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Byung-Ju Sohn

Abstract

Clear sky longwave radiation fluxes for the summer of 1988 and winter of 1989 have been simulated with a radiative transfer model that includes detailed treatment of atmospheric gas absorption. The input data to the model are humidity and temperature profiles from ECMWF analyses and surface temperature measurements from the International Satellite Cloud Climatology Project. To reduce the inherent uncertainties in humidity profiles, the ECMWF moisture fields have been adjusted, based on the climatological relationship between the moisture profile and total precipitable water (PW), so that the ECMWF PW is equal to that derived from Special Sensor Microwave Imager (SSMI) data. SSMI and ECMWF PW patterns are generally similar, but significant differences in magnitude are found in the low latitudes, particularly over the subtropical oceans in the Southern Hemisphere. At the same time the tropospheric brightness temperature (TB) at 53.74 GHz has been computed using a microwave transfer model with ECMWF temperature and humidity fields as inputs. The comparison of computed TB with the Microwave Sounding Unit (MSU) channel 2 TB revealed differences larger than 1.5 K over most of the oceans, suggesting that the ECMWF model atmosphere has a cold bias. Maximum biases greater than 3 K are found in low latitudes. After removing the bias from the ECMWF temperature field, a new set of temperature profiles yielding brightness temperature equal to MSU channel 2 TB has been obtained. Simulation results clearly demonstrate that inclusion of satellite estimates of PW and TB enhances the accuracy of the clear sky LW flux simulation, substantially reducing the differences from Earth Radiation Budget Experiment (ERBE) estimates. Global averages indicate that the model-derived top-of-atmosphere (TOA) clear sky fluxes from the adjusted moisture and temperature profiles are in very good agreement with satellite-measured ERBE values, differing by only 1.4 W m−2. By contrast, the calculation using only ECMWF analyses without any adjustments gives a significant underestimation of TOA clear sky fluxes up to 10.7 W m−2 compared to the satellite measurements, suggesting that the constraint method can significantly improve the accuracy in the radiation budget simulation. The results also indicate that the ECMWF-based simulation errors are, at least for the two seasons studied, mainly attributed to the error in the temperature field rather than to errors in the moisture field.

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Byung-Ju Sohn

Abstract

Three-dimensional global distributions of longwave radiative cooling for the summer of 1988 and the winter of 1989 are generated from radiative transfer calculations using European Centre for Medium-Range Weather Forecasts temperature and humidity profiles and International Satellite Cloud Climatology Project cloudiness as inputs. By adding the cooling of the clear atmosphere to the total radiative heating, cloud-induced atmospheric radiative heating has been obtained. Emphasis is placed on the impact of horizontal gradients of the cloud-generated radiative heating on the global atmospheric circulation. Cloud-induced heating, whose general pattern is well in agreement with total diabatic heating suggested by other studies, exhibits its maximum heating areas within the Indian Ocean and the western Pacific. By contrast, maximum cooling areas are found in the northern and southern flanks of the Indian Ocean, and over the eastern Pacific off the west coasts of both North and South America. The fact that these heating and cooling distributions reinforce the climatologically favored heating gradients both in the meridional and zonal directions indicates that cloud-radiative feedback can enhance the strength of both the north–south Hadley circulation and the east–west Walker circulation.

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Byung-Ju Sohn and Johannes Schmetz

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Subdividing the Indian Ocean domain into three areas: (i) a moist cloudy area due to tropical deep convection, (ii) a moist clear area fed by the evaporation of hydrometeors from adjacent high clouds, and (iii) a dry area represented by descending air over the subtropics, the relationships between upper-tropospheric humidity over these three areas and tropical convections are examined using the European Geostationary Meteorological Satellite (Meteosat-5) observations. It is observed that the clear dry area shrinks and becomes drier in response to expansion of the cloudy area in the Tropics and vice versa. This change in upper-tropospheric humidity over the subtropics appears to mitigate the increase (decrease) in water vapor greenhouse effect caused by the expansion (contraction) of moist convective areas.

A simple sensitivity test shows that the strength of the water vapor feedback due to changes in the spatial extent of tropical convection is benign, though slightly negative, if the changes in subtropical dryness are considered.

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Byung-Ju Sohn and Franklin R. Robertson

Despite the general agreement that clouds cool the earth–atmosphere, there are substantial differences in estimated magnitudes of the annual global mean of cloud radiative forcing. Recent estimates of globally averaged net cloud radiative forcing range from −2 to −27 W m−2. The reasons for these differences have not been clarified in spite of the important role of clouds in maintaining global heat balance. Here, three estimation methods [Earth Radiation Budget Experiment (ERBE), Regression I, and Regression II] are compared using the same data source and analysis period.

