ALBERT

Description: Changes in the Brewer-Dobson circulation (BDC) during the 30 years 1980–2009 are investigated using Modern Era Retrospective-analysis for Research and Applications (MERRA) reanalysis data. The mass streamfunction that is induced by wave forcings in the transformed Eulerian-mean (TEM) equation through the downward-control principle is used as a proxy for the BDC. The changes in the BDC are investigated using two aspects: the wave propagation conditions in the stratosphere and the wave activity in the upper troposphere. They are compared in the first (P1) and second (P2) 15-year periods. The resolved wave forcing, expressed by the Eliassen-Palm (EP) flux divergence (EPD), is significantly enhanced during the December-January-February (DJF) season in P2 in both the Northern Hemisphere (NH) high latitudes and the Southern Hemisphere (SH) mid- and high latitudes. The increased zonal mean zonal wind at high latitudes in the SH, caused by ozone depletion, leads to an upward shift of the Rossby-wave critical layer and this allows more transient planetary waves to propagate into the stratosphere. In the NH, the enhanced EPD in DJF leads to an increase in the frequency of Sudden Stratospheric Warming (SSW) events. The gravity wave drag (GWD) is smaller than the EPD and the change in it between the two time periods is insignificant. The residual term in the TEM equation is similar to the GWD in the two periods, but its change between the two periods is as large as the change in the EPD. Among the four components of the EP flux at 250 hPa, the meridional heat flux played a dominant role in the enhancement of the BDC in P2.

Description: The location and occurrence time of convective rainfalls have attracted great public concern as they can lead to terrible disasters. However, the simulation results of convective rainfalls in the Pearl River Delta region often show significant discrepancies from the observations. One of the major causes lies in the inaccurate geographic distribution of land surface properties used in the model simulation of the heavy precipitation. In this study, we replaced the default soil and vegetation datasets of Weather Research and Forecasting (WRF) model with two refined datasets, i.e. the GlobCover 2009 (GLC2009) land cover map and the Harmonized World Soil Database (HWSD) soil texture, to investigate the impact of vegetation and soil on the rainfall patterns. The result showed that the simulation patterns of convective rainfalls obtained from the coupled refined datasets are more consistent with the observations than those obtained from the default ones. By using the coupled refined land surface datasets, the overlap ratio of high precipitation districts reached 36.3% with a variance of 28.5 km from the observed maximum rainfall position, while those of the default United States Geological Survey (USGS) dataset and Moderate Resolution Imaging Spectroradiometer (MODIS) dataset are 17.0%/32.8 km and 24.9%/49.0 km, respectively. The simulated total rainfall amount and occurrence time using the coupled refined datasets are the closest to the observed peak values. In addition, the HWSD soil data has improved the accuracy of the simulated precipitation amount, and the GLC2009 land cover data also did better in catching the early peak time.

Description: This study proposes a model that estimates optimal expected temperature for thermal comfort at various levels of relative humidity, wind speed, mean radiation temperature, metabolic rate, and clothing insulation, while improving the variables of the mean radiation temperature. By including the results of numerical and observational data testing, the improved model considers mean radiant temperatures more comprehensively than the previous model. Changes in expected temperatures for thermal comfort at various mean radiation temperatures showed that the outdoor temperature must decrease as the length of outdoor exposure increases. The expected temperature for thermal comfort must be lower at higher metabolic rates. The expected temperature does not change as wind speed increases when the mean radiant temperature is low. However, as mean radiant temperature increases, the expected temperatures change depending on the wind speed.

