Changes In Cloudiness Research Articles

Comparison of Four Cloud Schemes in Simulating the Seasonal Mean Field Forced by the Observed Sea Surface Temperature

Abstract The impacts of four stratiform cloud parameterizations on seasonal mean fields are investigated using the global version of the Experimental Climate Prediction Center (ECPC) global-to-regional forecast system (G-RSM). The simulated fields are compared with the International Satellite Cloud Climatology Project (ISCCP) data for clouds, the Global Precipitation Climatology Project data for precipitation, the Earth Radiation Budget Experiment and the Surface Radiation Budget data for radiation, and the National Centers for Environmental Prediction (NCEP)–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) Reanalysis (R-2) for temperature. Compared to observations, no stratiform cloud parameterization performed better in simulating all aspects of clouds, temperature, precipitation, and radiation fluxes. There are strong interactions between parameterized stratiform clouds and boundary layer clouds and convection, resulting in changes in low-level cloudiness and precipitation in the simulations. When the simulations are compared with ISCCP cloudiness and cloud water, and the NCEP/DOE R-2 relative humidity, the cloud amounts simulated by all four cloud schemes depend mostly on relative humidity with less dependency on the model’s cloud water, while the observed cloud amount is more strongly dependent on cloud water than relative humidity, suggesting that cloud parameterizations and the simulation of cloud water require further improvement.

Luftverkehr und Klima. Atmosphärenforschung

AbstractDer globale Luftverkehr hat zum bisherigen Klimawandel nicht nur durch Ausstoß von Kohlendioxid, sondern auch durch Zusatzeffekte aus Stickoxiden, Kondensstreifen und Veränderungen der Bewölkung beigetragen. An der Gesamtheit der von Menschen verursachten Klimaänderung ist der Luftverkehr bisher mit etwa 3 % (2 bis 8 %) beteiligt. Der Anteil der Zusatzeffekte hängt vom Wachstum des Flugverkehrs und der betrachten Zeitspanne in der Zukunft ab und kann nicht mit einem konstanten Faktor bewertet werden. Bei raschem Verkehrswachstum sind die Zusatzeffekte groß, bei langsamerem Wachstum und über lange Zeiträume dominiert das ausgestoßene Kohlendioxid die Klimawirkung des Luftverkehrs.

Climate change and tropical Andean glaciers: Past, present and future

Observations on glacier extent from Ecuador, Peru and Bolivia give a detailed and unequivocal account of rapid shrinkage of tropical Andean glaciers since the Little Ice Age (LIA). This retreat however, was not continuous but interrupted by several periods of stagnant or even advancing glaciers, most recently around the end of the 20th century. New data from mass balance networks established on over a dozen glaciers allows comparison of the glacier behavior in the inner and outer tropics. It appears that glacier variations are quite coherent throughout the region, despite different sensitivities to climatic forcing such as temperature, precipitation, humidity, etc. In parallel with the glacier retreat, climate in the tropical Andes has changed significantly over the past 50–60 years. Temperature in the Andes has increased by approximately 0.1 °C/decade, with only two of the last 20 years being below the 1961–90 average. Precipitation has slightly increased in the second half of the 20th century in the inner tropics and decreased in the outer tropics. The general pattern of moistening in the inner tropics and drying in the subtropical Andes is dynamically consistent with observed changes in the large-scale circulation, suggesting a strengthening of the tropical atmospheric circulation. Model projections of future climate change in the tropical Andes indicate a continued warming of the tropical troposphere throughout the 21st century, with a temperature increase that is enhanced at higher elevations. By the end of the 21st century, following the SRES A2 emission scenario, the tropical Andes may experience a massive warming on the order of 4.5–5 °C. Predicted changes in precipitation include an increase in precipitation during the wet season and a decrease during the dry season, which would effectively enhance the seasonal hydrological cycle in the tropical Andes. These observed and predicted changes in climate affect the tropical glacier energy balance through its sensitivity to changes in atmospheric humidity (which governs sublimation), precipitation (whose variability induces a positive feedback on albedo) and cloudiness (which controls the incoming long-wave radiation). In the inner tropics air temperature also significantly influences the energy balance, albeit not through the sensible heat flux, but indirectly through fluctuations in the rain–snow line and hence changes in albedo and net radiation receipts. Given the projected changes in climate, based on different IPCC scenarios for 2050 and 2080, simulations with a tropical glacier–climate model indicate that glaciers will continue to retreat. Many smaller, low-lying glaciers are already completely out of equilibrium with current climate and will disappear within a few decades. But even in catchments where glaciers do not completely disappear, the change in streamflow seasonality, due to the reduction of the glacial buffer during the dry season, will significantly affect the water availability downstream. In the short-term, as glaciers retreat and lose mass, they add to a temporary increase in runoff to which downstream users will quickly adapt, thereby raising serious sustainability concerns.

