Changes In Radiative Fluxes Research Articles

Uncertainty in climate sensitivity: Causes, consequences, challenges

Fossil fuels supply about 85% of the world's primary energy, and future use would not appear limited by availability of reserves, especially of coal. Rather, future use of fossil fuels will likely be limited by controls on the emission of carbon dioxide into the atmosphere that are agreed to by the nations of the world. The increase in atmospheric CO2 over the past 200 years, mainly from fossil fuel combustion, is confidently thought to have increased global temperatures and induced other changes in Earth's climate, with the prospect of much more severe consequences from projected future emissions. Limiting such changes in Earth's climate would place major constraints on the combustion of fossil fuels and/or the emission of CO2 into the atmosphere. Developing effective and cost-effective strategies for limiting CO2 emissions requires the confident ability to project the changes in climate that would result from a given increase in atmospheric CO2. However, even the change in global mean surface temperature (GMST), the single most important index of climate change, that would result from a given increase in atmospheric CO2 remains uncertain to a factor of 2 or more, largely because of uncertainty in Earth's climate sensitivity, the change in GMST per change in radiative flux. This uncertainty in climate sensitivity, which gives rise to a comparable uncertainty in the shared global resource of the amount of fossil fuel that can be burned consonant with a given increase in global mean surface temperature, greatly limits the ability to effectively formulate strategies to limit climate change while meeting the world's future energy requirements. Key limits on determining climate sensitivity are the small change in downwelling longwave irradiance, less than one percent, that would give rise to changes in climate that reach the level of concern, the complexity of cloud processes and the difficulty of representing them in climate models, and limited understanding of the processes that control the radiative influences of atmospheric aerosols. A recent empirical calculation of Earth's climate sensitivity as the quotient of the relaxation time constant of GMST upon the effective heat capacity characterizing climate change on the multidecadal time scale points to a possible alternative approach to determining Earth's climate sensitivity. While improved knowledge of Earth's climate sensitivity is essential to development of optimal energy strategies, even for climate sensitivity at the low end of the range of present estimates, substantial reductions in CO2 emissions from their present values would be required to avert dangerous anthropogenic interference with the climate system that would otherwise occur well before the end of the present century.

Global model simulations of the impact of ocean-going ships on aerosols, clouds, and the radiation budget

Abstract. International shipping contributes significantly to the fuel consumption of all transport related activities. Specific emissions of pollutants such as sulfur dioxide (SO2) per kg of fuel emitted are higher than for road transport or aviation. Besides gaseous pollutants, ships also emit various types of particulate matter. The aerosol impacts the Earth's radiation budget directly by scattering and absorbing the solar and thermal radiation and indirectly by changing cloud properties. Here we use ECHAM5/MESSy1-MADE, a global climate model with detailed aerosol and cloud microphysics to study the climate impacts of international shipping. The simulations show that emissions from ships significantly increase the cloud droplet number concentration of low marine water clouds by up to 5% to 30% depending on the ship emission inventory and the geographic region. Whereas the cloud liquid water content remains nearly unchanged in these simulations, effective radii of cloud droplets decrease, leading to cloud optical thickness increase of up to 5–10%. The sensitivity of the results is estimated by using three different emission inventories for present-day conditions. The sensitivity analysis reveals that shipping contributes to 2.3% to 3.6% of the total sulfate burden and 0.4% to 1.4% to the total black carbon burden in the year 2000 on the global mean. In addition to changes in aerosol chemical composition, shipping increases the aerosol number concentration, e.g. up to 25% in the size range of the accumulation mode (typically >0.1 μm) over the Atlantic. The total aerosol optical thickness over the Indian Ocean, the Gulf of Mexico and the Northeastern Pacific increases by up to 8–10% depending on the emission inventory. Changes in aerosol optical thickness caused by shipping induced modification of aerosol particle number concentration and chemical composition lead to a change in the shortwave radiation budget at the top of the atmosphere (ToA) under clear-sky condition of about −0.014 W/m² to −0.038 W/m² for a global annual average. The corresponding all-sky direct aerosol forcing ranges between −0.011 W/m² and −0.013 W/m². The indirect aerosol effect of ships on climate is found to be far larger than previously estimated. An indirect radiative effect of −0.19 W/m² to −0.60 W/m² (a change in the atmospheric shortwave radiative flux at ToA) is calculated here, contributing 17% to 39% of the total indirect effect of anthropogenic aerosols. This contribution is high because ship emissions are released in regions with frequent low marine clouds in an otherwise clean environment. In addition, the potential impact of particulate matter on the radiation budget is larger over the dark ocean surface than over polluted regions over land.

