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Michael G. Bosilovich and Jiun-Dar Chern

Abstract

An atmospheric general circulation model simulation for 1948–97 of the water budgets for the MacKenzie, Mississippi, and Amazon River basins is presented. In addition to the water budget, passive tracers are included to identify the geographic sources of water for the basins, and the analysis focuses on the mechanisms contributing to precipitation recycling in each basin. While each basin’s precipitation recycling has a strong dependency on evaporation during the mean annual cycle, the interannual variability of the recycling shows important relationships with the atmospheric circulation. The MacKenzie River basin recycling has only a weak interannual correspondence with evaporation, where the variations in zonal moisture transport from the Pacific Ocean can affect the basin water cycle. On the other hand, the Mississippi River basin precipitation and recycling have strong interannual correlation on evaporation. The evaporation is related to the moist and shallow planetary boundary layer that provides moisture for convection at the cloud base. At global scales, high precipitation recycling is also found to be partly correlated to warm SSTs in the tropical Pacific Ocean. The Amazon River basin evaporation exhibits small interannual variations, so the interannual variations of precipitation recycling are related to atmospheric moisture transport from the tropical South Atlantic Ocean. Increasing SSTs over the 50-yr period are causing increased easterly transport across the basin. As moisture transport increases, the Amazon precipitation recycling decreases (without real-time varying vegetation changes). In addition, precipitation recycling from a bulk diagnostic method is compared to the passive tracer method used in the analysis. While the mean values of the different recycling methods are different, the interannual variations are comparable between each method. The methods also exhibit similar relationships to the terms of the basin-scale water budgets.

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Wen-Yih Sun and Jiun-Dar Chern

Abstract

Lee vortices have been frequently observed in the wake of mesoscale mountains under a low Froude number flow regime. During the Taiwan Area Mesoscale Experiment (TAMEX), a cyclonic vortex was observed to the Ice of Taiwan by a P-3 aircraft. In this paper a numerical simulation is carried out to study this event. It is shown that the numerical results are capable of recapturing the detailed features as observed by airplane and surface analysis. The simulated surface pressure, wind field, and Ice vortex are in good agreement with observations. The diurnal oscillation of cloudiness and precipitation in Taiwan is also consistent with the observations under undisturbed conditions during the TAMEX period.

Under a prevailing southwesterly-to-westerly summer monsson flow, numerical results demonstrate that the observed cyclonic vortex initially develops to the southeast of Taiwan after sunset, then drifts northeastward. The diurnal forcing not only generates land/sea breezes but also controls the vortex shedding. A sensitivity test without diurnal forcing indicates that the intrinsic vortex shedding period of Taiwan island is about 54 hours under the same initial condition. Due to the influence of diurnal forcing, however, the vortex shedding period

becomes 24 hours with the cyclonic vortex forming at 1700 LST and the anticyclonic vortex forming at 0500 LST. Moreover, the diurnal effect also influences the propagation of vortices, especially near the surface.

A vorticity budget study is also carded out to compare with the idealized case. The results show that the tilting term is important to generate vorticity over the Central Mountain Range. On the other hand, the stretching and advection terms are responsible for carrying and enhancing the vorticity to the lee side and are directly related to the initial development of the vortex. Moreover, each term in the vorticity budget is quite complicated, due to the existence of clouds, boundary-layer forcing, and the circulation of land/sea breezes.

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Wen-Yih Sun and Jiun-Dar Chern

Abstract

The Purdue Mesoscale Model (PMM) is applied to study the flow past large idealized mountains under a low Froude number. Results show that for Reynolds numbers in the range of 4 < Re < 1000, as long as the flow is symmetric to the central line of a symmetric mountain, two vortices remain stably attached to the mountain. For Re≤100, the size of the attached vortices after 120 hours of integration increases linearly with the increase of Re, but the size decreases slightly with Re for Re > 100.

Results also show that small perturbations in the oncoming wind, the inclination of the oncoming wind and major axis of the mountain, the mountain shape, and the Coriolis force all can contribute to atmospheric vortex shedding. The Reynolds number is not a good indicator of whether a vortex will stay or break away from the mountain in the atmosphere.