Temporal and spatial characteristics of positive and negative Indian Ocean dipole with and without ENSO

The differences in the temporal evolution and spatial characteristics of the Indian Ocean Dipole (IOD) between positive and negative events with and without ENSO have been investigated using observations for the period 1948–2002. To document such differences is particularly important for climate forecasts over far east Asia, since distinctly different monsoon activities over China, Korea, and Japan for different types of IOD are found in the composite maps of precipitation anomalies. The composite map of SST and wind during various stages of IOD and the ocean mixed layer heat budget showed that the IOD with and without ENSO has a large difference in its temporal evolutions and their triggering mechanisms. In both negative and positive IOD events without ENSO, the wind anomaly in the eastern Indian Ocean seems to be responsible for the formation of sea surface temperature anomalies, while the anomaly in the western Indian Ocean seems to be the oceanic dynamical response to the anomaly in the east. During the ENSO years, the temporal and spatial contrast of the asymmetry of the IOD evolution is smaller, and the SST anomaly is driven by the anomalies in incoming radiation due to changes in cloudiness caused by the ENSO associated anomalous atmospheric circulations and not by the local wind anomalies.

Spatial dependence of diurnal temperature range trends on precipitation from 1950 to 2004

This paper analyzes the spatial dependence of annual diurnal temperature range (DTR) trends from 1950- 2004 on the annual climatology of three variables: pre- cipitation, cloud cover, and leaf area index (LAI), by classifying the global land into various climatic regions based on the climatological annual precipitation. The regional average trends for annual minimum temperature (Tmin) and DTR exhibit significant spatial correlations with the climatological values of these three variables, while such correlation for annual maximum temperature (Tmax )i s very weak. In general, the magnitude of the downward trend of DTR and the warming trend of Tmin decreases with increasing precipitation amount, cloud cover, and LAI, i.e., with stronger DTR decreasing trends over drier regions. Such spatial dependence of Tmin and DTR trends on the climatological precipitation possibly reflects large-scale effects of increased global greenhouse gases and aerosols (and associated changes in cloudiness, soil moisture, and water vapor) during the later half of the twentieth century.

The exchange of carbon dioxide between wet arctic tundra and the atmosphere at the Lena River Delta, Northern Siberia

Abstract. The exchange fluxes of carbon dioxide between wet arctic polygonal tundra and the atmosphere were investigated by the micrometeorological eddy covariance method. The investigation site was situated in the centre of the Lena River Delta in Northern Siberia (72°22' N, 126°30' E). The study region is characterized by a polar and distinctly continental climate, very cold and ice-rich permafrost and its position at the interface between the Eurasian continent and the Arctic Ocean. The soils at the site are characterized by high organic matter content, low nutrient availability and pronounced water logging. The vegetation is dominated by sedges and mosses. The micrometeorological campaigns were performed during the periods July–October 2003 and May–July 2004 which included the period of snow and soil thaw as well as the beginning of soil refreeze. The main CO2 exchange processes, the gross photosynthesis and the ecosystem respiration, were found to be of a generally low intensity. The gross photosynthesis accumulated to −432 g m−2 over the photosynthetically active period (June–September). The contribution of mosses to the gross photosynthesis was estimated to be about 40%. The diurnal trend of the gross photosynthesis was mainly controlled by the incoming photosynthetically active radiation. During midday, the photosynthetic apparatus of the canopy was frequently near saturation and represented the limiting factor on gross photosynthesis. The synoptic weather conditions strongly affected the exchange fluxes of CO2 by changes in cloudiness, precipitation and pronounced changes of air temperature. The ecosystem respiration accumulated to +327 g m−2 over the photosynthetically active period, which corresponds to 76% of the CO2 uptake by photosynthesis. However, the ecosystem respiration continued at substantial rates during autumn when photosynthesis had ceased and the soils were still largely unfrozen. The temporal variability of the ecosystem respiration during summer was best explained by an exponential function with surface temperature, and not soil temperature, as the independent variable. This was explained by the major role of the plant respiration within the CO2 balance of the tundra ecosystem. The wet polygonal tundra of the Lena River Delta was observed to be a substantial CO2 sink with an accumulated net ecosystem CO2 exchange of −119 g m−2 over the summer and an estimated annual net ecosystem CO2 exchange of −71 g m−2.

