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Guoxing Chen and Wei-Chyung Wang

Abstract

Recently, Chen et al. used a combination of observations and WRF simulations to illustrate that the anthropogenic aerosol–cloud microphysics–radiation interactions over the southeast Pacific can potentially reduce the excessive shortwave radiation reaching the sea surface, a common bias identified in CMIP5 models. Here, with the aid of a mixed-layer ocean, the authors further study the implications of the shortwave radiation reduction to the underlying air–sea coupling, focusing on the SST sensitivity to the changes. Results show that responses of the air–sea coupling include two negative feedbacks (a large decrease in the latent heat flux and a small decrease in the sensible heat flux, both associated with the surface cooling) and a positive feedback (an increase in the cloud cover, caused by the increase in the relative humidity within the boundary layer, especially during the daytime). The 0.1°C (W m−2)−1 SST sensitivity is about half that documented in CMIP5 models. In addition, an effective daytime cloud fraction weighted with the solar diurnal cycle is proposed to facilitate diagnosing the intensity of cloud–radiation interactions in general circulation models.

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Wei-Chyung Wang and Peter H. Stone

Abstract

The feedback between ice albedo and temperature is included in a one-dimensional radiative-convective climate model. The effect of this feedback on global sensitivity to changes in solar constant is studied for the current climate conditions. This ice-albedo feedback amplifies global sensitivity by 26 and 39%, respectively, for assumptions of fixed cloud altitude and fixed cloud temperature. The global sensitivity is not affected significantly if the latitudinal variations of mean solar zenith angle and cloud cover are included in the global model.

The differences in global sensitivity between one-dimensional radiative-convective models and energy balance models are examined. It is shown that the models are in close agreement when the same feedback mechanisms are included.

The one-dimensional radiative-convective model with ice-albedo feedback included is used to compute the equilibrium ice line as a function of solar constant. It is found that the fixed cloud temperature parameterization breaks down before the completely ice-covered earth instability sets in, i.e., the lowest cloud layer intersects the ground.

In addition, it is shown that the ice-albedo feedback has a similar amplification effect on the global warming caused by increase in atmospheric carbon dioxide concentration as in the case of solar constant change.

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Zhe Li, Huiwen Xue, Jen-Ping Chen, and Wei-Chyung Wang

Abstract

This study investigates the effects of meteorological conditions and aerosols on marine stratocumulus in the southeastern Pacific using the Weather Research and Forecasting (WRF) Model. Two regimes with different temperature and moisture conditions in the finest model domain are investigated. The western regime is around 87°–79°W, while the eastern regime is around 79°–71°W. In both regimes, cloud fraction, liquid water path (LWP), cloud thickness, and precipitation show significant diurnal cycles. Cloud fraction can be 0.83 during the night and down to 0.29 during the day in the western regime. The diurnal cycles in the eastern regime have smaller amplitudes but are still very strong. Stratocumulus properties also differ in the two regimes. Compared to the western regime, the eastern regime has lower temperature, higher relative humidity, and a more coupled boundary layer, leading to higher cloud fraction (by 0.11) and lower cloud-base height. The eastern regime also has lower inversion height that causes lower cloud-top height and thinner clouds and, hence, lower LWP and less precipitation.

Cloud microphysical properties are very sensitive to aerosols in both regimes. Increasing aerosols greatly increase cloud number concentration, decrease cloud effective radius, and suppress precipitation. Cloud macrophysical properties (cloud fraction, LWP) are not sensitive to aerosols in either regime, most notably in the eastern regime where precipitation amount is less. The changes in cloud fraction and LWP caused by changes in aerosol concentrations are smaller than the changes in the diurnal cycle and the spatial variability between the two regimes.

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Wei-Chyung Wang, Joseph P. Pinto, and Yuk Ling Yung

Abstract

Using a one-dimensional radiative-convective model, we perform a sensitivity study of the effect of ozone depletion in the stratosphere on the surface temperature. There could be a cooling of the surface temperature by ∼0.2 K due to chlorofluoromethane-induced ozone depletion at steady state (assuming 1973 release rates). This cooling reduces significantly the greenhouse effect due to the presence of chlorofluoromethanes. Carbon tetrafluoride has a strong ν3 band at 7.8 μm, and the atmospheric greenhouse effect is shown to be 0.07 and 0.12 K (ppbv)−1 with and without taking into account overlap with CH4 and N2O bands. At concentration higher than l ppbv, absorption by the ν3 band starts to saturate and the greenhouse effect becomes less efficient.

