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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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Michael P. Dudek, Xin-Zhong Liang, and Wei-Chyung Wang

Abstract

The scale dependence of cloud-radiation interaction associated with the parameterizations for fractional cloudiness and radiation used in a global climate model is studied by examining the averages, for different spatial scales, of detailed structure of cloudiness and radiation simulated from a regional climate model that incorporates these parameterizations. The regional model simulation is conducted over an area about (360 km)2 located on the southern Great Plains for the period 10–17 April 1994 during which both satellite and surface measurements of radiation fluxes and clouds are available from the Intensive Observing Period of the Atmospheric Radiation Measurement program. The area corresponds approximately to one gridpoint size of a global climate model with horizontal resolution T31.

The regional model simulates well the overall cloud and radiation temporal features when averaged over the entire region. However, specific biases exist in the spatial patterns such as the high clouds, the TOA upwelling solar radiation under cloudy conditions, and the net longwave surface flux under clear conditions at night. The cloud and radiation parameterizations are found to be sensitive to the spatial scale of the computation. The diagnosed total cloudiness shows a strong horizontal resolution dependence that leads to large changes in the surface and TOA radiation budgets. An additional experiment, in which the diagnosed cloud at each level is held constant while the radiation parameterization is recalculated, still produces a substantial sensitivity to spatial scale in the calculated radiation quantities. This is because the nature of the cloud vertical overlapping assumption changes as the horizontal scale of the computation varies.

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Wei Wang, Ying-Hwa Kuo, and Thomas T. Warner

Abstract

An analysis of a diabatically driven and long-lived midtropospheric vortex in the lee of the Tibetan Plateau during 24–27 June 1987 is presented. The large-scale conditions were characterized by the westward expansion of the 500-mb western Pacific subtropical high and the amplification of a trough in the lee of the plateau. Embedded within the lee trough, three mesoscale convective systems (MCSs) developed. A vortex emerged following the dissipation of one MCS, with its strongest circulation located in the 400–500-mb layer. Low-level warm advection, and surface sensible and latent heating contributed to the convective initiation. Weak wind and weak ambient vorticity conditions inside the lee trough provided a favorable environment for these MCSs and the vortex to develop and evolve. The organized vortex circulation featured a coherent core of cyclonic vorticity extending from near the surface to 300 mb, with virtually no vertical tilt. The air in the vicinity of the vortex was very moist, and the temperature profile was nearly moist adiabatic, with moderate convective available potential energy. The wind near the vortex center was weak, with little vertical shear. These characteristics are similar to those of mesoscale convectively generated vortices found in the United States. The vortex circulation persisted in the same area for 3 days. The steadiness of large-scale circulation in the region, that is, the presence of the stationary lee trough and a geopotential ridge that developed to the east of the trough, likely contributed to the persistence of the vortex over the same area.

Potential vorticity (PV) diagnosis suggests that the significant increase in the relative vorticity associated with the vortex development was largely a result of diabatic heating associated with the MCS. An elevated PV anomaly was found near 400 mb in situ after the dissipation of the MCS. The PV anomaly was distinctly separated from those associated with baroclinic disturbances located to the north of the Tibetan Plateau, and the region of the PV anomaly was nearly saturated (with relative humidity exceeding 80%). Further support for this hypothesis was provided by the estimated heating profile and the rate of PV generation due to diabatic heating. The heating peaked at 300 mb, while the diabatic generation of PV reached its maximum at 500 mb. The preexisting ambient vorticity contributed about 20% to the total PV generation near the mature stage of the MCS.

The vortex was also associated with heavy precipitation over the western Sichuan Basin of China. The persistent, heavy rainfall took place in the southeasterly flow associated with the vortex circulation, about 300 km north of the vortex center.

