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W. Perrie and Y. Hu

Abstract

The energy exchange between ocean surface waves and ice floes in the marginal ice zone (MIZ) involves the scattering and attenuation of wave energy and the excitation of oscillation modes of the ice floes, as open ocean waves propagate into the MIZ. Heave, pitch, and roll modes of oscillation are linked to estimation of wave scattering and attenuation. A model is presented for wave attenuation and compared to measurements from the Marginal Ice Zone Experiments in the Greenland Sea, as reported by Wadhams et al.

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Y. Hu and X. Zou

Abstract

Determining tropical cyclone (TC) center positions is of interest to many researchers who conduct TC analysis and forecasts. In this study, we develop and apply a TC centering technique to Cross-Track Infrared Sounder (CrIS) and Advanced Technology Microwave Sounder (ATMS) observations of brightness temperature and report on an improvement of accuracy by adding a TC spectral analysis to the state of the art [Automated Rotational Center Hurricane Eye Retrieval (ARCHER)], especially for ATMS. We show that the ARCHER TC center-fixing algorithm locates TC centers more successfully based on the infrared channel with center frequency at 703.75 cm−1 (channel 89) of the CrIS than the ATMS channel 22 (183.31 ± 1.0 GHz) due to small-scale features in ATMS channel’s brightness temperature field associated with strong convective clouds. We propose to first apply the ARCHER TC center-fixing algorithm to ATMS channel 4 (51.76 GHz) that is less affected by small-scale convective clouds, and then to perform a set of the azimuthal spectral analysis of the ATMS channel-22 observations with tryout centers within a squared box centered at the ATMS channel-4-determined center. The center that gives the largest symmetric component is the final ATMS-determined center. Compared to the National Hurricane Center best track, the root-mean-square center-fixing errors determined from the two ATMS channels (one single CrIS channel) are 29.9 km (35.8 km) and 28.0 km (30.9 km) for 104 tropical storm and 81 hurricane cases, respectively, in the 2019 hurricane season.

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Y. X. Hu and K. Stamnes

Abstract

A new parameterization of the radiative properties of water clouds is presented. Cloud optical properties for both solar and terrestrial spectra and for cloud equivalent radii in the range 2.5–60 µm are calculated from Mie theory. It is found that cloud optical properties depend mainly on equivalent radius throughout the solar and terrestrial spectrum and are insensitive to the details of the droplet size distribution, such as shape, skewness, width, and modality (single or bimodal). This suggests that in cloud models, aimed at predicting the evolution of cloud microphysics with climate change, it is sufficient to determine the third and the second moments of the size distribution (the ratio of which determines the equivalent radius). It also implies that measurements of the cloud liquid water content and the extinction coefficient are sufficient to determine cloud optical properties experimentally (i.e., measuring the complete droplet size distribution is not required). Based on the detailed calculations, the optical properties are parameterized as a function of cloud liquid water path and equivalent cloud droplet radius by using a nonlinear least-square fitting. The parameterization is performed separately for the range of radii 2.5–12 µm, 12–30 µm, and 30–60 µm. Cloud heating and cooling rates are computed from this parameterization by using a comprehensive radiation model. Comparison with similar results obtained from exact Mic scattering calculations shows that this parameterization yields very accurate musts and that it is several thousand times faster. This parameterization separates the dependence of cloud optical properties on droplet size and liquid water content, and is suitable for inclusion into climate models.

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Y. Hu and R. T. Pierrehumbert

Abstract

The mixing of a passive tracer by realistic time-dependent stratospheric flow (European Centre for Medium-Range Weather Forecasts winds) on an isentropic surface (420 K) is studied. Simulations of the advection–diffusion problem for an initially large-scale tracer field are carried out in the limit of weak diffusivity. Owing to chaotic advection, tracer variance is cascaded to small scales, where it can be dissipated despite the weak diffusivity. The tracer fluctuations are characterized in terms of their probability distribution function (PDF). The PDFs are characterized by a Gaussian core and “fat tails,” which fall more slowly than a Gaussian, and indicate anomalously high probability of extreme concentration fluctuations. Given the nonlinearity of many chemical reactions of interest, the anomalous prevalence of extreme fluctuations could have a profound effect on reactive tracers.

