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Qing Cao and Guifu Zhang

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There have been debates and differences of opinion over the validity of using drop size distribution (DSD) models to characterize precipitation microphysics and to retrieve DSD parameters from multiparameter radar measurements. In this paper, simulated and observed rain DSDs are used to evaluate moment estimators. Seven estimators for gamma DSD parameters are evaluated in terms of the biases and fractional errors of five integral parameters: radar reflectivity (ZH), differential reflectivity (Z DR), rainfall rate (R), mean volume diameter (Dm), and total number concentration (NT). It is shown that middle-moment estimators such as M234 (using the second-third-fourth moments) produce smaller errors than lower- and higher-moment estimators if the DSD follows the gamma distribution. However, if there are model errors, the performance of M234 degrades. Even though the DSD parameters can be biased in moment estimators, integral parameters are usually not. Maximum likelihood (ML) and L-moment (LM) estimators perform similarly to low-moment estimators such as M012. They are sensitive to both model error and the measurement errors of the low ends of DSDs. The overall differences among M234, M246, and M346 are not substantial for the five evaluated parameters. This study also shows that the discrepancy between the radar and disdrometer observations cannot be reduced by using these estimators. In addition, the previously found constrained-gamma model is shown not to be exclusively determined by error effects. Rather, it is equivalent to the mean function of normalized DSDs derived through Testud’s approach, and linked to precipitation microphysics.

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Qing Zhang and Roland Stull

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Two alternative parameterizations for nonlocal turbulence mixing are tested in a 1D boundary-layer model against a dataset from the 1983 Boundary-Layer Experiment (BLX83) in Oklahoma. One method, proposed previously by Stull and Driedonks, is based on a nonlocal approximation to the turbulence kinetic energy (TKE) equation. An alternate method, based on a nonlocal approximation to the Richardson number, is simplified here from earlier parameterizations for transilient turbulence theory. Convective mixed-layer simulations of the vertical profiles of mean variables and fluxes using both methods are compared to the BLX83 observations and to simulations using a traditional slab model.

The TKE method develops a surface layer that is too thick compared to BLX83 data, particularly in the early morning. It also lacks the subadiabatic lapse rate that is observed in the top of the mixed layer. The Richardson number approach produces more accurate mixed-layer profiles, but lacks the general physical interpretation of the TKE method. Nonlocal spectral decompositions of the flux and intensity of mixing confirm that large-size eddies dominate within the middle of the mixed layer. Based on this limited validation, the Richardson number method is recommended for convective boundary layers, but the TKE approach should be used for modeling more general boundary layers that can include clouds and stable and/or windy conditions.

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He Zhang, Minghua Zhang, and Qing-cun Zeng

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The dynamical core of the Institute of Atmospheric Physics of the Chinese Academy of Sciences Atmospheric General Circulation Model (IAP AGCM) and the Eulerian spectral transform dynamical core of the Community Atmosphere Model, version 3.1 (CAM3.1), developed at the National Center for Atmospheric Research (NCAR) are used to study the sensitivity of simulated climate. The authors report that when the dynamical cores are used with the same CAM3.1 physical parameterizations of comparable resolutions, the model with the IAP dynamical core simulated a colder troposphere than that from the CAM3.1 core, reducing the CAM3.1 warm bias in the tropical and midlatitude troposphere. However, when the two dynamical cores are used in the idealized Held–Suarez tests without moisture physics, the IAP AGCM core simulated a warmer troposphere than that in CAM3.1. The causes of the differences in the full models and in the dry models are then investigated.

The authors show that the IAP dynamical core simulated weaker eddies in both the full physics and the dry models than those in the CAM due to different numerical approximations. In the dry IAP model, the weaker eddies cause smaller heat loss from poleward dynamical transport and thus warmer troposphere in the tropics and midlatitudes. When moist physics is included, however, weaker eddies also lead to weaker transport of water vapor and reduction of high clouds in the IAP model, which then causes a colder troposphere due to reduced greenhouse warming of these clouds. These results show how interactive physical processes can change the effect of a dynamical core on climate simulations between two models.