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Changlin Chen, Guihua Wang, Shang-Ping Xie, and Wei Liu

ABSTRACT

The Kuroshio and Gulf Stream, the subtropical western boundary currents of the North Pacific and North Atlantic, play important roles in meridional heat transport and ocean–atmosphere interaction processes. Using a multimodel ensemble of future projections, we show that a warmer climate intensifies the upper-layer Kuroshio, in contrast to the previously documented slowdown of the Gulf Stream. Our ocean general circulation model experiments show that the sea surface warming, not the wind change, is the dominant forcing that causes the upper-layer Kuroshio to intensify in a warming climate. Forced by the sea surface warming, ocean subduction and advection processes result in a stronger warming to the east of the Kuroshio than to the west, which increases the isopycnal slope across the Kuroshio, and hence intensifies the Kuroshio. In the North Atlantic, the Gulf Stream slows down as part of the Atlantic meridional overturning circulation (AMOC) response to surface salinity decrease in the high latitudes under global warming. The distinct responses of the Gulf Stream and Kuroshio to climate warming are accompanied by different regional patterns of sea level rise. While the sea level rise accelerates along the northeastern U.S. coast as the AMOC weakens, it remains close to the global mean rate along the East Asian coast as the intensifying Kuroshio is associated with the enhanced sea level rise offshore in the North Pacific subtropical gyre.

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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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Chuan Jiang Huang, Wei Wang, and Rui Xin Huang

Abstract

The circulation in the equatorial Pacific Ocean is studied in a series of numerical experiments based on an isopycnal coordinate model. The model is subject to monthly mean climatology of wind stress and surface thermohaline forcing. In response to decadal variability in the diapycnal mixing coefficient, sea surface temperature and other properties of the circulation system oscillate periodically. The strongest sea surface temperature anomaly appears in the geographic location of Niño-3 region with the amplitude on the order of 0.5°C, if the model is subject to a 30-yr sinusoidal oscillation in diapycnal mixing coefficient that varies between 0.03 × 10−4 and 0.27 × 10−4 m2 s−1. Changes in diapycnal mixing coefficient of this amplitude are within the bulk range consistent with the external mechanical energy input in the global ocean, especially when considering the great changes of tropical cyclones during the past decades. Thus, time-varying diapycnal mixing associated with changes in wind energy input into the ocean may play a nonnegligible role in decadal climate variability in the equatorial circulation and climate.

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Meilin Zhu, Tandong Yao, Wei Yang, Baiqing Xu, and Xiaojun Wang

Abstract

Accurate evaluations of incoming longwave radiation (L in) parameterization have practical implications for glacier and river runoff changes in high-mountain regions of the Tibetan Plateau (TP). To identify potential means of accurately predicting spatiotemporal variations in L in, 13 clear-sky parameterizations combined with 10 cloud corrections for all-sky atmospheric emissivity were evaluated at five sites in high-mountain regions of the TP through temporal and spatial parameter transfer tests. Most locally calibrated parameterizations for clear-sky and all-sky conditions performed well when applied to the calibration site. The best parameterization at five sites is Dilley and O’Brien’s A model combined with Sicart et al.’s A for cloud-correction-incorporated relative humidity. The performance of parameter transferability in time is better than that in space for the same all-sky parameterizations. The performance of parameter transferability in space presents spatial discrepancies. In addition, all all-sky parameterizations show a decrease in performance with increasing altitude regardless of whether the parameters of all-sky parameterizations were recalibrated by local conditions or transferred from other study sites. This may be attributable to the difference between screen-level air temperature and the effective atmospheric boundary layer temperature and to different cloud-base heights. Nevertheless, such worse performance at higher altitudes is likely to change because of terrain, underlying surfaces, and wind systems, among other factors. The study also describes possible spatial characteristics of L in and its driving factors by reviewing the few studies about L in for the mountain regions of the TP.