Un service d’observation des glaciers des alpes françaises « glacioclim-alpes », pour quoi faire ?

Glaciers act as very good climate indicators, and the relationship they develop with climate can be exploited at 2 levels. First of all, changes in meteorological conditions are directly recorded in glacier mass balance changes (in the form of either winter precipitation or summer ablation). Therefore, from separately measuring these two terms, climatic trends can be inferred and corresponding results are all the better as mass balance series are spanning longer periods. These mass balance changes then later express under the form of fluctuations in glacier length and thickness and/or ice surface velocities. Measuring these changes is another means of inferring climate change, although the relationship is much more complex because of a time-lagging non-linear response due to complicated processes such as ice rheology or basal gliding which only a deterministic ice-flow model can capture. Due to this specificity and complexity of the dynamical response of glaciers, using these glacier changes for climate reconstructions will also require as long as possible measurement series. It is therefore in this context of a better understanding of climate through glacier change that the glacier survey network was initiated. Three main types of measurements are undertaken ; (i) mass balance measurements with both winter accumulation and summer ablation, (ii) glacier dynamical response to mass balance change in the form of thickness and length changes as well as surface velocity measurements and last (iii) measurements of local meteorology with the help of Automatic Weather Stations. This paper describes the structure and the context in which the survey network was initiated as well as all the ensuing measurements. Some scientific applications are then presented as the results of these observations like the significant changes in surface mass balance connected to the strong changes in radiative and turbulent heat fluxes at the surface of glaciers over the last decades, or the better understanding of ice dynamics from direct inspection of glacier dynamical changes or from using these latter changes for tuning, validating and exploiting the results of an ice flow model.

Diagnosing Cloud Feedbacks in General Circulation Models

Abstract In this study, it is shown that the NCAR and GFDL GCMs exhibit a marked difference in climate sensitivity of clouds and radiative fluxes in response to doubled CO2 and ±2-K SST perturbations. The GFDL model predicted a substantial decrease in cloud amount and an increase in cloud condensate in the warmer climate, but produced a much weaker change in net cloud radiative forcing (CRF) than the NCAR model. Using a multiple linear regression (MLR) method, the full-sky radiative flux change at the top of the atmosphere was successfully decomposed into individual components associated with the clear sky and different types of clouds. The authors specifically examined the cloud feedbacks due to the cloud amount and cloud condensate changes involving low, mid-, and high clouds between 60°S and 60°N. It was found that the NCAR and GFDL models predicted the same sign of individual longwave and shortwave feedbacks resulting from the change in cloud amount and cloud condensate for all three types of clouds (low, mid, and high) despite the different cloud and radiation schemes used in the models. However, since the individual longwave and shortwave feedbacks resulting from the change in cloud amount and cloud condensate generally have the opposite signs, the net cloud feedback is a subtle residual of all. Strong cancellations between individual cloud feedbacks may result in a weak net cloud feedback. This result is consistent with the findings of the previous studies, which used different approaches to diagnose cloud feedbacks. This study indicates that the proposed MLR approach provides an easy way to efficiently expose the similarity and discrepancy of individual cloud feedback processes between GCMs, which are hidden in the total cloud feedback measured by CRF. Most importantly, this method has the potential to be applied to satellite measurements. Thus, it may serve as a reliable and efficient method to investigate cloud feedback mechanisms on short-term scales by comparing simulations with available observations, which may provide a useful way to identify the cause for the wide spread of cloud feedbacks in GCMs.

Interannual variability of smoke and warm cloud relationships in the Amazon as inferred from MODIS retrievals