Estimating sub‐canopy shortwave irradiance to melting snow on forested slopes

AbstractEstimates of shortwave irradiance energy beneath needle‐leaf forests over complex terrain are needed to drive energy balance snowmelt models and to evaluate the potential hydrological impacts of forest‐cover change in mountain regions. This paper outlines and evaluates a physically‐based model designed to estimate sub‐canopy shortwave irradiance to snowcover under needle‐leaf forest‐cover with respect to surface slope and azimuth. Transmission of above‐canopy irradiance was estimated using forest‐surveys and hemispherical photographs to determine the fractions of forest‐cover occupied by non‐transmitting trunks, partially‐transmitting crowns and fully‐transmitting gaps with respect to above‐canopy diffuse and direct beam shortwave irradiance. Simulations were conducted for continuous, uniform lodgepole pine forests on level site and a north facing slope and a discontinuous, non‐uniform forest on a southeast facing slope during snowmelt at the Marmot Creek Research Basin, Alberta, Canada. Mean observed daily transmissivity values of irradiance were 0·09 at the north‐facing forest, 0·21 at the level forest and 0·36 at the southeast‐facing forest. Modelled and observed results indicate that potential snowmelt energy from sub‐canopy shortwave irradiance is likely to exhibit the greatest variation with change in cloudiness and forest‐cover density under south‐facing forests and the least variation under north‐facing forests. Comparisons of simulations to observations indicate that the model can explain much of the difference in daily shortwave transmission amongst sites, performing relatively poorest at the north‐facing forest where fluxes were small and relatively best at the south‐east facing forest where fluxes were large. However, simulation errors in terms of absolute irradiance were greatest at the southeast‐facing forest, having a root mean square error (RMSE) 0·64 MJ m−2d−1 compared to 0·44 MJ m−2d−1 at the level forest and 0·27 MJ m−2d−1 at the north‐facing forest. Copyright © 2007 John Wiley & Sons, Ltd.

Estimating photosynthetically available radiation at the ocean surface from ADEOS-II global imager data

A simple, yet efficient and fairly accurate algorithm is presented to estimate photosynthetically available radiation (PAR) at the ocean surface from Global Imager (GLI) data. The algorithm utilizes plane-parallel radiation-transfer theory and separates the effects of the clear atmosphere and clouds, i.e., the planetary atmosphere is modeled as a clear atmosphere positioned above a cloud layer. PAR is computed as the difference between the incident 400–700 nm solar flux at the top of the atmosphere (known) and the solar flux reflected back to space by the atmosphere and surface (derived from GLI radiance), taking atmospheric absorption into account. Knowledge of pixel composition is not required, eliminating the need for cloud screening and arbitrary assumptions about sub-pixel cloudiness. For each GLI pixel, clear or cloudy, a daily PAR estimate is obtained. Diurnal changes in cloudiness are taken into account statistically, using a regional diurnal albedo climatology based on 5 years of Earth Radiation Budget Satellite (ERBS) data. The algorithm results are verified against other satellite estimates of PAR, the National Centers for Environmental Prediction (NCEP) reanalysis product, and in-situ measurements from fixed buoys. Agreement is generally good between GLI and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) estimates, with root-mean-squared (rms) differences of 7.9 (22%), 4.6 (13%), and 2.7 (8%) Einstein/m2/day on daily, weekly, and monthly time scales, and a bias of only 0.8–0.9 (about 2%) Einstein/m2/day. The rms differences between GLI and Visible and Infrared Spin Scan Radiometer (VISSR) estimates and between GLI and NCEP estimates are smaller and larger, respectively, on monthly time scales, i.e., 3.0 (7%) and 5.0 (14%) Einstein/m2/day, and biases are 1.1 (2%) and −0.2 (−1%) Einstein/m2/day. The comparison with buoy data also shows good agreement, with rms inaccuracies of 10.2 (23%), 6.3 (14%), and 4.5 (10%) Einstein/m2/day on daily, weekly, and monthly time scales, and slightly higher GLI values by about 1.0 (2%) Einstein/m2/day. The good statistical performance makes the algorithm suitable for large-scale studies of aquatic photosynthesis.