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Christopher Davis, Wei Wang, Jimy Dudhia, and Ryan Torn

Abstract

The representation of tropical cyclone track, intensity, and structure in a set of 69 parallel forecasts performed at each of two horizontal grid increments with the Advanced Research Hurricane (AHW) component of the Weather and Research and Forecasting Model (WRF) is evaluated. These forecasts covered 10 Atlantic tropical cyclones: 6 from the 2005 season and 4 from 2007. The forecasts were integrated from identical initial conditions produced by a cycling ensemble Kalman filter. The high-resolution forecasts used moving, storm-centered nests of 4- and 1.33-km grid spacing. The coarse-resolution forecasts consisted of a single 12-km domain (which was identical to the outer domain in the forecasts with nests). Forecasts were evaluated out to 120 h. Novel verification techniques were developed to evaluate forecasts of wind radii and the degree of storm asymmetry. Intensity (maximum wind) and rapid intensification, as well as wind radii, were all predicted more accurately with increased horizontal resolution. These results were deemed to be statistically significant based on the application of bootstrap confidence intervals. No statistically significant differences emerged regarding storm position errors between the two forecasts. Coarse-resolution forecasts tended to overpredict the extent of winds compared to high-resolution forecasts. The asymmetry of gale-force winds was better predicted in the coarser-resolution simulation, but asymmetry of hurricane-force winds was predicted better at high resolution. The skill of the wind radii forecasts decayed gradually over 120 h, suggesting a synoptic-scale control of the predictability of outer winds.

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Xinrong Wu, Wei Li, Guijun Han, Shaoqing Zhang, and Xidong Wang

Abstract

While fixed covariance localization can greatly increase the reliability of the background error covariance in filtering by suppressing the long-distance spurious correlations evaluated by a finite ensemble, it may degrade the assimilation quality in an ensemble Kalman filter (EnKF) as a result of restricted longwave information. Tuning an optimal cutoff distance is usually very expensive and time consuming, especially for a general circulation model (GCM). Here the authors present an approach to compensate the demerit in fixed localization. At each analysis step, after the standard EnKF is done, a multiple-scale analysis technique is used to extract longwave information from the observational residual (referred to the EnKF ensemble mean). Within a biased twin-experiment framework consisting of a global barotropical spectral model and an idealized observing system, the performance of the new method is examined. Compared to a standard EnKF, the hybrid method is superior when an overly small/large cutoff distance is used, and it has less dependence on cutoff distance. The new scheme is also able to improve short-term weather forecasts, especially when an overly large cutoff distance is used. Sensitivity studies show that caution should be taken when the new scheme is applied to a dense observing system with an overly small cutoff distance in filtering. In addition, the new scheme has a nearly equivalent computational cost to the standard EnKF; thus, it is particularly suitable for GCM applications.

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Yaru Guo, Yuanlong Li, Fan Wang, Yuntao Wei, and Zengrui Rong

Abstract

A high-resolution (3–8 km) regional oceanic general circulation model is utilized to understand the sea surface temperature (SST) variability of Ningaloo Niño in the southeast Indian Ocean (SEIO). The model reproduces eight Ningaloo Niño events with good fidelity and reveals complicated spatial structures. Mesoscale noises are seen in the warming signature and confirmed by satellite microwave SST data. Model experiments are carried out to quantitatively evaluate the effects of key processes. The results reveal that the surface turbulent heat flux (primarily latent heat flux) is the most important process (contribution > 68%) in driving and damping the SST warming for most events, while the roles of the Indonesian Throughflow (~15%) and local wind forcing are secondary. A suitable air temperature warming is essential to reproducing the reduced surface latent heat loss during the growth of SST warming (~66%), whereas the effect of the increased air humidity is negligibly small (1%). The established SST warming in the mature phase causes increased latent heat loss that initiates the decay of warming. A 20-member ensemble simulation is performed for the 2010/11 super Ningaloo Niño, which confirms the strong influence of ocean internal processes in the redistribution of SST warming signatures. Oceanic eddies can dramatically modulate the magnitudes of local SST warming, particularly in offshore areas where the “signal-to-noise” ratio is low, raising a caution for evaluating the predictability of Ningaloo Niño and its environmental consequences.

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