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국제학술지

공지 (2025. 10. 28.)

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Principal modes of climatological seasonal and intraseasonal variations of the Asian summer monsoon

Abstract: Principal modes of climatological variation of the Asian summer monsoon are investigated. Data used in this study include the high cloud fraction produced by the International Satellite Cloud Climatology Project and sea level pressure, and 850- and 200-mb geopotential heights from ECMWF analysis for the five summers of 1985–89. It is shown that the seasonal evolution of the Asian summer monsoon is adequately described by a few leading EOFs. These EOFs capture the variations of regional rainbands over the East Asian and Indian regions. The first mode is characterized by an increase in large-scale cloud over India and the subtropical western Pacific until mid-August. The second mode depicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the development of summer monsoon circulation: a low pressure system over the Asian continent and a subtropical high over the Pacific. The third eigenmode is characterized by zonal cloud bands from northern India, crossing the Korean peninsula to Japan, and dryness over the oceans in the south of cloud bands. This mode is related to the mature phase of Changma rainy season in Korea associated with the northward movement of cloud bands and circulation systems from the subtropical western Pacific. This mode appears as a first principal mode of climatological intraseasonal oscillation (CISO) over the entire Asian monsoon region. The CISO mode has a timescale of about 2 months. The northward moving CISO also appears in the 850- and 200-mb geopotential height fields as a first mode of each dataset. Based on the height variations of the CISO mode, it is suggested that the extratropical CISO during summer is related to a regional index cycle associated with the variation of north–south temperature gradient in East Asia.
- 작성자Kang et al.
- 작성일2024.06.14
- 조회수54
- 1999
Anomalous atmospheric hydrologic processes associated with ENSO: Mechanisms of hydrologic cycle-radiation interaction

Abstract: Using reanalysis data from the Goddard Earth Observing System (GEOS) Data Assimilation System, the authors have documented the basic three-dimensional features of anomalous atmospheric hydrologic processes observed during the El Niño–Southern Oscillation (ENSO). The most dominant anomaly pattern features a pair of subtropical temperature maxima straddling the equator in the upper troposphere coupled to a corresponding pair of temperature minima in the lower stratosphere in the form of a dipole. Over the Tropics and subtropics, the water vapor content is increased in regions of large-scale ascent with maximum response in the middle troposphere, whereas substantial drying is found in the descending branches of the Walker and Hadley circulations. While the temperature and moisture patterns in the lower troposphere are thermodynamically linked to the sea surface temperature anomaly pattern, the distribution of temperature and water vapor in the upper troposphere is largely controlled by dynamics and much less by thermodynamics. The troposphere–stratosphere temperature dipole is fundamentally due to the rising of the tropopause associated with hydrostatic expansion and vertical ascent in regions of enhanced deep convection. The rising motion pushes colder upper-tropospheric air into the lower stratosphere where the climatological temperature gradient reverses. No such dipole anomaly exists in the moisture field. Numerical experiments with the GEOS GCM show that while atmospheric dynamics are principally responsible for the generation of the basic structures of the temperature and moisture anomalies observed during ENSO, the interaction between the hydrologic cycle and radiation plays an important role in enhancing and modifying the response. The role of hydrologic cycle–radiation interaction is most important in rendering the atmosphere more unstable both columnwise and locally, through enhanced longwave heating in the middle and lower troposphere and cooling above. The enhanced instability leads to intensified Hadley and Walker circulations, which are accompanied by stronger latent heating and a more vigorous hydrologic cycle. The intensified hydrologic cycle promotes further warming and moistening of the middle and lower troposphere, and cooling and drying in the stratosphere. The radiation–dynamics feedback leads to a new equilibrium climate state in which the increased heat transport by convection into the upper troposphere and stratosphere is balanced by increased radiative cooling, which removes the local excessive heat buildup.
- 작성자Lau et al.
- 작성일2024.06.14
- 조회수248
- 1998
Comparison of model-calculated and ERBE-retrieved clear-sky outgoing longwave radiation

