Hwan-Jin Song

Characteristic Features of Warm-Type Rain Producing Heavy Rainfall over the Korean Peninsula Inferred from TRMM Measurements

AbstractIn contrast to the view that deep convection causes heavy rainfall, Tropical Rainfall Measuring Mission (TRMM) measurements demonstrate that heavy rainfall (ranging from moderate to extreme rain rate) over the Korean peninsula is associated more with low-level clouds (referred to as warm-type clouds in this study) than with conventional deep convective clouds (cold-type clouds). Moreover, it is noted that the low-level warm-type clouds producing heavy rainfall over Korea appear to be closely linked to the atmospheric river, which can form a channel that transports water vapor across the Korean peninsula along the northwestern periphery of the North Pacific high. Much water vapor is transported through the channel and converges on the Korean peninsula when warm-type heavy rain occurs there. It may be possible to produce abundant liquid water owing to the excess of water vapor; this could increase the rate and extent of raindrop growth, primarily below the melting layer, causing heavy rain when thes...

The Role of the Dry Static Stability for the Recent Change in the Pacific Walker Circulation

AbstractThere is an uncertainty in how the Pacific Walker circulation (PWC) will change in response to increased greenhouse gas (GHG) warming. On average, climate models predict that the PWC will weaken. Observational evidence is mixed, with some evidence supporting the models while others do not. In this study, insight into the PWC trend is provided by examining the tropical dry static stability, a quantity that is inversely proportional to the strength of the PWC. For the 1979–2012 period, the static stability increased markedly in all phase 5 of the Coupled Model Intercomparison Project (CMIP5) models, far more so than in the satellite and global reanalysis data, which show a strengthening of the PWC. The stabilization is greater for a subset of models that simulate a significant weakening of the PWC.With the observed sea surface temperature as the lower boundary condition, over the western tropical Pacific, atmospheric models that belong to the weakening-PWC-CMIP5 group produce greater stabilization t...

Two Heavy Rainfall Types over the Korean Peninsula in the Humid East Asian Summer Environment: A Satellite Observation Study

AbstractA total of 10 years (2002–11) of Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) reflectivities, signaling heavy rainfall (>10 mm h−1), were objectively classified by applying the K-means clustering method in order to obtain typical reflectivity profiles associated with heavy rainfall over East Asia. Two types of heavy rainfall emerged as the most important rain processes over East Asia: type 1 (cold type) characterized by high storm height and abundant ice water under convectively unstable conditions, developing mostly over inland China; and type 2 (warm type) associated with a lower storm height and lower ice water content, developing mostly over the ocean. These two types also show sharp contrasts in relation to their seasonal changes and in the diurnal variation of frequency maxima, in addition to other contrasting meteorological parameters. The PR-derived heavy rain events were observed over the Korean peninsula and their spatiotemporal evolution was examined using 10-yr c...

Idealized numerical experiments on the microphysical evolution of warm-type heavy rainfall

Recent satellite observations suggested that medium-depth heavy rain systems (i.e., warm-type heavy rainfall) were predominantly found in the Korean peninsula under moist-adiabatically near neutral conditions in contrast to the traditional view that deep convection induced by convective instability produced heavy rainfall (i.e., cold-type heavy rainfall). In order to examine whether a numerical model could explain the microphysical evolution of the warm-type as well as cold-type heavy rainfall, numerical experiments were implemented with idealized thermodynamic conditions. Under the prescribed humid and weakly unstable conditions, the warm-type experiments resulted in a lower storm height, earlier onset of precipitation, and heavier precipitation than was found for the cold-type experiments. The growth of ice particles and their melting process were important for developing cold-type heavy rainfall. In contrast, the collision and coalescence processes between liquid particles were shown to be the mechanism for increasing the radar reflectivity toward the surface in the storm core region for the warm-type heavy rainfall.

Physical explanation of the weakened brightness temperature difference signal over the yellow sea during a dust event: Case study for March 15–16, 2009

This paper attempts to explain the cause of weakening or disappearing brightness temperature difference (BTD) signatures, in particular, over the Yellow Sea during the March 15–16, 2009 dust event. Using a simple correction approach that removes the effects of emissivity difference and water vapor effect difference, we confirmed that the weakening or disappearing BTD signatures noted over the Yellow Sea are largely due to the spectral emissivity contrast between land and ocean. The weakening or disappearing dust is hypothesized to be pronounced when the dust loading is weak because of the surface contribution to the top of atmosphere radiance, and that it is mainly due to the difference in spectral emissivity over the window band between land and ocean. It is further suggested that water vapor may be considered as a correction factor in spite of its smaller contribution.

Validation of MODIS-derived Aerosol Optical Thickness Using SKYNET Measurements over East Asia

Using six-year (2004-2009) SKYNET measurements, MODIS-derived AOTs were validated at five SKYNET sites (Seoul, Chiba, Etchujima, Fukuejima, and Hedomisaki), in addition to climatological analysis of MODIS-derived optical properties over the East Asian domain (20-50 o N, 90-150 o E). In so doing MODIS-SKYNET collocated AOT data were constructed if two measurements are taken within 25 km distance and within 30 minute time difference. From the comparison of two measurements, it is demonstrated that aerosol type insignificantly affects the accuracy of MODIS AOT. It is because the aerosol model combining predefined fine aerosol model and coarse aerosol model is used for the retrieval. However, positive bias between MODIS and SKYNET increases as fraction of the coarse aerosol model increases. In addition, MODIS AOT appears to be overestimated in case of lower aerosol loading while the overestimation tends to decrease with increased aerosol loading. Regression analysis between MODIS AOT and SKYNET AOT for 550 nm band yields 0.86, 0.16, and 0.61 of regression slope, intercept, and coefficient of determination, respectively. Those statistical results may draw a conclusion that MODIS AOTs over East Asia carry a reasonable accuracy compared to ground-based SKYNET measurements.

An Evaluation of WRF Microphysics Schemes for Simulating the Warm-Type Heavy Rain over the Korean Peninsula

The Korean peninsula is the region of distinctly showing the heavy rain associated with relatively low storm height and small ice water content in the upper part of cloud system (i.e., so-called warm-type heavy rainfall). The satellite observations for the warm-type rain over Korea led to a conjecture that the cloud microphysics parameterization suitable for the continental deep convection may not work well for the warm-type heavy rainfall over the Korean peninsula. Therefore, there is a growing need to examine the performance of cloud microphysics schemes for simulating the warm-type heavy rain structures over the Korean peninsula. This study aims to evaluate the capabilities of eight microphysics schemes in the Weather Research and Forecasting (WRF) model how warm-type heavy rain structures can be simulated, in reference to the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) reflectivity measurements. The results indicate that the WRF Double Moment 6-class (WDM6) scheme simulated best the vertical structure of warm-type heavy rain by virtue of a reasonable collision-coalescence process between liquid droplets and the smallest amount of snow. Nonetheless the WDM6 scheme appears to have limitations that need to be improved upon for a realistic reflectivity structure, in terms of the reflectivity slope below the melting layer, discontinuity in reflectivity profiles around the melting layer, and overestimation of upper-level reflectivity due to high graupel content.