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Song-You Hong and Eugenia Kalnay

Abstract

This study presents results from mechanistic experiments to clarify the origin and maintenance of the Oklahoma–Texas (OK–TX) drought of the 1998 summer, using the National Centers for Environmental Prediction (NCEP) global and regional models. In association with this unprecedented drought, three major mechanisms that can produce extended atmospheric anomalies have been identified: (i) sea surface temperature (SST) anomalies, (ii) soil moisture anomalies, and (iii) atmospheric initial conditions favorable to such a climate extreme even in the absence of surface forcing (i.e., internal forcing).

The authors found that the SST anomalies during April–May 1998 established the large-scale conditions for the drought. However, the warm El Niño–Southern Oscillation (ENSO) SST anomalies over the central and eastern tropical Pacific alone did not play a major role in initiating the drought. The internal structure of atmospheric conditions played as significant a role as the SST anomalies over the globe. In June 1998, soil moisture anomalies started to play an important role in maintaining the drought, and the regional positive feedback associated with lower evaporation/lower precipitation explained most of the water deficit in July. After July, synoptic-scale disturbances overwhelmed the impact of dry soil moisture near the Gulf of Mexico states where above-normal precipitation occurred, but the regional feedback was still prominent over the OK–TX region, where the drought persisted until early October.

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Song-You Hong and Jimy Dudhia

No abstract available.

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Song-You Hong and Ants Leetmaa

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In this study, the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) has been evaluated as a means of enhancing the depiction of regional details beyond that which is capable in low-resolution global models. Three-month-long simulations driven by the NCEP–National Center for Atmospheric Research 40-yr reanalysis data are conducted with a horizontal resolution of about 50 km over the United States, for the two winters and summers. The selected winter cases are December–February (DJF) 1991/92 (warm eastern Pacific SST anomalies) and DJF 1992/93 (normal eastern Pacific SST anomalies). Summer cases are May–July (MJJ) 1988 (a drought in the Great Plains) and MJJ 1993 (a flooding).

Overall, the results from the model are very satisfactory in terms of the precipitation distribution for different seasons as well as the representation of large-scale features. Evaluation of simulated large-scale features reveals that the model does not exhibit a discernible synoptic-scale drift during the 3-month integration period, irrespective of the seasons. Surprisingly, the model simulation is found to correct some biases in the large-scale fields that exist in the reanalysis data. This bias reduction is attributed to the improved depiction of physical processes within the RSM. This finding indicates that one should take special care in the interpretation and validation of simulated results against the analyzed data.

Evaluation of the RSM simulated precipitation for the winter and summer cases generally agrees with results obtained from previous studies. For instance, the skill for simulated precipitation in the winter cases exceeds that of the summer cases by a factor of 2. Comparison of simulated precipitation with observations reveals the 3-month-long RSM simulated precipitation to be more skillful than that obtained from the reanalysis data (the 6-h forecast from the data assimilation system). In addition to seasonal variations in precipitation, daily variation in the simulated precipitation is quite good. However, detailed analysis points to the need for further RSM development, particularly in physics. In the summer cases the grid-resolvable precipitation physics simulate excessive precipitation over the northern United States. A more serious problem is found in the diurnal cycle of the simulation precipitation, in that the model initiates convection too early. Despite these deficiencies, it is concluded that the NCEP RSM is a very useful tool for regional climate studies.

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Song-You Hong, Yign Noh, and Jimy Dudhia

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This paper proposes a revised vertical diffusion package with a nonlocal turbulent mixing coefficient in the planetary boundary layer (PBL). Based on the study of Noh et al. and accumulated results of the behavior of the Hong and Pan algorithm, a revised vertical diffusion algorithm that is suitable for weather forecasting and climate prediction models is developed. The major ingredient of the revision is the inclusion of an explicit treatment of entrainment processes at the top of the PBL. The new diffusion package is called the Yonsei University PBL (YSU PBL). In a one-dimensional offline test framework, the revised scheme is found to improve several features compared with the Hong and Pan implementation. The YSU PBL increases boundary layer mixing in the thermally induced free convection regime and decreases it in the mechanically induced forced convection regime, which alleviates the well-known problems in the Medium-Range Forecast (MRF) PBL. Excessive mixing in the mixed layer in the presence of strong winds is resolved. Overly rapid growth of the PBL in the case of the Hong and Pan is also rectified. The scheme has been successfully implemented in the Weather Research and Forecast model producing a more realistic structure of the PBL and its development. In a case study of a frontal tornado outbreak, it is found that some systematic biases of the large-scale features such as an afternoon cold bias at 850 hPa in the MRF PBL are resolved. Consequently, the new scheme does a better job in reproducing the convective inhibition. Because the convective inhibition is accurately predicted, widespread light precipitation ahead of a front, in the case of the MRF PBL, is reduced. In the frontal region, the YSU PBL scheme improves some characteristics, such as a double line of intense convection. This is because the boundary layer from the YSU PBL scheme remains less diluted by entrainment leaving more fuel for severe convection when the front triggers it.