Intercomparison has been done for the time period of February and March 1985 over which major satellite radiation budget and cloudiness datasets (ERBE radiation budget, Nimbus-7, and ISCCP cloudiness) are contemporaneous. The global averages of five sets of net cloud radiative forcing by three independent methods agree to within 3.5 W m−2; four of five cases agree to within 1 W m−2. This suggests that differences in published global mean values of net cloud radiative forcing are mainly due to different data sources and analysis periods and a best estimated annual mean among all previous estimates appears to be the ERBE measurement, that is, −17.3 W m−2. In contrast to the close agreement in the net cloud radiative forcing estimates, both longwave and shortwave cloud radiative forcing show more dependence on the chosen method and dataset. The bias of regression-retrieved values between Nimbus-7 and ISCCP cloud climatology is largely attributed to the difference in total cloudiness between two climatologies whereas the discrepancies between the ERBE and regression method appear to be, in part, due to the conceptually different definition of clear-sky flux.

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Byung-Ju Sohn and Eric A. Smith

Abstract

This study examines the impact of differential net radiative heating on two-dimensional energy transports within the atmosphere-ocean system and the role of clouds on this process. Nimbus-7 earth radiation budget data show basic energy surpluses over the tropical oceans and relative or absolute energy deficits over low-latitude continental regions. The two-dimensional mean energy transports, in response to zonal and meridional gradients in the net radiation field, exhibit an east-west coupled dipole structure in which the west Pacific acts as the major energy source and North Africa as the major energy sink. It is shown that the dipole is embedded in the secondary energy transports arising mainly from the differential heating between land and means in the tropics in which the tropical cast-west (zonal) transports are up to 30% of the tropical north-south (meridional) transports. Thus, any perturbations to this dipole on an interannual basis due to regionally induced fluctuations of the net radiation balance give rise to low-latitude energy transport variations. In turn, the tropical variations lead to extratropical responses through alterations of requirements on both zonal and meridional transports at all positions on the globe. Cloud-induced transports, obtained by differentiating the cloud-free portion from the total transport field, indicate that year-to-year cloud amount changes are contributing to fluctuations of the global climate system through these mechanisms.

Increased cloudiness increases zonal available potential energy, thus increasing the intensity of the north-south transports while slightly weakening the dipole intensity. It would thus appear that the basic role of cloudiness is to diminish the role of differential heating between continents and oceans and force the globe toward a more meridionally distributed energy imbalance. This implies the radiative feedback effects of clouds, regardless of factors determining cloud amount variability, reduce the radiative decoupling of land and ocean. This conclusion cannot be arrived at heuristically because it pertains to the specific optical properties of continental and oceanic cloud systems and additional factors governing cloud amount variability over the landmasses and oceans themselves.

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Byung-Ju Sohn and Eric A. Smith

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The source and forcing mechanisms of radiation budget variability were examined over tropical latitudes by separating the variations into cloud- and surface-forced components. A zonal harmonic analysis of emitted longwave radiation emphasizes that these variations are largely controlled at the planetary wave scale. Positive total and cloud-forced longwave (LW) anomalies embedded within this planetary-scale structure show eastward movement from the Indian Ocean toward the eastern Pacific together with the easterly displacement of negative anomalies from the western Pacific toward Africa during the period prior to and after the active phase of the 1982–83 ENSO. The overall effect leads to an approximately 50° per year propagation phase speed that is considerably slower than the oceanic Kelvin wave capable of driving east-west LW anomalies through sea surface temperature (SST) feedback. The oceanic Kelvin wave speed is about 60° per month over the Pacific basin in the course of an ENSO cycle. This suggests there are longer time scales of climatic signals in the tropical radiation budget.

The examination of time-dependent radiative energetics over the tropics reveals that the aforementioned anomaly LW propagation is mainly due to cloud forcing associated with east-west circulation changes, although surface forcing contributes within the Pacific basin. Since cloud amount changes are directly linked to variations in latent heat release, diabatic heating associated with coupled ocean-atmosphere feedback appears to be largely responsible for the LW anomaly propagation. An examination of the complete radiation budget over the maritime continent and equatorial central Pacific during the 1982–83 ENSO event demonstrates that radiative forcing produces positive feedbacks in conjunction with the sea surface temperature anomalies that develop in both regions. Furthermore, surface forcing is found to be an important control on net radiation variability within this teleconnection. An examination of two additional tropical cast-west teleconnections shows that surface forcing is even more important than cloud forcing in controlling variations in the east-west net radiation gradients.

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Byung-Ju Sohn and Eric A. Smith