Annual and Interannual Behavior of Solar Ultraviolet Irradiance Revealed by Broadband Measurements¶

This research examines the behavior of ground-level solar UV radiation as measured by eight broadband meters in the continental United States during the period from late 1994 to late 1998. The goal is to define the variability that occurs in UV irradiance over time scales ranging from one to several years. The monthly integrated irradiances, from latitude 32°N to 47°N, contain large annual cycles and latitudinal gradients which depend on season. Seven of the eight sites show a maximum in July, a behavior related to proximity to the summer solstice, with modifications associated with the annual cycle in column ozone. A large interannual variability in monthly integrated irradiance appears over the 4 year period studied. A comparison of corresponding months during different years shows differences in irradiance of 20% or more in one-third of the cases analyzed. When the solar zenith angle (SZA) is held fixed in the range 60–65°, a substantial annual cycle in UV irradiance remains where the maximum monthly mean irradiance is 1.4–1.9 times the minimum, depending on location. Furthermore, the annual cycle at fixed SZA is not in phase with the normal seasonal cycle. Maximum irradiances at fixed SZA tend to occur in the October to December period, while minima cluster in April through July. The annual cycle in ozone, with maximum column values in spring and minima in autumn, explains the general character of the fixed-SZA data, although changes in cloudiness are significant contributors to interannual variability.

Changes in biologically-active ultraviolet radiation reaching the Earth’s surface

The Montreal Protocol is working. Concentrations of major ozone-depleting substances in the atmosphere are now decreasing, and the decline in total column amounts seen in the 1980s and 1990s at mid-latitudes has not continued. In polar regions, there is much greater natural variability. Each spring, large ozone holes continue to occur in Antarctica and less severe regions of depleted ozone continue to occur in the Arctic. There is evidence that some of these changes are driven by changes in atmospheric circulation rather than being solely attributable to reductions in ozone-depleting substances, which may indicate a linkage to climate change. Global ozone is still lower than in the 1970s and a return to that state is not expected for several decades. As changes in ozone impinge directly on UV radiation, elevated UV radiation due to reduced ozone is expected to continue over that period. Long-term changes in UV-B due to ozone depletion are difficult to verify through direct measurement, but there is strong evidence that UV-B irradiance increased over the period of ozone depletion. At unpolluted sites in the southern hemisphere, there is some evidence that UV-B irradiance has diminished since the late 1990s. The availability and temporal extent of UV data have improved, and we are now able to evaluate the changes in recent times compared with those estimated since the late 1920s, when ozone measurements first became available. The increases in UV-B irradiance over the latter part of the 20th century have been larger than the natural variability. There is increased evidence that aerosols have a larger effect on surface UV-B radiation than previously thought. At some sites in the Northern Hemisphere, UV-B irradiance may continue to increase because of continuing reductions in aerosol extinctions since the 1990s. Interactions between ozone depletion and climate change are complex and can be mediated through changes in chemistry, radiation, and atmospheric circulation patterns. The changes can be in both directions: ozone changes can affect climate, and climate change can affect ozone. The observational evidence suggests that stratospheric ozone (and therefore UV-B) has responded relatively quickly to changes in ozone-depleting substances, implying that climate interactions have not delayed this process. Model calculations predict that at mid-latitudes a return of ozone to pre-1980 levels is expected by the mid 21st century. However, it may take a decade or two longer in polar regions. Climate change can also affect UV radiation through changes in cloudiness and albedo, without involving ozone and since temperature changes over the 21st century are likely to be about 5 times greater than in the past century. This is likely to have significant effects on future cloud, aerosol and surface reflectivity. Consequently, unless strong mitigation measures are undertaken with respect to climate change, profound effects on the biosphere and on the solar UV radiation received at the Earth's surface can be anticipated. The future remains uncertain. Ozone is expected to increase slowly over the decades ahead, but it is not known whether ozone will return to higher levels, or lower levels, than those present prior to the onset of ozone depletion in the 1970s. There is even greater uncertainty about future UV radiation, since it will be additionally influenced by changes in aerosols and clouds.

