Evaluation of ACCMIP simulated fine-mode AOD and its implication for aerosol direct forcing

This study examines and evaluates simulated aerosol optical depth (AOD) and fine-mode AOD (fAOD) from the ACCMIP and CMIP5 global model archives. Satellite data nudged to AERONET data are used to construct reliable global observations of AOD and fAOD for validating the simulations. The difference in simulated global average AOD among models is of the order of a factor of 2, and the difference is even larger (～factor of 3) for fAOD. Compared to the observations, the models tend to underestimate AOD and fAOD significantly over eastern China. Another important discrepancy is that the models show larger fAOD over the Indus-Ganga Plain in summer than in winter, whereas the observations display an opposite feature. The models also overestimate the fAOD over the biomass burning regions of central Africa in DJF and underestimate the fAOD over the biomass burning regions of southern Africa in JJA. To evaluate the effect of the discrepancy between modeled and observed fAOD on aerosol direct radiative forcing, an offline radiation model is utilized. Comparing the model-fAODderived fine-mode forcing with the fine-mode forcing derived from the fAOD observation, the models tend to give too large (negative) value. This result implies that the calculated anthropogenic aerosol forcing in ACCMIP and CMIP5 models has a negative bias.     

A regional climate change simulation over East Asia

In this study, regional climate changes for seventy years (1980–2049) over East Asia and the Korean Peninsula are investigated using the Special Reports on Emission Scenarios (SRES) B1 scenario via a high-resolution regional climate model, and the impact of global warming on extreme climate events over the study area is investigated. According to future climate predictions for East Asia, the annual mean surface air temperature increases by 1.8°C and precipitation decreases by 0.2 mm day^?1 (2030–2049). The maximum wind intensity of tropical cyclones increases in the high wind categories, and the intra-seasonal variation of tropical cyclone occurrence changes in the western North Pacific. The predicted increase in surface air temperature results from increased longwave radiations at the surface. The predicted decrease in precipitation is caused primarily by northward shift of the monsoon rain-band due to the intensified subtropical high. In the nested higher-resolution (20 km) simulation over the Korean Peninsula, annual mean surface air temperature increases by 1.5°C and annual mean precipitation decreases by 0.2 mm day^?1. Future surface air temperature over the Korean Peninsula increases in all seasons due to surface temperature warming, which leads to changes in the length of the four seasons. Future total precipitation over the Korean Peninsula is decreased, but the intensity and occurrence of heavy precipitation events increases. The regional climate changes information from this study can be used as a fruitful reference in climate change studies over East Asia and the Korean peninsula.     

A global non-hydrostatic dynamical core using the spectral element method on a cubed-sphere grid

A new global model with a non-hydrostatic (NH) dynamical core is developed. It employs the spectral element method (SEM) in the horizontal discretization and the finite difference method (FDM) in the vertical discretization. The solver includes a time-split third-order Runge-Kutta (RK3) time-integration technique. Pursuing the quasi-uniform and pole singularity-free spherical geometry, a cubed-sphere grid is employed. To assess the performance of the developed dynamical solver, the results from a number of idealized benchmark tests for hydrostatic and non-hydrostatic flows are presented and compared. The results indicate that the non-hydrostatic dynamical solver is able to produce solutions with good accuracy and consistency comparable to reference solutions. Further evaluation of the model with a full-physics package demonstrates its capability in reproducing heavy rainfall over the Korean Peninsula, which confirms that coupling of the dynamical solver and full-physics package is robust.     

Structure of Eigenvalues in the Advection-Diffusion Equation by the Spectral Element Method on a Cubed-Sphere Grid

To fundamentally understand discretized equations and differential operators, this study investigates the eigenvalues of the spherical gradient matrix and spherical Laplacian matrix discretized by the spectral element method (SEM) on the cubed-sphere grid (CS) for solid-body rotation. The gradient matrix with prescribed wind for the solid-body rotation has small positive real numbers in the eigenvalues, which implies that the solutions of the advection equation for solid-body rotation can be unstable and distorted. However, the unstable model can be effectively controlled by the Laplacian matrix, and all eigenvalues of the Laplacian matrix are aligned along the negative part of the real axis. The maximum eigenvalues of the gradient matrix and Laplacian matrix exhibit the characteristic of a linear function of the resolution, which is important to determine the upper limit of the time-step size for stable time integration. This study suggests suitable e-folding time scale of the diffusion using the 6th order for applications of the advection-diffusion equation. Regarding the characteristics of the eigenvalues, the setting of the stable time step is discussed. A method to implement the diffusion operators in the third-order Runge-Kutta time integration scheme in the advection-diffusion model is also discussed.     

Analysis and simulation of mesoscale convective systems accompanying heavy rainfall: The goyang case

We investigated a torrential rainfall case with a daily rainfall amount of 379 mm and a maximum hourly rain rate of 77.5 mm that took place on 12 July 2006 at Goyang in the middlewestern part of the Korean Peninsula. The heavy rainfall was responsible for flash flooding and was highly localized. High-resolution Doppler radar data from 5 radar sites located over central Korea were analyzed. Numerical simulations using the Weather Research and Forecasting (WRF) model were also performed to complement the high-resolution observations and to further investigate the thermodynamic structure and development of the convective system. The grid nudging method using the Global Final (FNL) Analyses data was applied to the coarse model domain (30 km) in order to provide a more realistic and desirable initial and boundary conditions for the nested model domains (10 km, 3.3 km). The mesoscale convective system (MCS) which caused flash flooding was initiated by the strong low level jet (LLJ) at the frontal region of high equivalent potential temperature (θ_e) near the west coast over the Yellow Sea. The ascending of the warm and moist air was induced dynamically by the LLJ. The convective cells were triggered by small thermal perturbations and abruptly developed by the warm θ_e inflow. Within the MCS, several convective cells responsible for the rainfall peak at Goyang simultaneously developed with neighboring cells and interacted with each other. Moist absolutely unstable layers (MAULs) were seen at the lower troposphere with the very moist environment adding the instability for the development of the MCS.     