Aerosol and cloud data from the MODerate resolution Imaging Spectroradiometer (MODIS) onboard the Earth Observing System (EOS) Aqua are used to investigate interannual variability of smoke and warm cloud relationships during the dry-to-wet transition season (August–October) over the Amazon for two years and its association with meteorological conditions. In one year (2003), smoke aerosols are associated with an increase of cloud fraction and a decrease of cloud effective radius. These effects amplify the cooling at the surface and at the top of the atmosphere (TOA) caused by the aerosol extinction. However, in another year (2002) the cloud fraction decreases with increasing aerosol optical depth. Such a decrease of cloud fraction could offset the effect of increased reflection of solar radiation by the aerosols both at the surface and at TOA. The changes in radiative fluxes between these years would contribute to interannual changes of surface energy fluxes and radiative balance at the top of the atmosphere and influence variability of the wet season onset in the basin. In 2003, the atmosphere was more humid and less stable. These conditions may be relatively favorable for the activation of aerosol particles into cloud condensation nuclei and hence cloud droplets. In 2002, the clouds were less extensive and thinner in a relatively dry atmosphere and presumably dissipated more easily. This study suggests that the aerosol-cloud relation can be influenced by atmospheric structure and convective motions, in addition to changes in aerosols properties. An adequate characterization of aerosol-cloud relationship would require a longer time series of data that includes a variety of climate conditions. The caveat of this analysis is that differences in aerosol absorption and its vertical distribution may have contributed to the observed interannual change of smoke-cloud relationship but could not be determined due to lack of adequate measurements.

Phytoplankton and Cloudiness in the Southern Ocean

The effect of ocean biological productivity on marine clouds is explored over a large phytoplankton bloom in the Southern Ocean with the use of remotely sensed data. Cloud droplet number concentration over the bloom was twice what it was away from the bloom, and cloud effective radius was reduced by 30%. The resulting change in the short-wave radiative flux at the top of the atmosphere was -15 watts per square meter, comparable to the aerosol indirect effect over highly polluted regions. This observed impact of phytoplankton on clouds is attributed to changes in the size distribution and chemical composition of cloud condensation nuclei. We propose that secondary organic aerosol, formed from the oxidation of phytoplankton-produced isoprene, can affect chemical composition of marine cloud condensation nuclei and influence cloud droplet number. Model simulations support this hypothesis, indicating that 100% of the observed changes in cloud properties can be attributed to the isoprene secondary organic aerosol.

Reply to comment by T. Mölg et al. on “Recent glacial recession in the Rwenzori Mountains of East Africa due to rising air temperature”

[1] Debate persists as to the extent to which recent glacial recession observed in tropical highlands is driven primarily by changes in air temperature [e.g., Bradley et al., 2006; Thompson et al., 2006] and atmospheric humidity [e.g. Kaser et al., 2004; Molg and Hardy, 2004]. Uncertainty has also been expressed in the relationship between temperature trends at the surface and higher elevations in the tropical free troposphere [e.g., Christy et al., 2003; Christy and Norris, 2004; Douglass et al., 2004; Fu et al., 2004; Tett and Thorne, 2004] where alpine glaciers reside. We therefore welcome the constructive comments of Molg et al. [2006] regarding our original paper and appreciate the opportunity to clarify arguments made therein [Taylor et al., 2006]. We agree with Molg et al. that the surface energy balance and mass balance are best able to describe the relationship between climate parameters and glacier change [e.g., Wagnon et al., 1999; Molg and Hardy, 2004]. For the Rwenzori Mountains, measurements that would form the basis of a glacier mass balance model do not exist. This point was recognized explicitly in the original paper, ‘‘The absence of continuous and proximate meteorological observations in the Rwenzori Mountains prevents direct analysis of the climatic factors driving observed glacial recession.’’ Although a definitive, quantitative understanding of the climate variables responsible for glacier mass losses in the Rwenzori Mountains remains elusive, we dispute the assertion of Molg et al. that air temperature (Ta) is unlikely to be the main driver of observed glacial recession and argue that trends of increasing air temperature are better supported by currently available evidence than decreasing humidity posited by Molg et al. [2] The essential scientific criticism of our paper by Molg et al. [2006] is the validity of the assumption that Ta trends observed in gridded CRUTS 2.0 climate data sets [New et al., 2002] and at meteorological stations between 960 and 1869 meters above sea level (masl), reflect Ta trends in the middle troposphere (4800 to 5100 masl) where glaciers in the Rwenzori Mountains occur. Molg et al. suggest that we have disregarded evidence of inconsistencies between Ta trends at the surface and in the tropical troposphere, but the literature [Hense et al., 1988; Gaffen et al., 2000; Bradley et al., 2004] and evidence they cite is selective. Significant uncertainty persists in temperature data for the tropical troposphere whether these derive from satellite-borne Microwave Sounding Unit (MSU) observations or in situ measurements using radiosondes, particularly in data-poor regions like East Africa. Indeed, linear Ta trends in the tropical troposphere can vary significantly based simply upon choice of start and end date as is the case in the paper by Gaffen et al. [2000] using MSU data in which at 500 hPa a cooling trend is detected between 1979 and 1997 but an overall warming trend occurs between 1960 and 1997. Nevertheless, recent studies that employ diurnal corrections to MSU observations between 1979 and 2003 [Mears and Wentz, 2005] and homogenized radiosonde data sets (HadAT2) between 1958 and 2002 [Thorne et al., 2005], show that the middle troposphere warmed at a similar or slightly greater rate to the surface in the tropics [Fu and Johanson, 2005; Santer et al., 2005], consistent with the sign and (within error) magnitude of Ta trends (+0.13 C per decade) at the surface from climate model (HadCRU2v) predictions [Jones and Moberg, 2003]. [3] Molg et al. [2006] use NCEP reanalysis data [Kalnay et al., 1996] for the grid cell (30 E, 0 N) to support their claim that a discrepancy exists between Ta trends at the lower troposphere (850 hPa) and mid-troposphere (600 hPa) in the Rwenzori Mountains (their Figure 1). There is, however, widespread consensus within the climate community that reanalysis data are unsuitable for trend analysis in climate change studies as ‘‘. . .known discontinuities in reanalyzed data sets indicate that further research is required to reduce time-dependent errors to a level suitable for climate change studies’’ [Intergovernmental Panel on Climate Change, 2001, p. 120]. The existence of systematic, time-varying biases in reanalysis data is also highlighted by more recent studies [Bengtsson et al., 2004; Simmons et al., 2004; Sterl, 2004; Thorne et al., 2005]. Molg et al. consider biases in the NCEP data associated with the introduction of satellite observations in 1979 to reanalysis data sets (see caption in their Figure 1) but not other inconsistencies that arise from the wide range of data sources including modeled processes [Pepin and Seidel, 2005]. In contrast to inferences drawn by Molg et al. using NCEP data, upper air temperature records from gridded HadAT2 radiosonde data [Thorne et al., 2005] for the most proximate (and only) grid cell to the Rwenzori GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L20405, doi:10.1029/2006GL027606, 2006 Click Here for Full Article