Reconstruction of daily solar UV irradiation from 1893 to 2002 in Potsdam, Germany

Long-term records of solar UV radiation reaching the Earth's surface are scarce. Radiative transfer calculations and statistical models are two options used to reconstruct decadal changes in solar UV radiation from long-term records of measured atmospheric parameters that contain information on the effect of clouds, atmospheric aerosols and ground albedo on UV radiation. Based on earlier studies, where the long-term variation of daily solar UV irradiation was derived from measured global and diffuse irradiation as well as atmospheric ozone by a non-linear regression method [Feister et al. (2002) Photochem Photobiol 76:281-293], we present another approach for the reconstruction of time series of solar UV radiation. An artificial neural network (ANN) was trained with measurements of solar UV irradiation taken at the Meteorological Observatory in Potsdam, Germany, as well as measured parameters with long-term records such as global and diffuse radiation, sunshine duration, horizontal visibility and column ozone. This study is focussed on the reconstruction of daily broad-band UV-B (280-315 nm), UV-A (315-400 nm) and erythemal UV irradiation (ER). Due to the rapid changes in cloudiness at mid-latitude sites, solar UV irradiance exhibits appreciable short-term variability. One of the main advantages of the statistical method is that it uses doses of highly variable input parameters calculated from individual spot measurements taken at short time intervals, which thus do represent the short-term variability of solar irradiance.

Simulating the response of a closed‐basin lake to recent climate changes in tropical West Africa (Lake Bosumtwi, Ghana)

AbstractHistorical changes in the level of Lake Bosumtwi, Ghana, have been simulated using a catchment‐scale hydrological model in order to assess the importance of changes in climate and land use on lake water balance on a monthly basis for the period 1939–2004. Several commonly used models for computing evaporation in data‐sparse regions are compared, including the Penman, the energy budget, and the Priestley–Taylor methods. Based on a comparison with recorded lake level variations, the model with the energy‐budget evaporation model subcomponent is most effective at reproducing observed lake level variations using regional climate records. A sensitivity analysis using this model indicates that Lake Bosumtwi is highly sensitive to changes in precipitation, cloudiness and temperature. However, the model is also sensitive to changes in runoff related to vegetation, and this factor needs to be considered in simulating lake level variations. Both interannual and longer‐term changes in lake level over the last 65 years appear to have been caused primarily by changes in precipitation, though the model also suggests that the drop in lake level over the last few decades has been moderated by changes in cloudiness and temperature over that time. Based on its effectiveness at simulating the magnitude and rate of lake level response to changing climate over the historical record, this model offers a potential future opportunity to examine the palaeoclimatic factors causing past lake level fluctuations preserved in the geological record at Lake Bosumtwi. Copyright © 2006 John Wiley & Sons, Ltd.

