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Jiwang Ma and X. San Liang

Abstract

The wintertime atmospheric blocking over the Atlantic is investigated using a newly developed methodology—namely, localized multiscale energy and vorticity analysis (MS-EVA)—and the theory of canonical energy transfer. Through a multiscale window transform (MWT), the atmospheric fields from the ERA-40 data are reconstructed on three-scale ranges or scale windows: basic-flow window, blocking window, and synoptic window. The blocking event is obtained by compositing the wintertime blocking episodes, and a clear westward-retrograding signal is identified on the blocking window. Likewise, the local multiscale energetics following the signal are composited. It is found that a life cycle of the blocking-scale kinetic energy (KE) may be divided into three phases: onset phase, amplification phase, and decay phase. Different phases have different mechanisms in play. In general, pressure work and the canonical transfer from the synoptic eddies initiate the generation of the blocking, while the latter contributes to its amplification. The blocking decays as the system transports the KE away and as it converts the KE into available potential energy (APE) through buoyancy conversion. For the APE on the blocking window, its evolution experiences two maxima and, correspondingly, two phases can be distinguished. In the first maximum phase, the dominating mechanism is baroclinic instability; in the second, buoyancy conversion takes place. These are also the mechanisms that cause the warm core of the blocking in the troposphere.

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L. Xia, F. Zhao, Y. Ma, Z. W. Sun, X. Y. Shen, and K. B. Mao

Abstract

Cirrus clouds play an important role in the global radiation budget balance. However, the existing MODIS and Visible Infrared Imaging Radiometer Suite (VIIRS) cirrus cloud test algorithms struggle to provide accurate cirrus cloud information for the Tibetan Plateau region. In this study, the 1.38-μm cirrus cloud test was improved by adding 11-μm brightness temperature and a multiday average land surface temperature test. An algorithm sensitivity analysis indicated that the proposed algorithm lowered the threshold of the existing 1.38-μm algorithm to 0.005 in the winter and did not produce any observable misclassifications. Compared to the existing 1.38-μm cirrus test algorithm, the accuracy validation indicated that the improved algorithm detected 31.7% more cirrus clouds than the existing VIIRS 1.38-μm cirrus test and yielded 14% fewer misclassifications than the MODIS 1.38-μm cirrus test.

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H.-Y. Ma, X. Ji, J. D. Neelin, and C. R. Mechoso

Abstract

The present study examines the mechanisms for the connection between the precipitation variability in eastern Brazil and the South Atlantic convergence zone (SACZ) convective margin (eastern Brazil/SACZ convective margin) and the variability of low-level inflow on interannual time scales during austral summer. The authors' methodology is based on the analysis of observational datasets and simulations by the University of California, Los Angeles (UCLA) atmospheric general circulation model (AGCM) coupled to the Simplified Simple Biosphere Model.

It is demonstrated that the inflow variability is associated with the leading mode of wind variability over subtropical South America, and the connection is established through the mechanism of an analytic prototype for convective margin shifts proposed in previous studies. Over the eastern Brazil/SACZ convective margin, the weaker (stronger) convection tends to occur together with stronger (weaker) low-level inflows in reference to the mean easterly trades. By changing the “ventilation” effect, stronger (weaker) inflows with low moist static energy from the Atlantic Ocean suppress (promote) convection. The causal relationship is verified by AGCM mechanism-testing experiments performed in perpetual-February mode, in which low-level, nondivergent wind perturbations are imposed in a region overlapping eastern Brazil and the western Atlantic Ocean. With solely the imposed-wind perturbations acting on the moisture advection in the model equation, the AGCM can reproduce the precipitation variability in the eastern Brazil/SACZ convective margin. The capability of the AGCM in capturing such precipitation sensitivity to the low-level inflow variability also suggests that the mechanism can be applied to other regions of convective margins or to other time scales.

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A. D. Collard, S. A. Ackerman, W. L. Smith, X. Ma, H. E. Revercomb, R. O. Knuteson, and S-C. Lee

Abstract

During FIRE II, cirrus clouds were observed in the wavelength range 3–19, µm with two High Resolution Interferometer Sounders as described in the Part I companion paper. One, known as AC-HIS, was mounted on the NASA ER-2 aircraft in order to look down on the clouds; these results are described in the Part II companion paper. The other, GB-HIS, also known as the Atmospheric Emitted Radiance Interferometer (AERI), was ground based. The AERI observations have been simulated, assuming scattering from spherical ice particles, using a single-layer doubling model for the cloud, for two atmospheric windows at 700–1250 and 2650–3000 cm−1. The second of these windows is affected by scattered sunlight, which has been included in the calculations. The sensitivity of the cloud signal to quantities such as the ice water path (IWP) and effective radius (r eff) have been determined. Using the cloud model, best fits have been derived for IWP and r eff, for both windows individually and together. Possible errors in these derivations have been investigated.

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S. A. Ackerman, W. L. Smith, A. D. Collard, X. L. Ma, H. E. Revercomb, and R. O. Knuteson

Abstract

This paper parts analysis of cloud observations by the High-Resolution Interferometer Sounder made from the NASA ER-2 aircraft during FIRE II. Clear and cloudy sky radiance spectra are presented in terms of differences between observations and radiative transfer model simulations.