When the earth's rotation is included, the simulated pressure field and wind increase considerably on the left-hand side (facing downstream) of the mountain, which is quite different from that of an irrotational flow, although the pattern of vortex shedding is similar. It is also found that the Reynolds number and β effect can change the propagating speed but not the period of vortex shedding. On the other hand, the shape and size of the mountain and asymmetry of the oncoming wind can strongly influence the character of vortex shedding.

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Toshi Matsui, Jiun-Dar Chern, Wei-Kuo Tao, Stephen Lang, Masaki Satoh, Tempei Hashino, and Takuji Kubota

Abstract

A 14-yr climatology of Tropical Rainfall Measuring Mission (TRMM) collocated multisensor signal statistics reveals a distinct land–ocean contrast as well as geographical variability of precipitation type, intensity, and microphysics. Microphysics information inferred from the TRMM Precipitation Radar and Microwave Imager show a large land–ocean contrast for the deep category, suggesting continental convective vigor. Over land, TRMM shows higher echo-top heights and larger maximum echoes, suggesting taller storms and more intense precipitation, as well as larger microwave scattering, suggesting the presence of more/larger frozen convective hydrometeors. This strong land–ocean contrast in deep convection is invariant over seasonal and multiyear time scales. Consequently, relatively short-term simulations from two global storm-resolving models can be evaluated in terms of their land–ocean statistics using the TRMM Triple-Sensor Three-Step Evaluation Framework via a satellite simulator. The models evaluated are the NASA Multiscale Modeling Framework (MMF) and the Nonhydrostatic Icosahedral Cloud Atmospheric Model (NICAM). While both simulations can represent convective land–ocean contrasts in warm precipitation to some extent, near-surface conditions over land are relatively moister in NICAM than MMF, which appears to be the key driver in the divergent warm precipitation results between the two models. Both the MMF and NICAM produced similar frequencies of large CAPE between land and ocean. The dry MMF boundary layer enhanced microwave scattering signals over land, but only NICAM had an enhanced deep convection frequency over land. Neither model could reproduce a realistic land–ocean contrast in deep convective precipitation microphysics. A realistic contrast between land and ocean remains an issue in global storm-resolving modeling.

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Wen-Yih Sun, Jiun-Dar Chern, Ching-Chi Wu, and Wu-Ron Hsu

Abstract

Mesoscale circulation around Taiwan and the surrounding area has been investigated using the Purdue mesoscale model. The numerical results generated in an inviscid atmosphere show:

(a) A cyclonic vortex forms in the southeast and a slightly weaker anticyclonic vortex forms in the northeast of Taiwan uner a westerly or southwesterly wind. Subsidence warming also generates a relative low pressure on the southeastern coast.

(b) A low pressure associated with a cyclonic flow forms in the northwest and a slightly weaker anticyclonic flow forms in the southwest of Taiwan under an easterly mean flow. The easterly wind tends to turn northeasterly over the Taiwan Strait, with a stronger wind speed, due to the blocking effects of the mountains in Taiwan and along the Chinese coast.

(c) Under the existence of an easterly surface wind with a reverse shear, the horizontal temperature advection is not important in the formation of low pressure on the leeside, due to the small length scale of the island of Taiwan.

(d) The Froude number is an important parameter to estimate the blocking effect of the central mountain range; however, the flow pattern also depends on other parameters, such as the shape of mountains, the terrain of the surrounding areas, and other meteorological parameters.

(e) The budget study of the vorticity equation shows that stretching, tilting, and friction are important for the formation of lee vortices in our results.

These results may provide some physical explanations for the observed mesolow and cyclonic flow in the southeast and northwest of Taiwan during the late spring and early summer—a transitional period of the winter and summer monsoons in Taiwan.