UV climatology at McMurdo Station, Antarctica, based on version 2 data of the National Science Foundation's Ultraviolet Radiation Monitoring Network

Spectral ultraviolet (UV) and visible irradiance has been measured near McMurdo Station, Antarctica, between 1989 and 2004 with a SUV‐100 spectroradiometer. The instrument is part of the U.S. National Science Foundation's UV Monitoring Network. Here we present a UV climatology for McMurdo based on the recently produced “version 2” data edition. Compared to the previously published “version 0” data set, version 2 data differ by −5 to 12% in the UV, depending on wavelength, solar zenith angle (SZA), and year. A comparison with results of a radiative transfer model confirmed that measurements of different years are consistent to within ±5%. Clear‐sky spectra measured between October 1991 and March 1992 were significantly lower than spectra of other years because of the presence of volcanic aerosols. Total ozone column was calculated from UV spectra and was found in excellent agreement with collocated measurements of a Dobson spectrophotometer and satellite observations. Effective surface albedo was also estimated from clear‐sky spectra. Monthly average albedo ranges between 0.69 for March and 0.84 for October. Biologically effective UV radiation is largest in November and December when low total ozone amounts coincide with relatively small SZAs. During these months, the noon‐time UV Index typically varies between 2 and 5.5, but indices as high as 7.5 have been observed. The largest erythemal daily dose of 6.7 kJ/m2 was measured on 28 November 1998. Linear regression analyses did not indicate statistically significant trends in UV nor visible radiation for the months September to January. For February and March, we found large, statistically significant positive trends in the UV and visible as well as for short‐wave (0.3–3.0 μm) irradiance, ranging between 12 and 30% per decade. These trends are likely caused by changes in cloudiness and/or surface albedo, but the data do not allow unambiguous attribution of the increase to one of the two factors. On average, clouds reduce UV irradiance at 345 nm by 10% compared to clear‐sky levels. Reductions vary substantially by month and year, can exceed 60% on rare occasions, and generally increase with wavelength. Between September and November, the variability in UV introduced by changes in total ozone is about twice as high as the UV variability due to clouds.

Changes in Low Cloudiness over China between 1971 and 1996

Abstract The climatology and long-term trends of low-cloud conditions over China were examined using the Extended Edited Cloud Report Archive data from 1971 to 1996. Linear regression analysis was applied to time series of clear-sky frequencies and low-cloud frequencies, and low-cloud amounts when present. Over the 26-yr study period, the clear-sky frequency increased over northern China. During summer, the frequency of cumuliform clouds decreased over almost all of China. A significant decrease characterized the trend in cumulonimbus (Cb) frequency; however, the Cb cloud amount when present increased over the Yangtze River basin and southern China. Increasing trends in stratocumulus (Sc) cloud amount when present were also observed over much of China.

Results of Outgoing Longwave Radiation and Albedo over Saudi Arabia from Earth Radiation Budget Experiment (ERBE) Data

ABSTRACT. Changes in the albedo and outgoing longwave radiation at the top of the atmosphere derived from Earth Radiation Budget Experiment (ERBE) are specified as a function of atmospheric (all sky conditions) and surface properties (clear sky conditions). Albedo and outgoing longwave radiation observations are also used to relate cloudiness variations to regional features of the general circulations such as precipitation zones, persistent cloudiness, and areas of subsidence. A method requiring only measurements of outgoing long wave radiation and planetary albedo is applied to evaluate the relative importance of the albedo and long wave radiation on the atmospheric circulation. The approach of studying these observations in time and space is also used to examine the radiative energy budget of different geographic locations such as deserts, high land, and coastal areas. The results indicate that there are clear relationships between the variability in outgoing longwave radiation, albedo, and features of the atmospheric circulation, which appear to be linked to changes in cloudiness. In addition, the effect of geographical variations of Saudi Arabia on cloud is analyzed and related to circulation features. The longwave mean ranges from 257.1 Wm.2 to 307.7 Wm.2, and the longwave standard deviation ranges from 10.6 Wm.2 to 27.9 Wm.2 over the region. The albedo mean ranges from 20.9% to 30.6%, and the albedo standard deviation ranges from 5.5% to 8.7% over the region. The longwave cloud forcing at the top of the atmosphere equals 15 Wm.2 for January, 22 Wm.2 for April, 14 Wm.2 for July, and 10 Wm.2 for October 1989. Thus compared to a cloud free atmosphere, the January mean cloudiness, as it was present, reduces the outgoing longwave radiation for the total earth-atmosphere system by about 6%, for the month of April by about 8%, for the month of July by about 5%, and for the month of October by about 3%. For clear sky desert conditions, a weak relationship (R2 = 0.38) between the albedo and outgoing longwave radiation was found at some locations over the Kingdom. Whereas the relationship between all sky albedo and all sky outgoing longwave radiation results in a strong relationship (R2 = 0.80) at some locations.