Doubling/adding radiative transfer model simulations demonstrate that the magnitude of the brightness temperature differences (ΔBT) is a function of the cloud particle size distribution and the cloud ice water path. For effective radii greater than approximately 30 µm (size parameter of 18) there is little spectral variation in the brightness temperature (BT). An analysis of brightness temperature differences indicates that cirrus clouds over the FIRE II central site possessed a small-particle mode. The cases analyzed had similar appearances in a plot of ΔBT between 11 and 12 µm (BT11 – BT12) versus the observed ΔBT between 8 and 11 µm (BT8 – BT11), suggesting similarity in the microphysical properties of nongray cirrus. Brightness temperature differences between cirrus cloud over the central site and the Gulf of Mexico are presented to illustrate differences in the cirrus microphysical properties at the two different locations.

Cloud effective emissivities and effective radiative temperature were derived for observations over the FIRE central site using complementary lidar and radiosonde data. Small variations in these effective properties were seen on 5 December and 22 November. Although they had similar effective temperatures, the emissivities were very different. Very few clouds were observed to have an emissivity near unity.

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Q. S. He, C. C. Li, J. Z. Ma, H. Q. Wang, G. M. Shi, Z. R. Liang, Q. Luan, F. H. Geng, and X. W. Zhou

Abstract

As part of the Tibet Ozone, Aerosol and Radiation (TOAR) project, a micropulse lidar was operated in Naqu (31.5°N, 92.1°E; 4508 m MSL) on the Tibetan Plateau to observe cirrus clouds continuously from 19 July to 26 August 2011. During the experiment, the time coverage of ice clouds only was 15% in the upper troposphere (above 9.5 km MSL). The cirrus top/bottom altitudes (mean values of 15.6/14.7 km) are comparable to those measured previously at tropical sites but relatively higher than those measured at midlatitude sites. The majority of the cloud layers yielded a lidar ratio between 10 and 40 sr, with a mean value of 28 ± 15 sr, characterized by a bimodal frequency distribution. Subvisible, thin, and opaque cirrus formation was observed in 16%, 34%, and 50% of all cirrus cases, respectively. A mean cirrus optical depth of 0.33 was observed over the Tibetan Plateau, slightly higher than those in the subtropics and tropics. With decreasing temperature, the lidar ratio increased slightly, whereas the mean extinction coefficient decreased significantly. The occurrence of clouds is highly correlated with the outgoing longwave radiation and the strong cold perturbations in the upper troposphere. Deep convective activity and Rossby waves are important dynamical processes that control cirrus variations over the Tibetan Plateau, where both anvil cirrus outflowing from convective cumulonimbus clouds and large-scale strong cold perturbations in the upper troposphere should play an important role in cirrus formation.

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Noel E. Davidson, Yi Xiao, Yimin Ma, Harry C. Weber, Xudong Sun, Lawrie J. Rikus, Jeff D. Kepert, Peter X. Steinle, Gary S. Dietachmayer, Charlie C. F. Lok, James Fraser, Joan Fernon, and Hakeem Shaik

Abstract

The Australian Community Climate and Earth System Simulator (ACCESS) has been adapted for operational and research applications on tropical cyclones. The base system runs at a resolution of 0.11° and 50 levels. The domain is relocatable and nested in coarser-resolution ACCESS forecasts. Initialization consists of five cycles of four-dimensional variational data assimilation (4DVAR) over 24 h. Forecasts to 72 h are made. Without vortex specification, initial conditions usually contain a weak and misplaced circulation pattern. Significant effort has been devoted to building physically based, synthetic inner-core structures, validated using historical dropsonde data and surface analyses from the Atlantic. Based on estimates of central pressure and storm size, vortex specification is used to filter the analyzed circulation from the original analysis, construct an inner core of the storm, locate it to the observed position, and merge it with the large-scale analysis at outer radii.

Using all available conventional observations and only synthetic surface pressure observations from the idealized vortex to correct the initial location and structure of the storm, the 4DVAR builds a balanced, intense 3D vortex with maximum wind at the radius of maximum wind and with a well-developed secondary circulation. Mean track and intensity errors for Australian region and northwest Pacific storms have been encouraging, as are recent real-time results from the Australian National Meteorological and Oceanographic Centre. The system became fully operational in November 2011. From preliminary diagnostics, some interesting structure change features are illustrated. Current limitations, future enhancements, and research applications are also discussed.

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K.-M. Lau, V. Ramanathan, G.-X. Wu, Z. Li, S. C. Tsay, C. Hsu, R. Sikka, B. Holben, D. Lu, G. Tartari, M. Chin, P. Koudelova, H. Chen, Y. Ma, J. Huang, K. Taniguchi, and R. Zhang

Aerosol- and moonsoon-related droughts and floods are two of the most serious environmental hazards confronting more than 60% of the population of the world living in the Asian monsoon countries. In recent years, thanks to improved satellite and in situ observations, and better models, great strides have been made in aerosol and monsoon research, respectively. There is now a growing body of evidence suggesting that interaction of aerosol forcing with monsoon dynamics may alter the redistribution of energy in the atmosphere and at the Earth s surface, thereby influencing monsoon water cycle and climate. In this article, the authors describe the scientific rationale and challenges for an integrated approach to study the interactions between aerosol and monsoon water cycle dynamics. A Joint Aerosol-Monsoon Experiment (JAMEX) is proposed for 2007–11, with enhanced observations of the physical and chemical properties, sources and sinks, and long-range transport of aerosols, in conjunction with meteorological and oceanographic observations in the Indo-Pacific continental and oceanic regions. JAMEX will leverage on coordination among many ongoing and planned national research programs on aerosols and monsoons in China, India, Japan, Nepal, Italy, and the United States, as well as international research programs of the World Climate Research Program (WCRP) and the World Meteorological Organization (WMO).

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