In this study, we investigate the mechanism of the arctic warming pattern in surface air temperature (SAT) and sea ice concentrations over the last two decades in comparison with global warming since the 1970s. According to the analysis result, it is found that the patterns of SAT and sea ice before 1989 are mostly determined by the Arctic Oscillation (AO) in winter. In contrast, arctic warming patterns after 1989 are characterized by the intensification of the Beaufort High and the reduced sea-ice concentrations in summer induced by the positive ice-albedo feedback. It is concluded that the arctic warming before 1989 especially in winter was explained by the positive trend of the AOI. Moreover the intensified Beaufort High and the drastic decrease of the sea ice concentrations in September after 1989 were associated with the recent negative trend of the AOI. Since the decadal variation of the AO is recognized as the natural variability of the global atmosphere, it is shown that both of decadal variabilities before and after 1989 in the Arctic can be mostly explained by the natural variability of the AO not by the external response due to the human activity.
Large eddy simulations of dust devil-like vortices under various mean wind conditions were performed with the Weather Research and Forecasting model to investigate the effects of mean wind on the intensity and evolution of dust devils. It is shown that intense vortices are generated for all the mean-wind cases examined. Significant characteristics among the cases with the different mean winds are that vertical vortices are generated most preferably in medium-wind conditions in terms of their number and intensity, and that the lifetime of intense vortices becomes shortened with the increase in mean wind. It is also found that a stability parameter well describes the behavior of vortices in response to mean wind and provides a necessary condition for dust devil occurrence.
This paper introduces the East Asian drought monitoring system that has been operational since 2008 (http://atmos.pknu.ac.kr/~intra2). It provides the spatial and temporal distribution of droughts using variables such as the drought intensity and duration. The main feature of this system is the effective drought index (EDI; Byun and Wilhite 1999) in which the drought intensity is calculated by considering cumulative precipitation as a weighting function of time. In addition, this system provides an available water resources index (AWRI; Byun and Lee 2002), which represents the hydrological index. These indexes were calculated using the monthly precipitation data obtained from 298 stations in East Asia (Republic of Korea, Democratic People's Republic of Korea, Japan, and China). Information on drought is provided in terms of monthly spatial distribution and is presented in the text and a time series. The period of data collection differs according to the observation start dates of the stations, and the longest period of data collection (Seoul, Korea) started in 1778. This system will contribute to drought prevention by diagnosing drought in real time. It will also provide fundamental data for further studies on drought by detecting the time and location of droughts.
We investigated the impact of wave-ocean interaction on numerical predictions for Typhoon Hai-Tang in 2005 using a nonhydrostatic atmosphere model coupled with a third-generation ocean wave model and a mixed-layer ocean model. Here we address the effect of breaking surface waves on entrainment induced at the mixed-layer base, assuming that the turbulent transport due to breaking surface waves is expressed as a function of both wave-induced and surface wind stresses. The introduction of breaking surface waves into the coupled model enables us to reproduce the evolution of Hai-Tang's central pressure and sea-surface temperature (SST) distribution more realistically. SST decreases significantly (rarely) along Hai-Tang's track where the mixed-layer depth is relatively shallow (deep). Hai-Tang tends to intensify where the initial mixed layer is relatively deep along Hai-Tang's track. Introducing the wave-ocean interaction may lead to more precise tropical cyclone intensity prediction through more realistic reproduction of the SST distribution.
The Nested Air Quality Prediction Modeling System (NAQPMS), in coupling with the fifth-generation NCAR/Penn State Mesoscale Model (MM5), is employed to assess the impact of vehicle traffic restriction on air quality in Beijing within pre-Olympic environmental measures implemented from 17th to 20th August 2007. Predictions are compared against meteorological and air quality observed data and validation shows model good performance as a whole. Sensitivity experiments, including the baseline and traffic control scenarios, are designed to estimate the potential reduction of nitrogen dioxide (NO2) and particulate matter (PM10) concentrations during the traffic restriction. Results indicate that the NO2 concentration in Urban Beijing is reduced by 16%∼32%, with the average of 21%, while NOx emissions are lowered within 28%; the primary PM10 concentrations is also reduced by 6%∼15%, lower than the decreased percentages of NO2 concentration. The results show that the most significant reduction of air pollutants occurs in Urban Beijing where the restriction has been mainly imposed. This study demonstrates the efficiency of traffic restriction measure in air quality improvement over Beijing.
Substantial change was found in the El Niño/Southern Oscillation (ENSO) teleconnection during the boreal winter after the regime shift of the late 1970s. The ENSO teleconnection pattern of the earlier period is characterized by a wave-train-like structure over the Pacific/North American sector that also propagates into the stratosphere. In contrast, the recent ENSO-related extratropical circulation anomalies are characterized by changes in the tropospheric subtropical jet. The interaction of zonal winds with large-scale topography creates meridional dipole-type height anomalies downstream of the Rockies and the Tibetan Plateau, which may characterize the recent ENSO teleconnection pattern.
Cyclone Sidr had an intense rainband east of the cyclone center. The rainband exhibited two strong convective lines (band axes) composed of convective cells. To study the characteristics of the convective cells, a simulation was performed at 1-km resolution by using a cloud-resolving model. In both band axes, some cells showed the characteristic structure of a supercell. Supercells within the outer axis had stronger updraft, more intense precipitation, and a longer lifetime than those of the inner axis. The values of the CAPE and helicity are different in the strong vertical shear environment of the inner and outer axes. A large moisture flux was present throughout the troposphere on the east side of the outer axis. On the other hand, the upper level west of the inner axis was relatively dry. These are additional factors affecting the differences in cell characteristics in the two band axes.
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