Radiative Feedbacks and Monsoon Circulations: A View from Simplified Models

The modulation of radiative processes by changes in water vapor and cloudiness is at the origin of important feedbacks which control climate variability as well as climate changes. These feedbacks are especially active in the intertropical area, where it is possible to diagnose a combination of partially compensating positive and negative feedbacks. The characteristics and the strength of those feedbacks is closely associated with the dynamical regimes in which they develop. Reverse changes in dynamical patterns may cause a modulation of the radiative processes. A first approach to these problems is to distinguish between two ascending and subsiding circulation patterns. This bimodality of the circulation is well established in the tropical area, and favors the use of simplified models as an appropriate tool to carry out a first-order quantification of these processes. In particular, this combination of radiative and dynamical feedbacks characterizes the development of the monsoons and their variability. Simple conceptual models can thus serve to characterize some of the factors which will affect the intraseasonal variations of the monsoon.

Comments on “Contrails, Cirrus Trends, and Climate”

Minnis et al. (2004, hereafter MAPP) estimate that surface temperature trends over the United States due to aircraft-induced contrails and cirrus are in “good agreement” with corresponding observations over the period 1973 to 1994. MAPP reach this conclusion based on a simple model [their Eq. (3)] calibrated using general circulation model (GCM) results from Rind et al. (2000, hereafter RLS). Depending on the assumed cirrus optical depth, MAPP estimate an annual-mean surface warming trend over the United States of between 0.16 and 0.27 K decade , compared with Angell’s (1999) observed trend of 0.27 K decade . I believe that MAPP significantly overestimate the climate impact of aircraft-induced clouds. A useful first-order estimate for the equilibrium global-mean surface temperature change Ts comes from applying the approximate equation Ts F, where is a climate sensitivity parameter and F is the radiative forcing (e.g., Houghton et al. 2001). The range of values of found in current GCMs is about 0.4 to 1.2 K (W m ) , corresponding to an equilibrium surface warming due to a doubling of carbon dioxide from about 1.5 to 4.5 K (e.g., Houghton et al. 2001); RLS’s GCM has a climate sensitivity toward the upper end of this range. MAPP estimate a maximum global-mean radiative forcing due to aircraft-induced clouds (for 1992) of 0.0255 W m . With these values, the above equation yields a global-mean warming of 0.01 to 0.03 K, or about 0.003 to 0.01 K decade , assuming that the forcing grew quasi-linearly over the previous three decades. Hence, this estimate is one to two orders of magnitude lower than that given by MAPP. The realized warming is likely to be 25%–50% smaller than this because of the thermal inertia of the climate system. MAPP’s estimate is for the United States, rather than for the global mean, but evidence for significant localization of the climate response resulting from a geographically limited forcing, beyond a broad hemispheric scale effect, is scant. Plate 3 of RLS shows that their modeled surface warming due to aircraft-induced clouds over the United States as a whole differs by no more than 50% from the global mean. More importantly, RLS compare two experiments, both with a global-mean increase in aircraft-induced clouds of about 1%. In one experiment (which they label “1/99”), the spatial pattern of cloud change is related to flight path occurrence; in the other (labeled “scaled”), the spatial pattern is related to flight path density, yielding a far more inhomogeneous change in cloudiness (see RLS, Plate 1). The scaled experiment yields a cloud change over the eastern United States that reaches about a factor of 2 larger than 1/99—despite this, the pattern of surface temperature response in the two experiments is very similar and RLS conclude that “there is no obvious geographical bias in the warming” despite the localization of the forcing. Other GCM studies on the impact of inhomogeneous patterns of forcing reach similar conclusions. For example, Boer and Yu (2003) conclude that “temperature response patterns are at best a weak reflection of their parent forcing pattern.” Certainly the regional nature of the aircraft-induced cloud forcing cannot explain the order(s) of magnitude difference in surface temperature change between the estimate derived here and that derived by MAPP. I believe that the main cause of MAPP’s overestimate is that their use of Eq. (3) implicitly assumes that there is a 1% cloud cover change present globally, or at least hemispherically, rather than just over the United States. Indeed, RLS (their Table 3a) show that a 1% Corresponding author address: Dr. Keith P. Shine, Dept. of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, United Kingdom. E-mail: k.p.shine@reading.ac.uk 15 JULY 2005 N O T E S A N D C O R R E S P O N D E N C E 2781