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Jihyeon Jang and Song-You Hong

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This study examines the characteristics of a nonhydrostatic dynamical core compared to a corresponding hydrostatic dynamical core in the Regional Model Program (RMP) of the Global/Regional Integrated Model system (GRIMs), a spectral model for regional forecasts, focusing on simulated precipitation over Korea. This kind of comparison is also executed in the Weather Research and Forecasting (WRF) finite-difference model with the same physics package used in the RMP. Overall, it is found that the nonhydrostatic dynamical core experiment accurately reproduces the heavy rainfall near Seoul, South Korea, on a 3-km grid, relative to the results from the hydrostatic dynamical core in both models. However, the characteristics of nonhydrostatic effects on the simulated precipitation differ between the RMP and WRF Model. The RMP with the nonhydrostatic dynamical core improves the local maximum, which is exaggerated in the hydrostatic simulation. The hydrostatic simulation of the WRF Model displaces the major precipitation area toward the mountainous region along the east coast of the peninsula, which is shifted into the observed area in the nonhydrostatic simulation. In the simulation of a summer monsoonal rainfall, these nonhydrostatic effects are negligible in the RMP, but the simulated monsoonal rainfall is still influenced by the dynamical core in the WRF Model even at a 27-km grid spacing. One of the reasons for the smaller dynamical core effect in the RMP seems to be the relatively strong horizontal diffusion, resulting in a smaller grid size of the hydrostatic limit.

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Yoo-Bin Yhang and Song-You Hong

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This paper documents the sensitivity of the modeled evolution of the East Asian summer monsoon (EASM) to physical parameterization using the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM). To this end, perfect boundary condition experiments driven by analysis data are designed for August 2003 to investigate the individual role of the surface processes, boundary layer, and convection parameterization on the simulated monsoon. Also, 10-yr June–August (JJA) simulations from 1996 to 2005 are performed to evaluate the overall impacts of these revisions on the simulated EASM climatology.

The one-month simulation for August 2003 reveals that the experiment with a realistic distribution of land use conditions and vegetation and smaller thermal roughness length simulates higher temperature and geopotential height. On the other hand, in the experiment with an improved boundary layer scheme, the rainfall amount is slightly decreased due to reduced vertical mixing. The simulation with revised subgrid-scale processes in the cumulus parameterization scheme reproduces a rainband over the subtropics, which is weakly simulated by the default package. The overall large-scale distribution from the experiment, which includes all three revised physics processes, shows the same direction as that of the revised convection run in the middle and upper troposphere, but is improved further when other newly enhanced processes are combined. These improvements are also achieved in a 10-yr summer simulation. It is distinct that the revised physics package improves the large-scale patterns by strengthening the intensity of the North Pacific high and reducing the intensity of the lower-level jet, which are critical components in the EASM. The general patterns of the interannual and intraseasonal variation of precipitation are also improved, in particular, over land.

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Young Cheol Kwon and Song-You Hong

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A method that enables a mass-flux cumulus parameterization scheme (CPS) to work seamlessly in various model grids across CPS gray-zone resolutions is proposed. The convective cloud-base mass flux, convective inhibition, and convective detrainment in the simplified Arakawa–Schubert (SAS) scheme are modified to be functions of the convective updraft fraction. The combination of two updraft fractions is used to modulate the cloud-base mass flux; the first one depends on the horizontal grid space and the other is a function of the grid-scale and convective vertical velocity. The convective inhibition and detrainment of hydrometeors are also modified to be a function of the grid-size-dependent convective updraft fraction.

A set of sensitivity experiments with the Weather Research and Forecasting (WRF) Model is conducted for a heavy rainfall case over South Korea. The results show that the revised SAS CPS outperforms the original SAS. At 3 and 1 km, the precipitation core over South Korea is well reproduced by the experiments with the revised SAS scheme. On the contrary, the simulated precipitation is widespread in the case of the original SAS experiment and there are multiple spurious cores when the CPS is removed at those resolutions. The modified mass flux at the cloud base is found to play a major role in organizing the grid-scale precipitation at the convective core. A 1-month simulation at 3 km confirms that the revised scheme produces slightly better summer monsoonal precipitation results as compared to the typical model setup without CPS.