Abstract: In order to validate radiative transfer models and identify sources of errors in the satellite retrieval of radiation budgets, model calculations of clear-sky outgoing longwave radiation (OLR) over oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). The NASA Goddard Earth Observing System (GEOS-1) and the National Centers for Environmental Prediction and National Center for Atmospheric Research reanalyses of temperature and humidity and the Stratospheric Aerosol and Gas Experiment retrieval of the stratospheric humidity are used in the model calculations. Averaged over time (1985–1989) and space (60°S-60°N), the model-calculated clear-sky OLR has a positive bias of 1.9–2.3 W m−2 when compared with that of ERBE. Nearly all of the bias can be accounted for by the inclusion of the absorption due to CO2 in the 4.3-μm band and to the weak CO2 and O3 molecular lines distant from band centers. The use of the two different reanalyses has only a small effect on the flux calculations (∼0.4 W m−2). Consistent with suggestions by previous studies, ERBE is found to overestimate (underestimate) the clear-sky OLR over high humid (dry) regions due to incorrect identification of clear scenes in the ERBE retrievals. The importance of the upper tropospheric humidity in affecting the Earth radiation budget is also investigated. Although only ∼15% of the atmospheric humidity is contained in the region above the 600-hPa level, the upper troposphere is as important as the lower troposphere in contributing to the clear-sky OLR.
- 작성자Ho et al.
- 작성일2024.06.14
- 조회수66
- 1998
Minor trace gas radiative forcing calculations using the k-distribution method with one-parameter scaling

Abstract: The k distribution method with one-parameter pressure and temperature scaling, first developed for water vapor, has now been applied to the minor trace gas (N2O, CH4, CFCs, and two minor bands of CO2) absorption in the infrared window region (800–1380 cm−1). The derivation of the k distributions is based upon an exponential sum fitting to the monochromatically calculated transmission functions at a predetermined reference pressure and temperature. For nonhomogeneous path lengths, one-parameter scaling is utilized in conjunction with the k distribution method. To determine the accuracies of the k distribution method as compared to the monochromatic calculations, fluxes and cooling rates are calculated for a wide variety of atmospheric conditions. For the entire 800–1380 cm−1 spectral range the effect of the minor trace gases on the fluxes calculated using the k distribution method is within 2.3% of the monochromatic method. In addition to being accurate, this method is computationally very fast. When implemented into the Goddard EOS general circulation model, the computing time for the longwave flux calculations is increased by only 20% despite the inclusion of the minor trace gas absorption bands.
- 작성자Kratz et al.
- 작성일2024.06.14
- 조회수88
- 1998
Parameterizations for cloud overlapping and shortwave single-scattering properties for use in general circulation ...

Abstract: Parameterizations for cloud single-scattering properties and the scaling of optical thickness in a partial cloudiness condition have been developed for use in atmospheric models. Cloud optical properties are parameterized for four broad bands in the solar (or shortwave) spectrum; one in the ultraviolet and visible region and three in the infrared region. The extinction coefficient, single-scattering albedo, and asymmetry factor are parameterized separately for ice and water clouds. Based on high spectral-resolution calculations, the effective single-scattering coalbedo of a broad band is determined such that errors in the fluxes at the top of the atmosphere and at the surface are minimized. This parameterization introduces errors of a few percent in the absorption of shortwave radiation in the atmosphere and at the surface. Scaling of the optical thickness is based on the maximum-random cloud-overlapping approximation. The atmosphere is divided into three height groups separated approximately by the 400- and 700-mb levels. Clouds are assumed maximally overlapped within each height group and randomly overlapped among different groups. The scaling is applied only to the maximally overlapped cloud layers in individual height groups. The scaling as a function of the optical thickness, cloud amount, and the solar zenith angle is derived from detailed calculations and empirically adjusted to minimize errors in the fluxes at the top of the atmosphere and at the surface. Different scaling is used for direct and diffuse radiation. Except for a large solar zenith angle, the error in fluxes introduced by the scaling is only a few percent. In terms of absolute error, it is within a few watts per square meter. Full title: Parameterizations for cloud overlapping and shortwave single-scattering properties for use in general circulation and cloud ensemble models
- 작성자Chou et al.
- 작성일2024.06.13
- 조회수78
- 1998
Water vapor and cloud feedback over the tropical oceans: Can we use ENSO as a surrogate for the climate change?

Abstract: Based on experiments with the Goddard Earth Observing System (GEOS) global climate model we find that the basic patterns of anomalous water vapor greenhouse effect and cloud radiative forcing during ENSO are primarily determined by the basin-wide dynamical response to large scale sea surface temperature (SST) forcing. There is no supergreenhouse effect in the sense of unstable interaction due to local thermodynamics and water vapor radiative feedback on interannual time scales. About 80% of the clear sky water vapor greenhouse sensitivity to SST-deduced from ENSO anomalies are due to the transport of water vapor by the large scale circulation. The sensitivity of water vapor greenhouse effect to SST due to radiative feedback is found to be about 1.8 Wm −2/ °C, much smaller than the values of 6~9 Wm −2/ °C previously estimated from satellite observations from ENSO conditions. Our results show that regionally based interannual variability should not be used to infer radiative feedback sensitivity for climate change unless proper corrections are made for the effect of the large scale circulation.
- 작성자Lau et al.
- 작성일2024.06.13
- 조회수62
- 1996