Zonal variations of tracer are homogenized globally leading to a unimodal PDF. Initially meridional variations are strongly influenced by the presence of mixing barriers. Meridional gradients homogenize only within mixing zones of limited latitudinal extent, bounded by permeable mixing barriers. The PDF becomes multimodal, with distinct populations of air caused by blending of the concentration values within each mixing zone. The Tropics is a zone of weak mixing, and serves as an important repository of stratospheric tracers, which are episodically ejected into surf zones in the form of filaments bearing extreme concentration values.

The shapes of the PDFs are discussed in terms of theoretical methods developed in the context of highly idealized mixing models. It is shown that such methods retain their utility when applied to realistic stratospheric mixing. The role of the probability distribution of finite-time Lyapunov exponents for the underlying trajectory problem is highlighted. The use of conditional averages of diffusion and dissipation is also illustrated. PDFs yielded by the idealized advection–diffusion problem are found to resemble those appearing in a GCM simulation of N2O.

The theoretical arguments and numerical results imply that under the assumption that the diffusivity is set so that the dissipation scale is comparable to model resolution, the concentration PDF eventually reaches a universal shape independent of model resolution after an initial transient stage. However, the width of the distribution, or equivalently the variance of the tracer fluctuation, increases algebraically as model resolution is refined.

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Qi Hu, Y. Tawaye, and S. Feng

Abstract

Many studies have shown evidence of a major climate change in the late 1970s and early 1980s. The change comprises a reversal of the sea surface temperature anomaly pattern in the North Pacific Ocean, a lowering of the atmospheric geopotential height in the North Pacific, altered frequency and intensity of cyclones/anticyclones and severe storms in the mid-and high latitudes, along with an “abrupt” increase of the Northern Hemisphere (NH) average surface air temperature. What do these changes mean in terms of the nature of the NH atmospheric circulation in the warmer climate? Do they indicate changes of the atmospheric energy cycle and conversion/ exchange between various energy forms in the atmosphere? In which latitude regions are such changes in the energetics most significant? These are some of the questions examined in this study for the period 1948–2000 using the NCEP–NCAR reanalysis data. Major results of this study show significant increases of both the NH mean and eddy kinetic energy in boreal summer and winter in the recent two decades since 1980. The NH mean available potential energy has remained unchanged, however, even though the generation of the available potential energy has increased after 1980. The extra available potential energy generated is found to be converted to the kinetic energy by a more efficient conversion from the available potential energy to the kinetic energy in the warmer climate. The increase of the NH kinetic energy is attributed to an increase of the midlatitude kinetic energy because the kinetic energy in the tropical region has decreased slightly after 1980. Additional analysis of the NH energetics in wavenumber domain further reveals an increase of the kinetic energy for motions of planetary to regional scales. Increases of both the mean and eddy kinetic energy in the mid-and high latitudes is consistent with the reports of rising intensity of synoptic systems, cyclones and anticyclones, and severe storms in the recent two decades.

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W. Perrie, C. L. Tang, Y. Hu, and B. M. DeTracy

Abstract

Ocean models usually estimate surface currents without explicit modeling of the ocean waves. To consider the impact of waves on surface currents, here a wave model is used in a modified Ekman layer model, which is imbedded in a diagnostic ocean model. Thus wave effects, for example, Stokes drift and wave-breaking dissipation, are explicitly considered in conjunction with the Ekman current, mean currents, and wind-driven pressure gradient currents. It is shown that the wave effect on currents is largest in rapidly developing intense storms, when wave-modified currents can exceed the usual Ekman currents by as much as 40%. A large part of this increase in velocity can be attributed to the Stokes drift. Reductions in momentum transfer to the ocean due to wind input to waves and enhancements due to wave dissipation are each of the order 20%–30%. Model results are compared with measurements from the Labrador Sea Deep Convection Experiment of 1997.

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Y. P. Kim, S-G. Shim, K. C. Moon, C-G. Hu, C. H. Kang, and K. Y. Park

Abstract

Gaseous species and total suspended particles were measured at Kosan, Cheju Island, Korea, between 11 March and 19 April 1994. The concentrations of nonsea salt (nss) ions were higher than those measured in clean marine areas of Japan and other background marine areas of the world. In particular, the nss sulfate concentration was comparable to that measured in Seoul. The average SO2 and NOx concentrations were approximately 0.97 and 3.5 ppb, respectively, which were lower than those at other urban areas in Korea but higher than those of other remote areas in the world. In contrast, the average O3 concentration was approximately 55 ppb, which is comparable to or higher than those at remote sites in Japan that were influenced by long-range transport of air pollutants. Half of the air parcels during the period were from northern China and about 30% of the air parcels from southern China. The main difference of air pollutant levels between the two areas was higher crustal species and lower nss sulfate concentrations for air parcels from northern China. The nss SO2−4 concentrations had a strong correlation with nss K+, NH+4, and O3 concentrations. In addition, the nss Ca2+ concentrations had a strong correlation with the nss K+ and nss Mg2+ concentrations. It was suggested that nss K+ had two sources:anthropogenic and crustal.