Aerosol direct radiative effects over the northwest Atlantic, northwest Pacific, and North Indian Oceans: estimates based on in-situ chemical and optical measurements and chemical transport modeling

Abstract. The largest uncertainty in the radiative forcing of climate change over the industrial era is that due to aerosols, a substantial fraction of which is the uncertainty associated with scattering and absorption of shortwave (solar) radiation by anthropogenic aerosols in cloud-free conditions (IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on climate is critical to understanding climate change over the industrial period and to improving predictions of future climate change for assumed emission scenarios. Measurements of aerosol properties during major field campaigns in several regions of the globe during the past decade are contributing to an enhanced understanding of atmospheric aerosols and their effects on light scattering and climate. The present study, which focuses on three regions downwind of major urban/population centers (North Indian Ocean (NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates understanding gained from field observations of aerosol distributions and properties into calculations of perturbations in radiative fluxes due to these aerosols. This study evaluates the current state of observations and of two chemical transport models (STEM and MOZART). Measurements of burdens, extinction optical depth (AOD), and direct radiative effect of aerosols (DRE – change in radiative flux due to total aerosols) are used as measurement-model check points to assess uncertainties. In-situ measured and remotely sensed aerosol properties for each region (mixing state, mass scattering efficiency, single scattering albedo, and angular scattering properties and their dependences on relative humidity) are used as input parameters to two radiative transfer models (GFDL and University of Michigan) to constrain estimates of aerosol radiative effects, with uncertainties in each step propagated through the analysis. Constraining the radiative transfer calculations by observational inputs increases the clear-sky, 24-h averaged AOD (34±8%), top of atmosphere (TOA) DRE (32±12%), and TOA direct climate forcing of aerosols (DCF – change in radiative flux due to anthropogenic aerosols) (37±7%) relative to values obtained with "a priori" parameterizations of aerosol loadings and properties (GFDL RTM). The resulting constrained clear-sky TOA DCF is −3.3±0.47, −14±2.6, −6.4±2.1 Wm−2 for the NIO, NWP, and NWA, respectively. With the use of constrained quantities (extensive and intensive parameters) the calculated uncertainty in DCF was 25% less than the "structural uncertainties" used in the IPCC-2001 global estimates of direct aerosol climate forcing. Such comparisons with observations and resultant reductions in uncertainties are essential for improving and developing confidence in climate model calculations incorporating aerosol forcing.