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Stephen E. Lang, Wei-Kuo Tao, Jiun-Dar Chern, Di Wu, and Xiaowen Li

Abstract

Current cloud microphysical schemes used in cloud and mesoscale models range from simple one-moment to multimoment, multiclass to explicit bin schemes. This study details the benefits of adding a fourth ice class (frozen drops/hail) to an already improved single-moment three-class ice (cloud ice, snow, graupel) bulk microphysics scheme developed for the Goddard Cumulus Ensemble model. Besides the addition and modification of several hail processes from a bulk three-class hail scheme, further modifications were made to the three-ice processes, including allowing greater ice supersaturation and mitigating spurious evaporation/sublimation in the saturation adjustment scheme, allowing graupel/hail to transition to snow via vapor growth and hail to transition to graupel via riming, wet graupel to become hail, and the inclusion of a rain evaporation correction and vapor diffusivity factor. The improved three-ice snow/graupel size-mapping schemes were adjusted to be more stable at higher mixing ratios and to increase the aggregation effect for snow. A snow density mapping was also added.

The new scheme was applied to an intense continental squall line and a moderate, loosely organized continental case using three different hail intercepts. Peak simulated reflectivities agree well with radar for both the intense and moderate cases and were superior to earlier three-ice versions when using a moderate and large intercept for hail, respectively. Simulated reflectivity distributions versus height were also improved versus radar in both cases compared to earlier three-ice versions. The bin-based rain evaporation correction affected the squall line more but overall the agreement among the reflectivity distributions was unchanged. The new scheme also improved the simulated surface rain-rate histograms.

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Michael G. Bosilovich, Jiun-Dar Chern, David Mocko, Franklin R. Robertson, and Arlindo M. da Silva

Abstract

The assimilation of observations in reanalyses incurs the potential for the physical terms of budgets to be balanced by a term relating the fit of the observations relative to a forecast first guess analysis. This may indicate a limitation in the physical processes of the background model or perhaps assimilating data from an inconsistent observing system. In the MERRA reanalysis, an area of long-term moisture flux divergence over land has been identified over the central United States. Here, the water vapor budget is evaluated in this region, taking advantage of two unique features of the MERRA diagnostic output: 1) a closed water budget that includes the analysis increment and 2) a gridded diagnostic output dataset of the assimilated observations and their innovations (e.g., forecast departures).

In the central United States, an anomaly occurs where the analysis adds water to the region, while precipitation decreases and moisture flux divergence increases. This is related more to a change in the observing system than to a deficiency in the model physical processes. MERRA’s Gridded Innovations and Observations (GIO) data narrow the observations that influence this feature to the ATOVS and Aqua satellites during the 0600 and 1800 UTC analysis cycles, when radiosonde information is not prevalent. Observing system experiments further narrow the instruments that affect the anomalous feature to AMSU-A (mainly window channels) and Atmospheric Infrared Sounder (AIRS). This effort also shows the complexities of the observing system and the reactions of the regional water budgets in reanalyses to the assimilated observations.

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Wei-Kuo Tao, Jiun-Dar Chern, Robert Atlas, David Randall, Marat Khairoutdinov, Jui-Lin Li, Duane E. Waliser, Arthur Hou, Xin Lin, Christa Peters-Lidard, William Lau, Jonathan Jiang, and Joanne Simpson

A multiscale modeling framework (MMF), which replaces the conventional cloud parameterizations with a cloud-resolving model (CRM) in each grid column of a GCM, constitutes a new and promising approach for climate modeling. The MMF can provide for global coverage and two-way interactions between the CRMs and their parent GCM. The CRM allows for explicit simulation of cloud processes and their interactions with radiation and surface processes, and the GCM allows for global coverage.

A new MMF has been developed that is based on the NASA Goddard Space Flight Center (GSFC) finite-volume GCM (fvGCM) and the Goddard Cumulus Ensemble (GCE) model. This Goddard MMF produces many features that are similar to another MMF that was developed at Colorado State University (CSU), such as an improved surface precipitation pattern, better cloudiness, improved diurnal variability over both oceans and continents, and a stronger propagating Madden-Julian oscillation (MJO) compared to their parent GCMs using traditional cloud parameterizations. Both MMFs also produce a large and positive precipitation bias in the Indian Ocean and western Pacific during the Northern Hemisphere summer. However, there are also notable differences between the two MMFs. For example, the CSU MMF simulates less rainfall over land than its parent GCM. This is why the CSU MMF simulated less overall global rainfall than its parent GCM. The Goddard MMF simulates more global rainfall than its parent GCM because of the high contribution from the oceanic component. A number of critical issues (i.e., the CRM's physical processes and its configuration) involving the Goddard MMF are discussed in this paper.

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