu, and Renyi Zhang

Abstract

A continental cloud complex, consisting of shallow cumuli, a deep convective cloud (DCC), and stratus, is simulated by a cloud-resolving Weather Research and Forecasting Model to investigate the aerosol microphysical effect (AME) and aerosol radiative effect (ARE) on the various cloud regimes and their transitions during the Department of Energy Routine Atmospheric Radiation Measurement Aerial Facility Clouds with Low Optical Water Depths Optical Radiative Observations (RACORO) campaign. Under an elevated aerosol loading with AME only, a reduced cloudiness for the shallow cumuli and stratus resulted from more droplet evaporation competing with suppressed precipitation, but an enhanced cloudiness for the DCC is attributed to more condensation. With the inclusion of ARE, the shallow cumuli are suppressed owing to the thermodynamic effects of light-absorbing aerosols. The responses of DCC and stratus to aerosols are monotonic with AME only but nonmonotonic with both AME and ARE. The DCC is invigorated because of favorable convection and moisture conditions at night induced by daytime ARE, via the so-called aerosol-enhanced conditional instability mechanism. The results reveal that the overall aerosol effects on the cloud complex are distinct from the individual cloud types, highlighting that the aerosol–cloud interactions for diverse cloud regimes and their transitions need to be evaluated to assess the regional and global climatic impacts.

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Ruei-Fong Lin, David O'C. Starr, Paul J. DeMott, Richard Cotton, Kenneth Sassen, Eric Jensen, Bernd Kärcher, and Xiaohong Liu

Abstract

The Cirrus Parcel Model Comparison Project, a project of the GCSS [Global Energy and Water Cycle Experiment (GEWEX) Cloud System Studies] Working Group on Cirrus Cloud Systems, involves the systematic comparison of current models of ice crystal nucleation and growth for specified, typical, cirrus cloud environments. In Phase 1 of the project reported here, simulated cirrus cloud microphysical properties from seven models are compared for “warm” (−40°C) and “cold” (−60°C) cirrus, each subject to updrafts of 0.04, 0.2, and 1 m s−1. The models employ explicit microphysical schemes wherein the size distribution of each class of particles (aerosols and ice crystals) is resolved into bins or the evolution of each individual particle is traced. Simulations are made including both homogeneous and heterogeneous ice nucleation mechanisms (all-mode simulations). A single initial aerosol population of sulfuric acid particles is prescribed for all simulations. Heterogeneous nucleation is disabled for a second parallel set of simulations in order to isolate the treatment of the homogeneous freezing (of haze droplets) nucleation process. Analysis of these latter simulations is the primary focus of this paper.

Qualitative agreement is found for the homogeneous-nucleation-only simulations; for example, the number density of nucleated ice crystals increases with the strength of the prescribed updraft. However, significant quantitative differences are found. Detailed analysis reveals that the homogeneous nucleation rate, haze particle solution concentration, and water vapor uptake rate by ice crystal growth (particularly as controlled by the deposition coefficient) are critical components that lead to differences in the predicted microphysics.

Systematic differences exist between results based on a modified classical theory approach and models using an effective freezing temperature approach to the treatment of nucleation. Each method is constrained by critical freezing data from laboratory studies, but each includes assumptions that can only be justified by further laboratory research. Consequently, it is not yet clear if the two approaches can be made consistent. Large haze particles may deviate considerably from equilibrium size in moderate to strong updrafts (0.2–1 m s−1) at −60°C. The equilibrium assumption is commonly invoked in cirrus parcel models. The resulting difference in particle-size-dependent solution concentration of haze particles may significantly affect the ice particle formation rate during the initial nucleation interval. The uptake rate for water vapor excess by ice crystals is another key component regulating the total number of nucleated ice crystals. This rate, the product of particle number concentration and ice crystal diffusional growth rate, which is particularly sensitive to the deposition coefficient when ice particles are small, modulates the peak particle formation rate achieved in an air parcel and the duration of the active nucleation time period. The consequent differences in cloud microphysical properties, and thus cloud optical properties, between state-of-the-art models of ice crystal initiation are significant.

Intermodel differences in the case of all-mode simulations are correspondingly greater than in the case of homogeneous nucleation acting alone. Definitive laboratory and atmospheric benchmark data are needed to improve the treatment of heterogeneous nucleation processes.

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