Gordon Labow

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.

Climate simulations for 1880–2003 with GISS modelE

We carry out climate simulations for 1880–2003 with GISS modelE driven by ten measured or estimated climate forcings. An ensemble of climate model runs is carried out for each forcing acting individually and for all forcing mechanisms acting together. We compare side-by-side simulated climate change for each forcing, all forcings, observations, unforced variability among model ensemble members, and, if available, observed variability. Discrepancies between observations and simulations with all forcings are due to model deficiencies, inaccurate or incomplete forcings, and imperfect observations. Although there are notable discrepancies between model and observations, the fidelity is sufficient to encourage use of the model for simulations of future climate change. By using a fixed well-documented model and accurately defining the 1880–2003 forcings, we aim to provide a benchmark against which the effect of improvements in the model, climate forcings, and observations can be tested. Principal model deficiencies include unrealistically weak tropical El Nino-like variability and a poor distribution of sea ice, with too much sea ice in the Northern Hemisphere and too little in the Southern Hemisphere. Greatest uncertainties in the forcings are the temporal and spatial variations of anthropogenic aerosols and their indirect effects on clouds.

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.

Northern hemisphere atmospheric effects due to the July 2000 Solar Proton Event

The third largest solar proton event in the past thirty years took place during July 14-16, 2000, and had a significant impact on the earths atmosphere. These energetic protons produced both HO x (H, OH, HO 2 ) and NO x (N, NO, NO 2 ) constituents in the mesosphere and upper stratosphere at polar latitudes (> 60° geomagnetic) of both hemispheres. The temporal evolution of increases in NO and NO 2 during the event at northern polar latitudes were measured by the UARS HALOE instrument. Increases in mesospheric NO x of over 50 ppbv were found in the HALOE measurements. Measurements from the UARS HALOE and NOAA 14 SBUV/2 instruments indicate short-term (∼day) middle mesospheric ozone decreases of over 70% caused by short-lived HO x during the event with a longer-term (several days) upper stratospheric ozone depletion of up to 9% caused by longer-lived NO x . We believe this is the first time that the three constituents NO, NO 2 , and ozone were all measured simultaneously during a proton event. The observations constitute a dramatic confirmation of the impact of a large particle event in the control of ozone in the polar middle atmosphere and offer the opportunity to test theories of constituent changes driven by particle precipitation.

An assessment of the accuracy of 14.5 years of Nimbus 7 TOMS version 7 ozone data by comparison with the Dobson network

A Version 7 algorithm and calibration have been applied to the 14.5 year Nimbus 7 TOMS ozone record (1978–1993). The ozone retrieval algorithm has been significantly improved for cloudy conditions and for high solar zenith angles, and the radiative transfer used in the algorithm is more accurate. New calibration techniques have been used that produce a very stable data set even after 1990 when TOMS degradation became significant. TOMS ozone now agrees with average ozone from an ensemble of 30 northern hemisphere ground stations (Dobsons and Brewers) to within ±1% throughout most of the 14.5 year record. The time-dependent drift relative to Dobson is 0.29% per decade through the end of the data record. There is almost no solar zenith angle dependence in the comparison for angles below about 80°, but data should be used with caution for larger solar zenith angles. There is also a residual total ozone dependence in the TOMS-Dobson difference, of about 1% per 100 DU.

A satellite‐derived ozone climatology for balloonsonde estimation of total column ozone

The standard technique for computing total column ozone from a balloonsonde measurement includes an extrapolation of the measured ozone profile to altitudes above the balloon burst altitude. This total column calculation can be improved by using a monthly average ozone climatology based on ozone profile measurements from the solar backscattered ultraviolet instrument on the Nimbus 7 satellite. For each month and 10° latitude zone we provide the column ozone above 30 distinct pressure levels in the middle and upper atmosphere (from 1 through 30 mbar) that can be added to the measured balloon profile. Use of this climatology reduces the uncertainty in the column ozone estimate when the sonde data are compared with simultaneous Dobson observations at Boulder and at Mauna Loa.

An assessment of the world ground-based total ozone network performance from the comparison with satellite data

Total ozone data available from the World Ozone and Ultraviolet Radiation Data Centre (WOUDC) have been compared with Version 7 of the Nimbus 7 (for the 1978-1993 period) and Meteor 3 total ozone mapping spectrometer (TOMS) data sets (for the 1991-1994 period) and with Version 6 of the Nimbus 7 solar backscattered ultraviolet (SBUV) data set (for the 1978-1990 period). Comparison between the ground-based and satellite observations resulted in parameters such as bias, scattering, relative trend, and seasonal component. About 80% of all Dobson, Brewer, and filter ozonometer stations have standard deviations of monthly mean differences with TOMS that are less than 2.5%. Typically, results of the comparisons between ground-based stations and SBUV are similar to those for ground-based and TOMS comparisons. For the ground-based direct sun total ozone measurements the standard deviations between TOMS and Dobson daily mean values are about 2.4%. The standard deviations for Brewer and filter ozonometer stations are 2.2 and 3.5%, respectively. For the less accurate zenith sky measurements, standard deviations are 3.8% for Dobsons, 4% for Brewers, and 4.7% for filter ozonometer data. Comparisons of individual ground-based measurements with satellite overpasses yield standard deviations which for middle latitudes increase approximately linearly from about 2% at coincidence to 6% for a 24 hour difference in time or a 600 km spatial difference. Typical problems with ground-based observations and results of the comparisons for individual stations are also discussed.

Aerosol ultraviolet absorption experiment (2002 to 2004), part 2: absorption optical thickness, refractive index, and single scattering albedo

Compared to the visible spectral region, very little is known about aerosol absorption in the UV. