D. Larko

Record Low Global Ozone in 1992

The 1992 global average total ozone, measured by the Total Ozone Mapping Spectrometer (TOMS) on the Nimbus-7 satellite, was 2 to 3 percent lower than any earlier year observed by TOMS (1979 to 1991). Ozone amounts were low in a wide range of latitudes in both the Northern and Southern hemispheres, and the largest decreases were in the regions from 10�S to 20�S and 100N to 60�N. Global ozone in 1992 is at least 1.5 percent lower than would be predicted by a statistical model that includes a linear trend and accounts for solar cycle variation and the quasi-biennial oscillation. These results are confirmed by comparisons with data from other ozone monitoring instruments: the SBUV/2 instrument on the NOAA-11 satellite, the TOMS instrument on the Russian Meteor-3 satellite, the World Standard Dobson Instrument 83, and a collection of 22 ground-based Dobson instruments.

UV-B increases (1979–1992) from decreases in total ozone

Increases in ultraviolet fluxes (300 nm to 340 nm) reaching the ground between 1979 and 1992 are estimated using measured stratospheric ozone amounts and reflectivity data from Nimbus-7/TOMS (Total Ozone Mapping Spectrometer). The UV-increases are estimated from an ozone data set obtained using a new algorithm incorporating improved in-flight instrument calibration. The 380 nm radiance data are used to show that there were no changes in ultraviolet atmospheric albedo due to clouds and aerosols from 1979 to 1992 within the 1% uncertainty of the measurements. Linear least squares fits to the monthly and annual increases in UV exposure since 1979 are given for 3 wavelengths (300 nm, 310 nm, and 320 nm) that are strongly, moderately, and weakly absorbed by ozone. The estimated linear changes for the 3 wavelengths become significant (2 standard deviations) poleward of about 40° latitude. In the 45°±5° latitude band, the slope of linear fits to the annual zonally averaged changes for these wavelengths are about 13%, 3%, and 1% per decade in the southern hemisphere, and 10%, 3%, and 1% per decade in the northern hemisphere. Similarly derived values are estimated for DNA, plant, and erythema action spectra. Monthly values of exposure changes are larger towards higher latitudes and during the spring and winter months (e.g., 8.6%, 9.8%, and 5.1% per decade during April at 45°N). Erythemal UV-increases obtained from TOMS data disagree with previously determined ground based UV-decreases from an average of 8 U.S. Robertson-Berger sites.

Distribution of UV radiation at the Earth's surface from TOMS-measured UV-backscattered radiances

Daily global maps of monthly integrated UV-erythemal irradiance (290–400 nm) at the Earths surface are estimated using the ozone amount, cloud transmittance, aerosol amounts, and surface reflectivity from the solar UV radiation backscattered from the Earths atmosphere as measured by the total ozone mapping spectrometer (TOMS) and independently measured values of the extraterrestrial solar irradiance. The daily irradiance values at a given location show that short-term variability (daily to annual) in the amount of UV radiation, 290–400 nm, reaching the Earths surface is caused by (1) partially reflecting cloud cover, (2) haze and absorbing aerosols (dust and smoke), and (3) ozone. The reductions of UV irradiance estimated from TOMS data can exceed 50 ± 12% underneath the absorbing aerosol plumes in Africa and South America (desert dust and smoke from biomass burning) and exceeded 70 ± 12% during the Indonesian fires in September 1997 and again during March 1998. Recent biomass burning in Mexico and Guatemala have caused large smoke plumes extending into Canada with UV reductions of 50% in Mexico and 20% in Florida, Louisiana, and Texas. Where available, ground-based Sun photometer data show similar UV irradiance reductions caused by absorbing aerosol plumes of dust and smoke. Even though terrain height is a major factor in increasing the amount of UV exposure compared to sea level, the presence of prolonged clear-sky conditions can lead to UV exposures at sea level rivaling those at cloudier higher altitudes. In the equatorial regions, ±20°, the UV exposures during the March equinox are larger than during the September equinox because of increased cloudiness during September. Extended land areas with the largest erythemal exposure are in Australia and South Africa where there is a larger proportion of clear-sky days. The large short-term variations in ozone amount which occur at high latitudes in the range ±65° cause changes in UV irradiance comparable to clouds and aerosols for wavelengths between 280 nm and 300 nm that are strongly absorbed by ozone. The absolute accuracy of the TOMS monthly erythemal exposure estimates over a TOMS field of view is within ±6%, except under UV-absorbing aerosol plumes (dust and smoke) where the accuracy is within ±12%. The error caused by aerosols can be reduced if the height of the aerosol plume is more accurately known. The TOMS estimated irradiances are compared with ground-based Brewer spectroradiometer data obtained at Toronto, Canada. The Brewer irradiances are systematically 20% smaller than TOMS irradiance estimates during the summer months. An accounting of systematic errors brings the Brewer and TOMS irradiances into approximate agreement within the estimated instrumental uncertainties for both instruments.

Pyro‐cumulonimbus injection of smoke to the stratosphere: Observations and impact of a super blowup in northwestern Canada on 3–4 August 1998

[1] We report observations and analysis of a pyro-cumulonimbus event in the midst of a boreal forest fire blowup in Northwest Territories Canada, near Norman Wells, on 3–4 August 1998. We find that this blowup caused a five-fold increase in lower stratospheric aerosol burden, as well as multiple reports of anomalous enhancements of tropospheric gases and aerosols across Europe 1 week later. Our observations come from solar occultation satellites (POAM III and SAGE II), nadir imagers (GOES, AVHRR, SeaWiFS, DMSP), TOMS, lidar, and backscattersonde. First, we provide a detailed analysis of the 3 August eruption of extreme pyro-convection. This includes identifying the specific pyro-cumulonimbus cells that caused the lower stratospheric aerosol injection, and a meteorological analysis. Next, we characterize the altitude, composition, and opacity of the post-convection smoke plume on 4–7 August. Finally, the stratospheric impact of this injection is analyzed. Satellite images reveal two noteworthy pyro-cumulonimbus phenomena: (1) an active-convection cloud top containing enough smoke to visibly alter the reflectivity of the cloud anvil in the Upper Troposphere Lower Stratosphere (UTLS) and (2) a smoke plume, that endured for at least 2 hours, atop an anvil. The smoke pall deposited by the Norman Wells pyro-convection was a very large, optically dense, UTLS-level plume on 4 August that exhibited a mesoscale cyclonic circulation. An analysis of plume color/texture from SeaWiFS data, aerosol index, and brightness temperature establishes the extreme altitude and ‘‘pure’’ smoke composition of this unique plume. We show what we believe to be a first-ever measurement of strongly enhanced ozone in the lower stratosphere mingled with smoke layers. We conclude that two to four extreme pyro-thunderstorms near Norman Wells created a smoke injection of hemispheric scope that substantially increased stratospheric optical depth, transported aerosols 7 km above the tropopause (above 430 K potential temperature), and also perturbed lower stratospheric ozone.

Ozone depletion at Northern and Southern latitudes derived from January 1979 to December 1991 Total Ozone Mapping Spectrometer data

Long-term ozone depletion rates (percentage change) have been computed from 13 years of Nimbus 7/Total Ozone Mapping Spectrometer (TOMS) data as a function of latitude, longitude, and month for the period January 1, 1979, to December 31, 1991. In both hemispheres the amount of ozone has decreased at latitudes above 30° by amounts that are larger than predicted by homogeneous chemistry models for the 13-year time period. The largest rates of ozone decrease occur in the southern hemisphere during winter and spring, with partial recovery during the summer and autumn. Outside of the Antarctic ozone hole region, the 12-year ozone depletion rates reach 8–10% per decade during the winter and spring at 55°S. Ozone depletion rates in excess of 7% per decade occur over populated regions in the southern hemisphere poleward of 45°S (southern Argentina and the tip of New Zealand) for 7 months of the year. Similar rates of decrease occur during northern winter and spring (6–8% at 55°N) over large populated regions. The enhanced zonal average ozone depletion rates at northern mid-latitudes (40–50°N) during January, February, and March, that correspond to five geographically localized regions of high ozone depletion rates, are probably associated with long-term dynamical or temperature changes. Only the equatorial band between ±20° shows little or no long-term ozone change since January 1979. Ozone time series data have been examined for effect of volcanic eruptions on stratospheric ozone observed by TOMS, with only the Mount Pinatubo stratospheric aerosol injection affecting ozone amounts for a few months after the eruption in June 1991. The TOMS data show no ozone perturbation after the El Chichon eruption, or after any of the other smaller equatorial eruptions, that cannot be explained by interference effects between the annual, El Nino/Southern Oscillation, and Quasi-Biennial Oscillation cycles. The effect of the stratospheric aerosol scattering phase function is clearly seen in the high spatial resolution TOMS ozone data after El Chichon and Mount Pinatubo. Errors caused by the short-term presence of stratospheric aerosols in the TOMS zonally averaged ozone data are less than 1% before correction, and have no significant effect on ozone trend determination.

UV 380 nm reflectivity of the Earth's surface, clouds and aerosols

The 380 nm radiance measurements of TOMS (Total Ozone Mapping Spectrometer) have been converted into a global data set of daily (1979 to 1992) Lambert equivalent reflectivities R of the Earths surface and boundary layer (clouds, aerosols, surface haze, and snow/ice). Since UV surface reflectivity is between 2 and 8% for both land and water during all seasons of the year (except for ice and snow cover), reflectivities larger than the surface value indicates the presence of clouds, haze, or aerosols in the satellite field of view. Statistical analysis of 14 years of daily data show that most snow/ice-free regions of the Earth have their largest fraction of days each year when the reflectivity is low (R less than 10%). The 380 nm reflectivity data shows that the true surface reflectivity is 2 to 3% lower than the most frequently occurring reflectivity value for each TOMS scene. The most likely cause of this could be a combination of frequently occurring boundary-layer water or aerosol haze. For most regions, the observation of extremely clear conditions needed to estimate the surface reflectivity from space is a comparatively rare occurrence. Certain areas (e.g., Australia, southern Africa, portions of northern Africa) are cloud-free more than 80% of the year, which exposes these regions to larger amounts of UV radiation than at comparable latitudes in the Northern Hemisphere. Regions over rain-forests, jungle areas, Europe and Russia, the bands surrounding the Arctic and Antarctic regions, and many ocean areas have significant cloud cover (R greater than 15%) more than half of each year. In the low to middle latitudes, the areas with the heaviest cloud cover (highest reflectivity for most of the year) are the forest areas of northern South America, southern Central America, the jungle areas of equatorial Africa, and high mountain regions such as the Himalayas or the Andes. The TOMS reflectivity data show the presence of large nearly clear ocean areas and the effects of the major ocean currents on cloud production.

Global average ozone change from November 1978 to May 1990

A recent recalibration and reprocessing of the total ozone mapping spectrometer (TOMS) data have made possible a new determination of the global average (69°S to 69°N) total ozone decrease of 3.5% over the 11-year period, January 1, 1979, to December 31, 1989, with a 2σ error of 1.4%. The revised TOMS ozone trend data are in agreement, within error limits, with the average of 39 ground-based Dobson stations and with the world standard Dobson spectrometer 83 at Mauna Loa, Hawaii. Superimposed on the 11-year ozone trend is a possible solar cycle effect, quasi-biennial oscillation (QBO), annual, and semiannual cycles. Using solar 10.7-cm flux data and 30-mbar Singapore wind data (QBO), a time series has been constructed that reproduces the long-term behavior of the globally averaged ozone. Removal of the apparent solar cycle effect from the global average reduces the net ozone loss to 2.66±1.4% per decade. The precise value of the global average ozone trend depends on the latitude range selected, with ranges greater than ±69° emphasizing the larger variations at high latitudes.

Changes in the Earth's UV reflectivity from the surface, clouds, and aerosols

Measurements of the Earths 380 nm UV reflectivity combine the effects of surface reflectivity, aerosols, haze, cloud optical thickness, and the fraction of the scene covered by clouds. Changes in UV cloud and aerosol reflectivity would imply similar changes over a wide range of wavelengths, UV, visible, and near infrared (at least 0.3 to 2 μm), affecting both the transmission of radiation to the Earths surface and the reflection back to space. Using the TOMS (Total Ozone Mapping Spectrometer) 380 nm reflectivity data, the 14-year annual mean power reflected back to space is 385.3±31 W/m2, mostly by clouds, aerosols, and snow/ice. On the basis of measured long-term changes in global reflectivity, it is estimated that there is an additional 2.8±2.8 W/m2 per decade reflected back to space (2 standard deviation error estimate) during the TOMS observing period of 1979–1992. Since the 380 nm surface reflectivity is low (2–8%) over most surfaces, water and land, the observed reflectivity changes are mostly caused by changes in the amount of snow/ice, cloudiness, and aerosols. Time series analysis of TOMS reflectivity over the period from 1979 to 1992 shows that there were no significant changes in annually averaged zonal-average reflectivity at latitudes within 60°S–60°N, even though there were changes at higher latitudes (e.g., 3% per decade, in reflectivity units, between 60°N and 70°N). When the effects of the 11.3-year solar cycle and ENSO (El Nino-Southern Oscillation) are removed from the data, statistically significant reflectivity changes are observed poleward of both 40°S and 40°N. The presence of a statistically significant 11.3-year periodicity in the reflectivity time series correlates with the solar cycle and suggests a possible Sun-weather relationship. There are significant regional changes in reflectivity R over land and ocean areas that affect the amount of solar radiation reaching the surface. The largest of these regions have decreases in R of 3 to 6 ± 1% per decade in central Europe, the western United States, central China, and western Russia. These decreases are offset by increases in the same latitude bands mostly over the oceans. The largest regions showing an increase in R are off the western coast of South America (near Chile and Peru), 5 to 8 ± 1%/decade and over the Weddell Sea in Antarctica of 10%/decade, but no change over the ice shelf and continent. The largest increase in R occurs over the ocean just to the north of Antarctica. This change is important because it reduces UV radiation overall (290–400 nm) and partially offsets the effect of the increased amount of UV-B radiation (290–320 nm) caused by decreasing Antarctic ozone.

Interannual variability of ozone and UV-B ultraviolet exposure

Zonal averages of annual and seasonal averages of ozone amounts from Nimbus 7/TOMS (1979–1992) have been examined to estimate the systematic interannual variability of UV-B (290–320 nm) exposure to solar radiation between ±60° latitude. As shown from statistical modeling, clear-sky interannual UV-B changes can be ascribed mainly to the quasi-biennial oscillation (QBO) driven by stratospheric winds. The QBO oscillations can cause interannual changes in UV-B exposure of ±15% at 300 nm and ±5% at 310 nm at the equator and at middle latitudes. In addition to QBO effects, there are larger interannual changes in ozone and UV-B associated with dynamical effects at higher latitudes. When UV-B attenuation from clouds is included, the general latitudinal structure of the interannual variability is maintained. At the equator the interannual variability of ozone amounts and UV exposure caused by the combination of the 2.3 year QBO and annual cycles implies that there is about a 5 year periodicity in UV-B variability caused by dynamical effects. At higher latitudes the appearance of the interannual UV-B maximum is predicted by the QBO but without the regular periodicity. The QBO effects on UV-B irradiance are larger than the long-term changes caused by the decrease in ozone amounts.

Diurnal variation of 340 nm Lambertian equivalent reflectivity due to clouds and aerosols over land and oceans

Received 30 August 2010; revised 2 March 2011; accepted 9 March 2011; published 8 June 2011. [1] Thirty years of satellite measurements of Lambert equivalent reflectivity (LER) at 340 nm have been analyzed to show the changes in diurnal LER that are associated with cloud and aerosol amounts. Five degree zonal mean diurnal variations of LER are obtained from multiple NASA and NOAA satellites making 340 nm LER measurements from 0600 to 1800 local solar time. These zonal means were calculated separately over water and land. The results show different behavior of clouds over oceans with LER peaking in the morning compared to LER over land, which peaks in the afternoon. Over the oceans the cloud amount increases as a function of latitude in both hemispheres, with the exception of the midtropics (10–20°). Over land, the amount of cloudiness significantly decreases by almost a factor of 2 from the equator to ∼25° and then, in the Northern Hemisphere, increases as a function of latitude. The zonal means represent measured cloud amounts, and thus a quantification of energy reflected back into space as a function of time of day. Cloud fraction measurements made by Aqua and Terra Moderate Resolution Imaging Spectroradiometer show a signature that is similar to the measured ratios of LER in morning to afternoon. The similarity in pattern demonstrates that the LER is measuring a quantity directly related to cloud fraction.