Abstract

Required global energy transports determined from Nimbus-7 satellite net radiation measurements have been separated into atmospheric and oceanic components by applying a maximum entropy production principle to the atmospheric system. Strong poleward fluxes by the oceans in the Northern Hemisphere exhibit a maximum of 2.4 1015W at 18°N, whereas maximum atmospheric transports are found at 37°N with a magnitude of 4.5 1015W. These results are in good agreement with other published results. In the Southern Hemisphere, atmospheric transports are found to be considerably stronger than oceanic transports, and this finding corroborates findings based on other published direct estimates. Maximum atmospheric energy transports are found at 37°S with a magnitude of 4.7 × 1015 W; two local oceanic transport maxima are shown at 1 8°S and 45°S with magnitudes of 1.3 × 1O15 W and 1.1 × 1015 W, respectively. There is also evidence of net cross-equatorial) transport in which the Southern Hemisphere oceans give rise to a net transfer of beat northward across the equator that exceeds a net transfer from Northern to Southern Hemisphere by the atmosphere. Since Southern Hemisphere results in this study should have the same degree of accuracy as in the Northern Hemisphere, these findings suggest that Southern Ocean transports are weaker than previously reported. A main implication of the study is that a maximum entropy production principle can serve as a governing rule on macroscale global climate, and in conjunction with conventional satellite measurements of the net radiation balance, provides a means to decompose atmosphere and ocean transports from the total transport field. Furthermore, the modeling methodology provides a possible means to partition the transports in a two-dimensional framework; this approach is tested on the separate ocean basins with qualified success.

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Byung-Ju Sohn and Eric A. Smith

Abstract

Cloud–radiative forcing calculations based on Nimbus-7 radiation budget and cloudiness measurements reveal that cloud-induced longwave (LW) warming (cloud greenhouse influence) is dominant over the tropics, whereas cloud-induced shortwave (SW) cooling (cloud albedo influence) is dominant over mid- and high latitudes. The average SW cloud cooling taken over the area of the globe from 65°N to 65°S is −27.8 W m−2. This magnitude slightly overcomes LW cloud warming (−25.7 W m−2), resulting in a small net cooling effect of −2.1 W m−2 over 93% of the earth.

A 6-year zonally averaged mean cloudy- and clear-sky net radiation flux analysis shows that there are three distinct regimes in terms of net cloud warming or cooling, that is, warming in the tropics (between 20°N and 20°S) and in the high latitudes (poleward of 55°) and cooling in the extratropical latitudes between 20° and 55° in both hemispheres. These distributions reinforce the intensities of the Hadley and Ferrel meridional circulation cells. This stems from strong warming due to high-level clouds in the tropics and strong cooling due to mid- and low-level clouds at extratropical latitudes. The magnitude of the contribution by cloud forcing is found to be of the same order as eddy heat and momentum flux forcing to the maintenance of the mean meridional circulation.

Surface–atmosphere forcing obtained by differentiating the cloud-induced effects from the measured radiative fluxes indicates that an east–west coupled North Africa–western Pacific energy transport dipole is maintained mainly by low-latitude land–ocean contrasts associated with shortwave radiation but supported by cloud controls on tropical longwave radiation. This implies that interannual variations in the net radiation balance associated with these two regions can give rise to fluctuations of the basic dipole structure and thus fundamental changes in low-latitude climate.

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Hwan-Jin Song and Byung-Ju Sohn

Abstract

A 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 composites of 11-μm brightness temperature from geostationary satellites and Interim ECMWF Re-Analysis (ERA-Interim) data. Cold-type heavy rainfall over Korea is characterized by an eastward moving cloud system with an oval shape while the warm type shows a comparatively wide spatial distribution over an area extending from the southwest to northeast. Overall the warm-type process appears to link the low-level moisture convergence area to the vertically aligned divergence area formed over the jet stream level. This setup continuously pushes air upward under moist-adiabatically near-neutral conditions and thus yields heavy rainfall. As warm-type heavy rainfall persists longer, it is considered to be more responsible for flood events occurring over the Korean peninsula.

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Byung-Ju Sohn and Eric A. Smith

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This study examines a mix of seven statistical and physical Special Sensor Microwave Imager (SSM/I) passive microwave algorithms that were designed for retrieval of over-ocean precipitable water (PW). The aim is to understand and explain why the algorithms exhibit a range of discrepancies with respect to measured PWs and with respect to each other, particularly systematic regional discrepancies that would produce substantive uncertainties in water vapor transports and radiative cooling in the context of climate dynamics. Data analysis is used to explore the nature of the algorithm differences, while radiative transfer analysis is used to explore the influence of several environmental variables (referred to as tangential environmental factors) that affect the PW retrievals. These are sea surface temperature (SST), surface wind speed (U s), cloud liquid water path (LWP), and vertical profile structure of water vapor [q(z)]. The main datasets include the Wentz matched radiosonde–SSM/I point database consisting of 42 months of globally distributed oceanic radiosonde profiles paired with coincident SSM/I brightness temperatures, and globally compiled instantaneous orbit-swath maps of SSM/I brightness temperatures for January and July 1990.

Results demonstrate that the seemingly good agreement found in past studies and herein, within the conventional framework of scatter diagram analysis that ignores regional classification, gives way to poor agreement in the framework of monthly and zonally averaged differences. It is shown how much of the disagreement inherent to statistical algorithms is due to disjoint training datasets used in deriving algorithm regression coefficients. The investigation also explores how tangential environmental factors composed of variations in SST, U s, cloud LWP, and q(z) structure impart dissimilar errors to retrieved PWs, according to the design of the retrieval algorithms. A discussion on implications of the discrepancies vis-à-vis the Global Energy and Water Cycle Experiment program is given, with suggestions on mitigating discrepancies in algorithm designs.

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