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Guoxing Chen, Wei-Chyung Wang, and Jen-Ping Chen

Abstract

Atmosphere–ocean general circulation models tend to underestimate the solar radiative forcing by stratocumulus over the southeast Pacific, contributing to a warm sea surface temperature (SST) bias. The underestimation may be caused by biases in either macro- or micro- (or both) physical properties of clouds. This study used the WRF Model (incorporated with a physics-based two-moment cloud microphysical scheme) together with the 2008 Variability of the American Monsoon Systems Ocean–Cloud–Atmosphere–Land Study (VOCALS) field observations to investigate the effects of anthropogenic aerosols on the stratocumulus properties and their subsequent effects on the surface radiation balance. The effects were studied by comparing two cases: a control case with the anthropogenic aerosols and a sensitivity case without the anthropogenic aerosols. Results show that the control case produced cloud properties comparable with the measurements by aircraft and that aerosol–cloud microphysical interactions play an important role in regulating solar cloud radiative forcing. As expected, the anthropogenic aerosols increase the cloud droplet number and decrease the cloud droplet size, resulting in an enhancement of solar cloud radiative forcing and a reduction in solar radiation reaching the sea surface, up to a maximum of about 30 W m−2 near the coast. Results also show that aerosol–cloud microphysics–radiation interactions are sensitive to cloud fraction, thus highlighting the role of cloud diurnal variation in studying the cloud–radiation interactions. Analysis of the high-resolution (3 km) model simulations reveals that there exists an inherent scale dependence of aerosol–cloud–radiation interactions, with coarser horizontal resolution yielding a weaker variability.

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Chia-Chi Wang, Chia Chou, and Wei-Liang Lee

Abstract

The effects of moisture on the intertropical convergence zone (ITCZ) over the eastern Pacific on the synoptic time scale are investigated using an intermediate complexity atmospheric circulation model, the quasi-equilibrium tropical circulation model (QTCM1), on an aquaplanet.

The dry simulation shows results consistent with those of simple dynamic models, except that a slightly stronger heating rate is needed owing to different model designs. In the moist simulations, the most important result is the formation of a tail southwest of a vortex during and after the ITCZ breakdown. This tail may extend zonally more than 60° longitude and last for more than two weeks in an idealized simulation. In the eastern North Pacific, this phenomenon is often observed in cases that involve easterly waves. In a sense, the formation of the tail suggests a possible mechanism that forms an ITCZ efficiently.

This study shows that the surface convergent flow induced by a disturbance initializes a positive wind–evaporation feedback that forms the tail. In the tail, the most important energy source is surface evaporation, and the latent heat is nicely balanced by an adiabatic cooling of the ascending motion. In other words, the energy is redistributed vertically by vertical energy convergence.

The lifespan of the tail is controlled by the propagation of tropical waves that modify the surface wind pattern, leading to a decrease in surface wind speed and corresponding surface fluxes. It may explain the absence of the tail in some of the events in the real atmosphere.

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Hans R. Schneider, Malcolm K. W. Ko, Nien Dak Sze, Guang-Yu Shi, and Wei-Chyung Wang

Abstract

The effect of eddy diffusion in an interactive two-dimensional model of the stratosphere is reexamined. The model consists of a primitive equation dynamics module, a simplified HOx ozone model and a full radiative transfer scheme. The diabatic/residual circulation in the model stratosphere is maintained by the following processes: 1) nonlocal forcing resulting from dissipation in the parameterized model troposphere and frictional drag at mesospheric levels, 2) mechanical damping within the stratosphere itself, and 3) potential vorticity flux due to large scale waves. The net effect of each process is discussed in terms of the efficiency of the induced circulation in transporting ozone from the equatorial lower stratosphere to high latitude regions. The same eddy diffusion coefficients are used to parameterize the flux of quasi-geostrophic potential vorticity and diffusion in the tracer transport equation. It is shown that the ozone distributions generated with the interactive two-dimensional model are very sensitive to the choice of values for the friction and the eddy diffusion coefficients. The strength of the circulation increases with the mechanical damping and Kyy. At the same time, larger diffusion in the tracer transport equation reduces the equator to pole transport (Holton 1986). Depending on the amount of friction assumed in the stratosphere, increasing eddy diffusion can lead to an increase as well as a decrease in the net transport. It is shown that reasonable latitudinal gradients of ozone can be obtained by using small values for the mechanical damping [≈1/(100 days)] and Kyy (order 104 m2 s−1) for the mid- and high-latitude stratosphere.