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Hann-Ming Henry Juang and Song-You Hong

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This paper evaluates the performance of the National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) based on the sensitivities of different model domain sizes and horizontal resolutions. The perturbation method and the spectral computation in the NCEP RSM construct the nesting strategy as a “domain nesting” in physical space as well as a “spectral nesting” in spectral space, instead of the conventional “lateral boundary nesting” as used in most regional models. The NCEP RSM has the same model structure, dynamics, and physics as its outer coarse-resolution global model, and it also has a terrain blending along the lateral boundary at the initial time. Both together result in a smooth lateral boundary behavior through one-way nesting. An optimal lateral boundary relaxation reduces the influence of lateral boundary error and generates more areas with small-scale features. The treatment of numerical stabilities, such as a semi-implicit time scheme, time filter, and horizontal diffusion, are applied in perturbation without recomputing or disturbing the large-scale waves. The combination of the aforementioned methods is the uniqueness of the NCEP RSM, which demonstrates the capabilities to conserve the large-scale waves, resolve the mesoscale features, and minimize the lateral boundary errors.

A case of winter cyclogenesis with propagation of the synoptic-scale disturbances through the lateral boundaries was selected to investigate the sensitivities of the NCEP RSM based on different nesting strategies. The results from the experiment over a quarter-sphere domain with similar resolutions between RSM and T126 global model demonstrated that the domain nesting was a success, because the lateral boundary error and perturbation were negligibly small. The experiments in a 48-km resolution with different sizes of the model domain had mixed results. The continental domain had the best performance but inclined to generate erroneous large-scale waves that degraded its performance after 60 h. The results of the regional and subregional domains were proximity to their base field, T126, in terms of root-mean-square differences. They had similar mesoscale features in a 48-km horizontal resolution regardless of the different model domain sizes. The results from the experiments with nesting in different coarse grids over the radar-range domain imply that it can use either a T126 or subregional domain as its base field for similar performances. Nevertheless, more mesoscale features were obtained by the experiment with the base field at higher resolution. The results from the experiments, with 30-day integration, reveal that the performance of the experiment in the subregional domain was much closer to its base field than that in the continental domain. It indicates that the predictability of the global model is the predictability of the NCEP RSM in the regional domain; however, the regional domain could generate higher-resolution features than its base field. This successful long-range integration with the regional domain is because the lateral boundary errors are relatively small and the large-scale waves are preserved through the domain and spectral nesting.

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Jung-Eun Kim and Song-You Hong

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Numerous modeling studies have shown that soil moisture anomalies in later spring have a significant effect on the summer rainfall anomalies in North America. On the other hand, the role of soil moisture in forming monsoonal precipitation in East Asia has not been identified. This study attempts to clarify the importance of soil moisture on the summer rainfall in late spring in East Asia. The National Centers for Environmental Prediction (NCEP) Regional Spectral Model (RSM) is utilized for 3-month (June–August) simulations in 1998 (above-normal precipitation year) and 1997 (below-normal precipitation year). Initial and boundary conditions are derived from the NCEP–Department of Energy (DOE) reanalysis. The control run uses the initial soil moisture from the reanalysis, whereas it is set as a saturation and wilting point for “wet” and “dry” experiments, respectively.

The impact of soil moisture anomalies on the simulated summer rainfall in East Asia is not significant. The change in precipitation between the wet and dry experiments is about 10%. A conflict between the local feedback of soil moisture and a change in large-scale circulations associated with the summertime monsoonal circulation in East Asia can be attributed as a reason for this anomaly. It is found that enhanced (suppressed) evaporation from the soil to the atmosphere in wet (dry) initial soil moisture reduces (increases) the land–sea contrast between East Asia and the Pacific Ocean, leading to a weakened sensitivity of the monsoonal circulations to the initial soil moisture. It can be concluded that the impact of the initial soil moisture is significant on the dynamic circulation in East Asia.

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Jung-Eun Kim and Song-You Hong

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A global atmospheric analysis dataset is constructed via a spectral nudging technique. The 6-hourly National Centers for Environmental Prediction (NCEP)–Department of Energy (DOE) reanalysis from January 1979 to February 2011 is utilized to force large-scale information, whereas a higher-resolution structure is resolved by a global model with improved physics. The horizontal resolution of the downscaled data is about 100 km, twice that of the NCEP–DOE reanalysis.

A comparison of the 31-yr downscaled data with reanalysis data and observations reveals that the downscaled precipitation climatology is improved by correcting inherent biases in the lower-resolution reanalysis, and large-scale patterns are preserved. In addition, it is found that global downscaling is an efficient way to generate high-quality analysis data due to the use of a higher-resolution model with improved physics. The uniqueness of the obtained data lies in the fact that an undesirable decadal trend in the analysis due to a change in the amount of observations used in reanalysis is avoided. As such, a downscaled dataset may be used to investigate changes in the hydrological cycle and related mechanisms.

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