Recycling of suspended particulates by the interaction of sea-land breeze circulation and complex coastal terrain

Summary The dispersion of recycled particulates in the complex coastal terrain surrounding Kangnung city, Korea was investigated using a three-dimensional non-hydrostatic numerical model and lagrangian particle model (or random walk model). The results show that particulates at the surface of the city that float to the top of thermal internal boundary layer (TIBL) are then transported along the eastern slope of the mountains with the sea breeze passage and nearly reach the top of the mountains. Those particulates then disperse eastward at this upper level over the coastal sea and finally spread out over the open sea. Total suspended particulate (TSP) concentration near the surface of Kangnung city is very low. At night, synoptic scale westerly winds intensify due to the combined effect of the synoptic scale wind and land breeze descending the eastern slope of the mountains toward the coast and further seaward. This increase in speed causes development of internal gravity waves and a hydraulic jump up to a height of about 1km above the surface over the city. Particulate matter near the top of the mountains also descends the eastern slope of the mountains during the day, reaching the central city area and merges near the surface inside the nocturnal surface inversion layer (NSIL) with a maximum ground level concentration of TSP occurring at 0300 LST. Some particulates are dispersed following the propagation area of internal gravity waves and others in the NSIL are transported eastward to the coastal sea surface, aided by the land breeze. The following morning, particulates dispersed over the coastal sea from the previous night, tend to return to the coastal city of Kangnung with the sea breeze, developing a recycling process and combine with emitted surface particulates during the morning. These processes result in much higher TSP concentration. In the late morning, those particulates float to the top of the TIBL by the intrusion of the sea breeze and the ground level TSP concentration in the city subsequently decreases.     

Technical note on a track-pattern-based model for predicting seasonal tropical cyclone activity over the western North Pacific

Recently, the National Typhoon Center (NTC) at the Korea Meteorological Administration launched a track-pattern-based model that predicts the horizontal distribution of tropical cyclone (TC) track density from June to October. This model is the first approach to target seasonal TC track clusters covering the entire western North Pacific (WNP) basin, and may represent a milestone for seasonal TC forecasting, using a simple statistical method that can be applied at weather operation centers. In this note, we describe the procedure of the track-pattern-based model with brief technical background to provide practical information on the use and operation of the model. The model comprises three major steps. First, long-term data of WNP TC tracks reveal seven climatological track clusters. Second, the TC counts for each cluster are predicted using a hybrid statistical-dynamical method, using the seasonal prediction of large-scale environments. Third, the final forecast map of track density is constructed by merging the spatial probabilities of the seven clusters and applying necessary bias corrections. Although the model is developed to issue the seasonal forecast in mid-May, it can be applied to alternative dates and target seasons following the procedure described in this note. Work continues on establishing an automatic system for this model at the NTC.     

Effects of climate change on the thermal structure of lakes in the Asian Monsoon Area

This research investigates the effect of climate change on the thermal structure of lakes in response to watershed hydrology. We applied a hydrodynamic water quality model coupled to a hydrological model with a future climate scenario projected by a GCM A2 emission scenario to the Yongdam Reservoir, South Korea. In the climate change scenario, the temperature will increase by 2.1°C and 4.2°C and the precipitation will increase by 178.4?mm and 464.4?mm by the 2050 and 2090, respectively, based on 2010. The pattern changes of precipitation and temperature increase due to climate change modify the hydrology of the watershed. The hydrological model results indicate that they increase both surface runoff itself and temperature. The reservoir model simulation with the hydrological model results showed that increasing air temperature is related to higher surface water temperature. Surface water temperature is expected to increase by about 1.2°C and 2.2°C from the 2050 and 2090, respectively, based on the 2010 results. The simulation results of the effects of climate warming on the thermal structure of the Asian Monsoon Area Lake showed consistent results with those of previous studies in terms of greater temperature increases in the epilimnion than in the hypolimnion, increased thermal stratification, and decreasing thermocline depths during the summer and fall. From this study, it was concluded that the hydrodynamic water quality model coupled to the hydrological model could successfully simulate the variability of the epilimnetic temperature, changed depth and magnitude of the thermocline and the changed duration of summer stratification.     

Antibiotics in the Hong Kong metropolitan area: Ubiquitous distribution and fate in Victoria Harbour

We investigated the presence and fate of 16 antibiotics belonging to seven groups (β-lactams, fluoroquinolones, macrolides, sulfonamides, tetracyclines, trimethoprim and amphenicols) in effluents of sewage plants and receiving waters in Hong Kong. Cefalexin, amoxicillin, ofloxacin and erythromycin-H₂O were ubiquitous in sea water throughout Victoria Harbour, indicating continuous discharge to the environment. This is one of the few studies reporting the frequent occurrence of cefalexin and amoxicillin in sewage effluents and sea water (170-5070 and 64-1670 ng/L in sewage; 6.1-493 and 0.64-76 ng/L in sea water, respectively). Mass flows from seven sewage plants discharged an estimated total of 14.4 kg/day to the Harbour. Typhoon shelters also appeared to play an important role as sources of antibiotics, as evidenced by elevated concentrations within their boundaries. Mass balance estimations suggested significant quantities of antibiotics are discharged to the Harbour without passage through treatment plants.