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Wei-Chyung Wang, William B. Rossow, Mao-Sung Yao, and Marilyn Wolfson

Abstract

We illustrate the potential complexity of the feedback between global mean cloud amount and global mean surface temperature when variations of the vertical cloud distribution are included by studying the behavior of a one-dimensional radiative–convective model with two types of cloud variation: 1) variable cloud cover with constant optical thickness and 2) variable optical thickness with constant cloud cover. The variable parameter is calculated assuming a correlation between cloud amount and precipitation or the vertical flux convergence of latent heat. Since the vertical latent heat flux is taken to be a fraction of the total heat flux, modeled by convective adjustment, we examine the sensitivity of the results to two different critical lapse rates, a constant 6.5 K km−1 lapse rate and a temperature-dependent, moist adiabatic lapse rate. The effects of the vertical structure of climate perturbations on the nature of the cloud feedback are examined using two cases: a 2% increase in the solar constant and a doubling of the atmospheric carbon dioxide concentration. The model results show that changes in the vertical cloud distribution and mean cloud optical thickness can be as important to climate variations as are changes in the total cloud cover. Further the variety and complexity of the feedbacks exhibited even by this simple model suggest that proper determination of cloud feedbacks must include the effects of varying vertical distribution.

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Liqi Chen, Wei Li, Jianqiong Zhan, Jianjun Wang, Yuanhui Zhang, and Xulin Yang

Abstract

To investigate the concentrations, sources, and temporal variations of atmospheric black carbon (BC) in the summer Arctic, routine ground-level observations of BC by optical absorption were made in the summer from 2005 to 2008 at the Chinese Arctic “Yellow River” Station (78°55′N, 11°56′E) at Ny-Ålesund on the island of Spitsbergen in the Svalbard Archipelago. Methods of the ensemble empirical-mode decomposition analysis and back-trajectory analysis were employed to assess temporal variation embedded in the BC datasets and airmass transport patterns. The 10th-percentile and median values of BC concentrations were 7.2 and 14.6 ng m−3, respectively, and hourly average BC concentrations ranged from 2.5 to 54.6 ng m−3. A gradual increase was found by 4 ng m−3 a−1. This increase was not seen in the Zeppelin Station and it seemed to contrast with the prevalent conception of generally decreasing BC concentration since 1989 in the Arctic. Factors responsible for this increase such as changes in emissions and atmospheric transport were taken into consideration. The result indicated that BC from local emissions was mostly responsible for the observed increase from 2005 to 2008. BC temporal variation in the summer was controlled by the atmospheric circulation, which presented a significant 6–14-day variation and coherent with 1–3- and 2–5-day and longer cycle variation. Although the atmospheric circulation changes from 2005 to 2008, there was not a marked trend in long-range transportation of BC. This study suggested that local emissions might have significant implication for the regional radiative energy balance at Ny-Ålesund.

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Yachao Hu, Greg M. McFarquhar, Wei Wu, Yongjie Huang, Alfons Schwarzenboeck, Alain Protat, Alexei Korolev, Robert M Rauber, and Hongqing Wang

Abstract

High ice water content (HIWC) regions above tropical mesoscale convective systems are investigated using data from the second collaboration of the High Altitude Ice Crystals and High Ice Water Content projects (HAIC-HIWC) based in Cayenne, French Guiana, in 2015. Observations from in situ cloud probes on the French Falcon 20 determine the microphysical and thermodynamic properties of such regions. Data from a 2D stereo probe and precipitation imaging probe show how statistical distributions of ice crystal mass median diameter (MMD), ice water content (IWC), and total number concentration (Nt) for particles with maximum dimension (Dmax) > 55 μm vary with environmental conditions, temperature (T), and convective properties such as vertical velocity (w), MCS age, distance away from convective peak (L), and surface characteristics. IWC is significantly correlated with w, whereas MMD decreases and Nt increases with decreasing T consistent with aggregation, sedimentation, and vapor deposition processes at lower altitudes. MMD typically increases with IWC when IWC < 0.5 g m−3, but decreases with IWC when IWC > 0.5 g m−3 for −15° ≤ T ≤ −5°C. Trends also depend on environmental conditions, such as the presence of convective updrafts that are the ice crystal source, MMD being larger in older MCSs consistent with aggregation and less injection of small crystals into anvils, and IWCs decrease with increasing L at lower T. The relationship between IWC and MMD depends on environmental conditions, with correlations decreasing with decreasing T. The strength of correlation between IWC and Nt increases as T decreases.

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