The initial dispersal and radiative forcing of a Northern Hemisphere mid-latitude super volcano: a model study

Abstract. The chemistry climate model MAECHAM4/ CHEM with interactive and prognostic volcanic aerosol and ozone was used to study the initial dispersal and radiative forcing of a possible Northern Hemisphere mid-latitude super eruption. Tropospheric climate anomalies are not analysed since sea surface temperatures are kept fixed. Our experiments show that the global dispersal of a super eruption located at Yellowstone, Wy. is strongly dependent on the season of the eruption. In Northern Hemisphere summer the volcanic cloud is transported westward and preferentially southward, while in Northern Hemisphere winter the cloud is transported eastward and more northward compared to the summer case. Aerosol induced heating leads to a more global spreading with a pronounced cross equatorial transport. For a summer eruption aerosol is transported much further to the Southern Hemisphere than for a winter eruption. In contrast to Pinatubo case studies, strong cooling tendencies appear with maximum peak values of less than −1.6 K/day three months after the eruption in the upper tropical stratosphere. This strong cooling effect weakens with decreasing aerosol density over time and initially prevents the aerosol laden air from further active rising. All-sky net radiative flux changes of less than −32 W/m2 at the surface are about a factor of 6 larger than for the Pinatubo eruption. Large positive flux anomalies of more than 16 W/m2 are found in the first months in the tropics and sub tropics. These strong forcings call for a fully coupled ocean/atmosphere/chemistry model to study climate sensitivity to such a super-eruption.

The Sensitivity of the Tropical-Mean Radiation Budget

AbstractA key disagreement exists between global climate model (GCM) simulations and satellite observations of the decadal variability in the tropical-mean radiation budget. Measurements from the Earth Radiation Budget Experiment (ERBE) over the period 1984–2001 indicate a trend of increasing longwave emission and decreasing shortwave reflection that no GCM can currently reproduce. Motivated by these results, a series of model sensitivity experiments is performed to investigate hypotheses that have been advanced to explain this discrepancy. Specifically, the extent to which a strengthening of the Hadley circulation or a change in convective precipitation efficiency can alter the tropical-mean radiation budget is assessed. Results from both model sensitivity experiments and an empirical analysis of ERBE observations suggest that the tropical-mean radiation budget is remarkably insensitive to changes in the tropical circulation. The empirical estimate suggests that it would require at least a doubling in strength of the Hadley circulation in order to generate the observed decadal radiative flux changes. In contrast, rather small changes in a model’s convective precipitation efficiency can generate changes comparable to those observed, provided that the precipitation efficiency lies near the upper end of its possible range. If, however, the precipitation efficiency of tropical convective systems is more moderate, the model experiments suggest that the climate would be rather insensitive to changes in its value. Further observations are necessary to constrain the potential effects of microphysics on the top-of-atmosphere radiation budget.

Preliminary study of cosmic rays, geomagnetic field changes and possible climate changes

The possibility of a connection between cosmic radiation and climate has intrigued scientists for the past several decades. The studies of Friis-Christensen and Svensmark reported a variation of 3–4% in the global cloud cover between 1980 and 1995 that appeared to be directly correlated with the change in galactic cosmic radiation flux over the solar cycle. However, not only the solar cycle modulation of cosmic radiation must be considered, but also the changes in the cosmic radiation impinging at the top of the atmosphere as a result of the long-term evolution of the geomagnetic field. We present preliminary results of an on-going study of geomagnetic cutoff rigidities over a 400-year interval. These results show: (1) the change in cutoff rigidity is sufficiently large over this 400-year period that the change in cosmic radiation flux impacting the earth is approximately equal to the relative change in flux over a solar cycle, and (2) the changes in cutoff rigidity are non-uniform over the globe with both significant increases and decreases at mid-latitude locations.

Large Aerosol Radiative Forcing due to the 1997 Indonesian Forest Fire

During the last decade, the feedback between El Niño and biomass burning caused the Indonesia's forest fire aerosols to be the second most significant source of anthropogenic aerosol over the tropical Indian Ocean after the South Asian Haze. In this paper, the estimates of the radiative forcing during the 1997 Indonesia's forest fire have been obtained by integrating satellite derived aerosol optical depths and cloud cover with in‐situ observations of single scattering albedo and a Monte‐Carlo Aerosol‐Cloud radiation model. The haze reduced the seasonal average solar radiation absorbed by the equatorial Indian ocean by as much as 30 to 60 W m−2 during September to November 1997, and increased the atmospheric solar heating by as much as 50% to 100% within the first 3 kilometers. The radiative forcing at the top of the atmosphere (TOA) was in the range of 5 to 15 W m−2 under cloudy skies. The significance of such large radiative flux changes to the tropical ocean‐atmosphere heat budget and climate needs to be examined with climate models.