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Haifeng Zhang, Qing Wu, and Ge Chen

Abstract

The Haiyang-2A (HY-2A; HaiYang means ocean in Chinese) satellite was successfully launched in China on 16 August 2011, carrying the nation’s first operational radar altimeter along with three other microwave sensors. In this study, HY-2A altimeter significant wave height (SWH) data have been validated against National Data Buoy Center (NDBC) buoy and Jason-2 altimeter SWH data over a period of 27 months (from 1 October 2011 to 31 December 2013). During the collocation, the effects of different thresholds of several flags are carefully studied. These flags prove to be useful for the SWH selection and different thresholds are observed to change the results remarkably. The final results show that HY-2A SWHs, with a 0.339-m root-mean-square (RMS) difference and a negative bias of 0.231 m in buoy comparison, have reached the mission target (0.5-m RMS). Nonetheless, the Jason-2 altimeter performs better with a lower RMS difference of 0.292 m and a positive bias of only 0.016 m. In addition, by analyzing the residuals (altimeter minus buoy), the bias for the HY-2A altimeter is found to decline monotonically over the whole range with an overestimation at low sea state (SWH < 1 m), a minor underestimation at middle sea state (1 m < SWH < 5 m), and a severe underestimation at high sea state (SWH > 5 m). However, only an underestimation at high sea state is found for the Jason-2 altimeter. A linear regression is also proposed. The 20 days of the newly processed HY-2A SWHs are investigated and discussed as well, and a slight quality improvement has been observed using these data.

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Qing Yan, Ting Wei, and Zhongshi Zhang

Abstract

Simulations of past warm climate provide an opportunity to better understand how the climate system may respond to increased greenhouse gas emissions. Using the ~25-km-resolution Community Atmosphere Model, version 4 (CAM4), we examine climate change over China in the Late Pliocene warm period (3.264–3.025 Ma) and further explore the influences of different sea surface temperature (SST) forcings and model horizontal resolutions. Initial evaluation shows that the high-resolution CAM4 performs well in capturing the climatological distribution of present-day temperature, precipitation, and low-level monsoon circulations over China. Based on the standard Pliocene Research, Interpretation and Synoptic Mapping (version 4; PRISM4) boundary conditions, CAM4 predicts an increase of annual mean temperature by ~0.5°C over China in the Late Pliocene relative to the preindustrial era, with the greatest warming in northwest China but cooling in southwest China. Enhanced annual mean precipitation is observed in the Late Pliocene over most of China except for northwest China where precipitation is decreased. The East Asian summer (winter) monsoon is intensified (weakened) in the Late Pliocene as suggested by geological evidence, which is attributed to the enhanced (reduced) land–sea thermal contrast. The East Asian monsoon domain exhibits a northwestward expansion in the Late Pliocene, especially over the Tibetan Plateau. Additionally, our results indicate that the modeled climate change is sensitive to the Late Pliocene SST forcings and model resolution. Particularly, different SST forcings [PRISM4-based vs Pliocene Model Intercomparison Project (PlioMIP)-based SSTs] affect the modeled phase change of summer monsoon and the associated precipitation change, while model resolution (~25 vs 400 km) mainly impacts precipitation change.

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Qing Yan, Robert Korty, and Zhongshi Zhang

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Using a coupled global climate model, Community Earth System Model (CESM), the authors investigate the response of tropical cyclone (TC) genesis factors (i.e., potential intensity, vertical wind shear, midtropospheric moisture content, and absolute vorticity) to external forcings in the last two millennia (L2M). They then examine how the large-scale conditions that favor TC activity varied using a genesis potential index (GPI). These large-scale genesis factors generally exhibit no long-term trend in the simulation of the L2M prior to the industrial revolution, and the spread in the interannual variability lies within a small window. The estimated TC activity is highly variable from region to region on multidecadal time scales. Conditions appear to be more favorable for TC genesis in the twentieth century in the Northern Hemisphere relative to earlier centuries of the L2M. Additionally, conditions in this simulation are not more favorable for TC formation during the Medieval Climate Anomaly (AD 1000–1200) relative to the Little Ice Age (AD 1500–1700) except in the eastern North Pacific and south Indian Ocean. Although a comparison of conditions simulated in the model with proxy-based reconstructions of prehistoric storm activity finds agreement during several active periods in the western North Pacific, the time series of simulated genesis factors does not match that of proxy reconstructions over the entire interval in either the western North Pacific or North Atlantic; this discrepancy likely arises from uncertainties in both the model and reconstructions.

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Qing Cao, Guifu Zhang, and Ming Xue

Abstract

This study presents a two-dimensional variational approach to retrieving raindrop size distributions (DSDs) from polarimetric radar data in the presence of attenuation. A two-parameter DSD model, the constrained-gamma model, is used to represent rain DSDs. Three polarimetric radar measurements—reflectivity ZH, differential reflectivity Z DR, and specific differential phase K DP—are optimally used to correct for the attenuation and retrieve DSDs by taking into account measurement error effects. Retrieval results with simulated data demonstrate that the proposed algorithm performs well. Applications to real data collected by the X-band Center for Collaborative Adaptive Sensing of the Atmosphere (CASA) radars and the C-band University of Oklahoma–Polarimetric Radar for Innovations in Meteorology and Engineering (OU-PRIME) also demonstrate the efficacy of this approach.