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T. Auligné, A. Lorenc, Y. Michel, T. Montmerle, A. Jones, M. Hu, and J. Dudhia

Abstract

No Abstract available.

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S. Chen, P. E. Kirstetter, Y. Hong, J. J. Gourley, Y. D. Tian, Y. C. Qi, Q. Cao, J. Zhang, K. Howard, J. J. Hu, and X. W. Xue

Abstract

In this paper, the authors estimate the uncertainty of the rainfall products from NASA and Japan Aerospace Exploration Agency's (JAXA) Tropical Rainfall Measurement Mission (TRMM) Precipitation Radar (PR) so that they may be used in a quantitative manner for applications like hydrologic modeling or merging with other rainfall products. The spatial error structure of TRMM PR surface rain rates and types was systematically studied by comparing them with NOAA/National Severe Storms Laboratory's (NSSL) next generation, high-resolution (1 km/5 min) National Mosaic and Multi-Sensor Quantitative Precipitation Estimation (QPE; NMQ/Q2) over the TRMM-covered continental United States (CONUS). Data pairs are first matched at the PR footprint scale (5 km/instantaneous) and then grouped into 0.25° grid cells to yield spatially distributed error maps and statistics using data from December 2009 through November 2010. Careful quality control steps (including bias correction with rain gauges and quality filtering) are applied to the ground radar measurements prior to considering them as reference data. The results show that PR captures well the spatial pattern of total rainfall amounts with a high correlation coefficient (CC; 0.91) with Q2, but this decreases to 0.56 for instantaneous rain rates. In terms of precipitation types, Q2 and PR convective echoes are spatially correlated with a CC of 0.63. Despite this correlation, PR's total annual precipitation from convection is 48.82% less than that by Q2, which points to potential issues in the PR algorithm's attenuation correction, nonuniform beam filling, and/or reflectivity-to-rainfall relation. Finally, the spatial analysis identifies regime-dependent errors, in particular in the mountainous west. It is likely that the surface reference technique is triggered over complex terrain, resulting in high-amplitude biases.

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Bryan A. Baum, Ping Yang, Andrew J. Heymsfield, Steven Platnick, Michael D. King, Y-X. Hu, and Sarah T. Bedka

Abstract

This study examines the development of bulk single-scattering properties of ice clouds, including single-scattering albedo, asymmetry factor, and phase function, for a set of 1117 particle size distributions obtained from analysis of the First International Satellite Cloud Climatology Project Regional Experiment (FIRE)-I, FIRE-II, Atmospheric Radiation Measurement Program intensive observation period, Tropical Rainfall Measuring Mission Kwajalein Experiment (KWAJEX), and the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL) Florida Area Cirrus Experiment (FACE) data. The primary focus is to develop band-averaged models appropriate for use by the Moderate Resolution Imaging Spectroradiometer (MODIS) imager on the Earth Observing System Terra and Aqua platforms, specifically for bands located at wavelengths of 0.65, 1.64, 2.13, and 3.75 μm. The results indicate that there are substantial differences in the bulk scattering properties of ice clouds formed in areas of deep convection and those that exist in areas of much lower updraft velocities. Band-averaged bulk scattering property results obtained from a particle-size-dependent mixture of ice crystal habits are compared with those obtained assuming only solid hexagonal columns. The single-scattering albedo is lower for hexagonal columns than for a habit mixture for the 1.64-, 2.13-, and 3.75-μm bands, with the differences increasing with wavelength. In contrast, the asymmetry factors obtained from the habit mixture and only the solid hexagonal column are most different at 0.65 μm, with the differences decreasing as wavelength increases. At 3.75 μm, the asymmetry factor results from the two habit assumptions are almost indistinguishable. The asymmetry factor, single-scattering albedo, and scattering phase functions are also compared with the MODIS version-1 (V1) models. Differences between the current and V1 models can be traced to the microphysical models and specifically to the number of both the smallest and the largest particles assumed in the size distributions.

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