Enhanced climatic warming in the Tibetan Plateau due to doubling CO2: a model study

The National Center for Atmospheric Research (NCAR) regional climate model (RegCM2), together with initial conditions and time-dependent lateral boundary conditions provided by a 130-year transient increasing CO2 simulation of the NCAR Climate System Model (CSM), has been used to investigate the mechanism of ground warming over the Tibetan Plateau (TP). The model results show that when CO2 in the atmosphere is doubled, a strong ground warming occurs in the TP. Two regions within it with the largest warming are in the eastern TP (region I) and along the southwestern and western slopes (region II). Moreover, in region I the ground warming in the winter half year is stronger than that in the summer half year, but in region II the warming difference between the seasons becomes opposite to that in region I, i.e., the warming is strong in the summer half year and weak in the winter half year. There are indications that the summer monsoon enhances but the winter monsoon weakens when CO2 is doubled. A strong elevation dependency of ground warming is found in region I for the winter half year, and in region II for both winter and summer half years at elevations below 5 km. The simulated characteristics of ground warming in the TP are consistent with the observations. In region I, when CO2 is doubled, the cloud amount increases at lower elevations and decreases at higher elevation for the winter half year. As a consequence, at lower elevations the short wave solar radiation absorbed at the surface declines, and the downward long wave flux reaching the surface enhances; on the other hand, at higher elevations the surface solar radiation flux increases and the surface infrared radiation flux shows a more uniform increase. The net effect of the changes in both radiation fluxes is an enhanced surface warming at higher elevations, which is the primary cause of the elevation dependency in the surface warming. In the summer half year the cloud amount reduces as a result of doubling CO2 in region I for all elevations, and there is no elevation dependency detected in the ground warming. Furthermore, there is little snow existing in region I for both summer and winter half years, and the impact of snow-albedo feedback is not significant. In region II, although the changes in the cloud amount bear a resemblance to those in region I, the most significant factor affecting the surface energy budget is the depletion of the snow cover at higher elevations, which leads to a reduction of the surface albedo. This reduction in turn leads to an enhancement in the solar radiation absorbed in the surface. The snow-albedo feedback mechanism is the most essential cause of the elevation dependency in the surface warming for region II.

The roles of radiation and dynamical processes in the El Niño-like response to global warming

The current understanding of ENSO does not foreshadow how it might change as a consequence of global warming due to an increase in atmospheric greenhouse gas (GHG) concentration. A number of global coupled climate models simulate a "mean" El Nino-like change in tropical Pacific temperatures, precipitation, and winds but at least one model exhibits a La Nina-like pattern and others a more or less homogeneous warming in the tropics with little of either pattern. The mechanisms leading to a mean positive El Nino-like pattern (PEP) are studied in simulations with the Canadian Centre for Climate Modeling and Analysis (CCCma) coupled general circulation model. The changes associated with the PEP are compared with, and are shown to closely resemble, those observed for the positive El Nino phase of the ENSO oscillation in the current climate including the anomalies in SST, precipitation and atmospheric circulation, the changes of vertically integrated energy and heat transports in the atmosphere, and changes in the sign and magnitude of radiative energy balance terms. The PEP in the model is supported by changes in oceanic heat transport and surface longwave radiative flux in the face of solar radiative flux and evaporative flux changes which act to damp it away. There is negative cloud-radiation feedback associated with the PEP, as with the observed El Nino. Negative cloud feedback by itself does not, therefore, preclude the existence of a PEP response to GHG forcing. The climatological PEP does not exhibit an oceanic export of energy from the tropical Pacific, as inferred for the regular El Nino event, but rather an oceanic import of energy. Nevertheless the PEP provides an effective means of regulating climate warming and the energy budget in the tropical Pacific which is accomplished through energy transports out of the region by the atmosphere. The PEP is seen as a more or less straightforward manifestation of the feedback mechanism proposed by Bjerknes and as a physically plausible response to GHG-induced climate warming.