The Ultraviolet Radiation Environment of High Southern Latitudes: Springtime Behavior over a Decadal Timescale¶

ABSTRACTFour spectroradiometers located at latitudes from 55° to 90°S conducted near‐continuous measurements of ground‐level solar ultraviolet irradiance from 1990 through 2001. The behavior during months from October through December is of special interest because this period includes the springtime loss in column ozone and the naturally large irradiances of early summer. Monthly integrated irradiances using biological weightings for erythema and damage to DNA show a distortion of the normal annual cycle in irradiance, with enhanced values occurring in October and November. In some cases, these irradiances exceed those near summer solstice in December. Changes in local cloudiness and column ozone both contribute significantly to interannual variability in erythemal irradiance. This is particularly the case at Palmer Station, near 65°S, where the monthly integrated erythemal irradiance in November 1997 was more than double that observed 5 years earlier. In general, at sites on the Antarctic continent, interannual variability in monthly integrated erythemal irradiance is greatest in November, when the observation for any given year can fall 40% above or below the multiyear mean. Near the tip of South America, interannual variability is approximately half that seen in Antarctica.

The Ultraviolet Radiation Environment of High Southern Latitudes: Springtime Behavior over a Decadal Timescale

Four spectroradiometers located at latitudes from 55 degrees to 90 degrees S conducted near-continuous measurements of ground-level solar ultraviolet irradiance from 1990 through 2001. The behavior during months from October through December is of special interest because this period includes the springtime loss in column ozone and the naturally large irradiances of early summer. Monthly integrated irradiances using biological weightings for erythema and damage to DNA show a distortion of the normal annual cycle in irradiance, with enhanced values occurring in October and November. In some cases, these irradiances exceed those near summer solstice in December. Changes in local cloudiness and column ozone both contribute significantly to interannual variability in erythemal irradiance. This is particularly the case at Palmer Station, near 65 degrees S, where the monthly integrated erythemal irradiance in November 1997 was more than double that observed 5 years earlier. In general, at sites on the Antarctic continent, interannual variability in monthly integrated erythemal irradiance is greatest in November, when the observation for any given year can fall 40% above or below the multiyear mean. Near the tip of South America, interannual variability is approximately half that seen in Antarctica.

The Ultraviolet Radiation Environment of High Southern Latitudes: Springtime Behavior over a Decadal Timescale¶

Four spectroradiometers located at latitudes from 55° to 90°S conducted near-continuous measurements of ground-level solar ultraviolet irradiance from 1990 through 2001. The behavior during months from October through December is of special interest because this period includes the springtime loss in column ozone and the naturally large irradiances of early summer. Monthly integrated irradiances using biological weightings for erythema and damage to DNA show a distortion of the normal annual cycle in irradiance, with enhanced values occurring in October and November. In some cases, these irradiances exceed those near summer solstice in December. Changes in local cloudiness and column ozone both contribute significantly to interannual variability in erythemal irradiance. This is particularly the case at Palmer Station, near 65°S, where the monthly integrated erythemal irradiance in November 1997 was more than double that observed 5 years earlier. In general, at sites on the Antarctic continent, interannual variability in monthly integrated erythemal irradiance is greatest in November, when the observation for any given year can fall 40% above or below the multiyear mean. Near the tip of South America, interannual variability is approximately half that seen in Antarctica.