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Kelly Schrieber, Roland Stull, and Qing Zhang

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Onset and coverage of small cumulus clouds depend on the relative abundance of surface-layer air parcels possessing favourable buoyancy and moisture—two variables that are coupled through the surface energy budget. This abundance is described using a joint frequency distribution (JFD) as a function of virtual potential temperature θv. and height of lifting condensation level z LCL. It is shown analytically that the shape and spread of this JFD depends on die ranges of Bowen ratios and solar forcings (albedoes, cloud shading, etc.) that exist within a domain of heterogeneous land use.

To sample the character of such JFDs in the real atmosphere, a case study is presented using turbulence data gathered by aircraft flying in the surface layer of southwest France. This case study includes 4 days of clear skies during the Hydrologic Atmospheric Pilot Experiment (HAPEX) of 1986. The full flight track during HAPEX overflew a wide range of land use including evergreen forest, corn, vineyards, pastures, and irrigated fields over varied topography. The JFDs from these full tracks are found to he quite complex, being frequently multimodal with a convoluted perimeter. However. when a full track is broken into segments, each over a subdomain of quasi-homogeneous land use, the resulting segment JFDs are mono-modal with simpler topology.

Such a characterization of IFDs provides guidance toward eventual subgrid cumulus parameterization in large-scale forecast models, with associated impacts in aviation forecasting, pollutant venting and chemical reactions, vertical dispersion and turbulence modulation, and radiation balance in climate-change models.

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Maurice Danard, Qing Zhang, and John Kozlowski

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Corby et al. present a finite-difference expression for the horizontal pressure gradient force in sigma coordinates that, in a barotropic atmosphere where the temperature varies linearly with logarithm of pressure, has the same net truncation error as the centered finite-difference approximation for the isobaric geopotential gradient. The requirement that the temperature vary linearly with logarithm of pressure is imposed on analyzed isobaric heights and temperatures using a variational procedure. This reduces the errors in geostrophic winds computed using sigma coordinates. Initial surface pressures and temperatures are calculated in a mesoscale model, assuming the temperature varies linearly with logarithm of pressure and linearly with height. The first method (linear variation with logarithm of pressure) results in smaller errors in computed initial surface geostrophic winds. The structure of a sigma coordinate model is described in which temperature varies linearly with logarithms of pressure. Analytical expressions are derived for the truncation error in the case of temperature varying linearly with height. It is concluded that if a linear variation of temperature with logarithm of pressure is imposed and Corby et al.'s finite difference is employed, then truncation error in the horizontal pressure gradient force will be reduced.

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Nanxuan Jiang, Qing Yan, Zhiqing Xu, Jian Shi, and Ran Zhang

Abstract

To advance our knowledge of the response of midlatitude westerlies to various external forcings, we investigate the meridional shift of midlatitude westerlies over arid central Asia (ACA) during the past 21 000 years, which experienced more varied forcings than the present day based on a set of transient simulations. Our results suggest that the evolution of midlatitude westerlies over ACA and driving factors vary with time and across seasons. In spring, the location of midlatitude westerlies over ACA oscillates largely during the last deglaciation, driven by meltwater fluxes and continental ice sheets, and then shows a long-term equatorward shift during the Holocene controlled by orbital insolation. In summer, orbital insolation dominates the meridional shift of midlatitude westerlies, with poleward and equatorward migration during the last deglaciation and the Holocene, respectively. From a thermodynamic perspective, variations in zonal winds are linked with the meridional temperature gradient based on the thermal wind relationship. From a dynamic perspective, variations in midlatitude westerlies are mainly induced by anomalous sea surface temperatures over the Indian Ocean through the Matsuno–Gill response and over the North Atlantic Ocean by the propagation of Rossby waves, or both, but their relative importance varies across forcings. Additionally, the modeled meridional shift of midlatitude westerlies is broadly consistent with geological evidence, although model–data discrepancies still exist. Overall, our study provides a possible scenario for a meridional shift of midlatitude westerlies over ACA in response to various external forcings during the past 21 000 years and highlights important roles of both the Indian Ocean and the North Atlantic Ocean in regulating Asian westerlies, which may shed light on the behavior of westerlies in the future.

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