The effect of the solar eclipse on the air temperature near the ground

This paper describes the effect of the air temperature decrease in low boundary layer during the solar eclipse with special regard to influence on convectional events. The phenomenon progress was modelled to predict solar radiation flux changes. Then the basic model of local ground and air temperature changes was constructed. The qualitative features of air temperature–time curve during the eclipse were explained. The effect was investigated experimentally on the example of the partial eclipse observed from Szczawnica, Poland on 11th August, 1999. The results of the precise air temperature measurements were presented. The general shape of temperature curve was confirmed. The problem of convection intensity and temporal scales of convectional events was examined. It was observed that the temperature variance decreased over a factor of 2 in the maximum eclipse- centred, 50 min long time interval which depicts the reduced convection regime. In addition, the temperature spectrum for long periods obtained for this time range seem to differ significantly from one registered before and after. The convection near the maximum eclipse is characterised by a dominant temporal scale of 22 min , while before and after 11– 13 min scale is the most important.

Aerosol Radiative Forcing Derived from SeaWiFS-Retrieved Aerosol Optical Properties

To understand climatic implications of aerosols over global oceans, the aerosol optical properties retrieved from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) are analyzed, and the effects of the aerosols on the Earth's radiation budgets (aerosol radiative forcing, ARF) are computed using a radiative transfer model. It is found that the distribution of the SeaWiFS-retrieved aerosol optical thickness is distinctively zonal. The maximum in the equatorial region coincides with the Intertropical Convergence Zone, and the maximum in the Southern Hemispheric high latitudes coincides with the region of prevailing westerlies. The minimum aerosol optical thickness is found in the subtropical high pressure regions, especially in the Southern Hemisphere. These zonal patterns clearly demonstrate the influence of atmospheric circulation on the oceanic aerosol distribution. Over global oceans, aerosols reduce the annual mean net downward solar flux by 5.4 W m-2 at the top of the atmosphere and by 6.1 W m-2 at the surface. The largest ARF is found in the tropical Atlantic, Arabian Sea, Bay of Bengal, the coastal regions of Southeast and East Asia, and the Southern Hemispheric high latitudes. During the period of the Indonesian big fires (September-December 1997), the cooling due to aerosols is greater than 15 W m-2 at the top of the atmosphere and greater than 30 W m(exp -1) at the surface in the vicinity of the maritime continents. The atmosphere receives extra solar radiation by greater than 15 W m(exp -1) over a large area. These large changes in radiative fluxes are expected to have enhanced the atmospheric stability, weakened the atmospheric circulation, and augmented the drought condition during that period. It would be very instructive to simulate the regional climatic. The model-calculated clear sky solar flux at the top of the atmosphere is compared with that derived from the Clouds and the Earth's Radiant Energy System (CERES). The net downward solar flux of CERES is systematically larger than the model calculations by -3 W M-2. In the equatorial region, the CERES-derived net downward solar flux is even larger than the model calculations without including aerosols. It is possible that the CERES incorrectly identified regions of high humidity and high aerosol concentration as being cloud contaminated and, hence, overestimated the clear sky net downward solar flux.

赤外放射加熱される平板状含水物中の放射照度及び発熱パワーの分布特性

Short-, medium-, and long-wave infrared emitters have different effects on the temperature-rise and drying-rate characteristics in infrared radiation heating of water containing flat material. Accordingly they also significantly affect thermal efficiency. The heating characteristics also depend on the thickness of the heated material. It is known that absorbed radiation flux changes to heat within the heated substance, the absorption depends to a large extent on the amount of water in the substance, and the Lambert absorption coefficient of water varies significantly between the three wave-length regions. The coefficient is higher in the long-wave region, moderate in the medium one, and much lower in the short one. We assumed that the infrared emitter used was a black or perfectly grey body and that the heated substance acted as free water, and calculated the irradiance and generated heating-power distributions within the substance. The thickness in millimeters of the substance needed to absorb about 90% of the incident radiation flux is indicated by D's in the Figure and Table. The heating-power generated at the irradiated surface when heated by a long-wave emitter was noticeably higher. The high drying-rate, which is characteristic of long-wave emitter heating, is attributed to the concentrated heating-power at the irradiated surface. This high heating-power provides large latent heat for evaporation. The obtained results of Figures and Tables give basic and useful Informations for choice in advance of the emitter kind between the three ones in infrared heating of a water contained substance with a given thickness.