Description: Bifurcation or bi-modal tropical cyclone intensity forecasts may arise due to uncertainty in the timing of formation, timing and magnitude of rapid intensification periods, or track forecast uncertainty leading to landfall or non-landfall or leading to interaction with warm- or cold-ocean eddies. An objective technique is developed and tested to detect these intensity bifurcation situations in our weighted-analog intensity (WANI) forecasts that are based on the 10 best historical analogs to the Joint Typhoon Warning Center (JTWC) official track forecasts. About 19% of the overall sample of 1136 WANI forecasts in the western North Pacific during the 2010–2012 seasons met the criteria for a substantial intensity bifurcation situation. Using a hierarchical clustering technique, two clusters of the 10 best analogs are defined and separate WANI forecasts and intensity spreads are calculated for the two clusters. If an always perfect selection of the correct cluster WANI forecast of each bifurcation situation is made, a substantial improvement in the intensity mean absolute errors is achieved relative to the original WANI forecasts based on all 10 of the best analogs. These perfect-cluster selection WANI forecasts have smaller bias errors and are more highly correlated with the verifying intensities at all forecast intervals through 120 h. Without further bias correction and calibration, the cluster WANI intensity spreads are under-determined as the Probability of Detections are smaller than the desired 68%. Four examples of WANI cluster predictions of intensity bifurcation situations are provided to illustrate how a correct choice of the intensity forecast and the intensity spread can be the basis for improved warnings of the threat from western North Pacific tropical cyclones.

Description: PM2.5 is a big issue as it is considerably more harmful than other sizes of particulate matter. World Health Organization (WHO) recommends 25 μg m −3 as the daily average concentration, and 10 μg m −3 per day as an annual average. To keep up with global trends, it is first necessary to understand the current status and characteristics of PM2.5 concentrations in Korea. Using the PM2.5 data measured by Seoul Metropolitan City from November 2005 to March 2012, the author analyzed its statistical characteristics and correlations with other air pollutants. For the time period from 2005 to 2012, the annual average concentration of PM2.5 was 27 μg m −3 , three times the WHO standard. Also, the daily average PM2.5 concentration of 215 days per year also exceeded the WHO standard. However, the number days exceeding the Korean daily average standard of 50 μg m −3 to be enacted in 2014 was only three. PM2.5 concentration had a high correlation (r = 0.84) with PM10, and also showed high correlations with gaseous pollutants, such as SO 2 , NO 2 , and CO, but not O 3 . This study suggests that the Korean government should strengthen their standard to match the criteria used by WHO.

Description: How well the climate models simulate extreme temperature over East Asia and how the extreme indices would change under anthropogenic global warming are investigated. The indices studied include hot days (HD), tropical nights (TN), growing degree days (GDD), and cooling degree days (CDD) in summer and heating degree days (HDD) and frost days (FD) in winter. The representative concentration pathway 4.5 (RCP 4.5) experiments for the period of 2075-2099 are compared with historical simulations for the period of 1979–2005 from 15 coupled models that are participated in phase 5 of the Coupled Model Intercomparison Project (CMIP5). To optimally estimate future change and its uncertainty, groups of best models are selected based on Taylor diagrams, relative entropy, and probability density function (PDF) methods previously suggested. Overall, the best models’ multi-model ensemble based on Taylor diagrams has the lowest errors in reproducing temperature extremes in the present climate among three methods. Selected best models in three methods tend to project considerably different changes in the extreme indices from each other, indicating that the selection of reliable models are of critical importance to reduce uncertainties. Three groups of best models show significant increase of summerbased indices but decrease of the winter-based indices. Over East Asia, the most significant increase is seen in the HD (336 ± 23.4% of current climate) and the most significant decrease is appeared in the HDD (82 ± 4.2%). It is suggested that the larger future change in the HD is found over in the Southeastern China region, probably due to a higher local maximum temperature in the present climate. All of the indices show the largest uncertainty over Southeastern China, particularly in the TN (~3.9 times as large as uncertainty over East Asia) and in the HD (~2.4). It is further noted that the TN reveals the largest uncertainty over three East Asian countries (~1.7 and 1.4 over Korea and Japan, respectively). These future changes in extreme temperature events have an important implication for energy-saving applications and human molarity in the future.