Thermodynamic and Radiative Impact of the Correction of Sounding Humidity Bias in the Tropics

Accurate measurements of atmospheric water vapor are crucial to many aspects of climate research and atmospheric science. This paper discusses some of the meteorological implications of a bias discovered in the measurement of water vapor in widely deployed radiosonde systems. This problem apparently arose in the early 1990s and a correction scheme has been recently developed that intends to remove the bias. The correction scheme also includes improvements in the humidity measurements in the upper troposphere and near the surface. This scheme has been applied to data taken during the Tropical Ocean and Global Atmosphere Coupled Ocean­Atmosphere Response Experiment (TOGA COARE). The impact of the bias on the general stability of the tropical atmosphere to deep convection, as measured by the convective available potential energy (CAPE) and the convective inhibition (CIN), is quite large. On the basis of the uncorrected data set, one might erroneously conclude that it is difficult to trigger deep convection over the region. When the correction is taken into account, the atmosphere over the tropical western Pacific becomes typically unstable to deep convection, with convective instability similar to that measured from aircraft in the vicinity of active convective sytems. Radiative fluxes are also significantly modified. For clear sky conditions, it is found that, on average, the net surface radiative flux increases by 4 W m-2 due to the humidity correction. Under more realistic cloudy conditions, the differences are weaker, but still significant. Changes in radiative fluxes are explained at first order by the the precipitable water increase.

Case studies of the vertical structure of the direct shortwave aerosol radiative forcing during TARFOX

The vertical structure of aerosol‐induced radiative flux changes in the Earth's troposphere affects local heating rates and thereby convective processes, the formation and lifetime of clouds, and hence the distribution of chemical constituents. We present observationally based estimates of the vertical structure of direct shortwave aerosol radiative forcing for two case studies from the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) which took place on the U.S. east coast in July 1996. The aerosol radiative forcings are computed using the Fu‐Liou broadband radiative transfer model. The aerosol optical properties used in the radiative transfer simulations are calculated from independent vertically resolved estimates of the complex aerosol indices of refraction in two to three distinct vertical layers, using profiles of in situ particle size distributions measured aboard the University of Washington research aircraft. Aerosol single‐scattering albedos at 450 nm thus determined range from 0.9 to 0.985, while the asymmetry factor varies from 0.6 to 0.8. The instantaneous shortwave aerosol radiative forcings derived from the optical properties of the aerosols are of the order of −36 W m−2 at the top of the atmosphere and about −56 W m−2 at the surface for both case studies.

Retrieving the vertical structure of the effective aerosol complex index of refraction from a combination of aerosol in situ and remote sensing measurements during TARFOX

The largest uncertainty in estimates of the effects of atmospheric aerosols on climate stems from uncertainties in the determination of their microphysical properties, including the aerosol complex index of refraction, which in turn determines their optical properties. A novel technique is used to estimate the aerosol complex index of refraction in distinct vertical layers from a combination of aerosol in situ size distribution and remote sensing measurements during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX). In particular, aerosol backscatter measurements using the NASA Langley LASE (Lidar Atmospheric Sensing Experiment) instrument and in situ aerosol size distribution data are utilized to derive vertical profiles of the “effective” aerosol complex index of refraction at 815 nm (i.e., the refractive index that would provide the same backscatter signal in a forward calculation on the basis of the measured in situ particle size distributions for homogeneous, spherical aerosols). A sensitivity study shows that this method yields small errors in the retrieved aerosol refractive indices, provided the errors in the lidar‐derived aerosol backscatter are less than 30% and random in nature. Absolute errors in the estimated aerosol refractive indices are generally less than 0.04 for the real part and can be as much as 0.042 for the imaginary part in the case of a 30% error in the lidar‐derived aerosol backscatter. The measurements of aerosol optical depth from the NASA Ames Airborne Tracking Sunphotometer (AATS‐6) are successfully incorporated into the new technique and help constrain the retrieved aerosol refractive indices. An application of the technique to two TARFOX case studies yields the occurrence of vertical layers of distinct aerosol refractive indices. Values of the estimated complex aerosol refractive index range from 1.33 to 1.45 for the real part and 0.001 to 0.008 for the imaginary part. The methodology devised in this study provides, for the first time, a complete set of vertically resolved aerosol size distribution and refractive index data, yielding the vertical distribution of aerosol optical properties required for the determination of aerosol‐induced radiative flux changes.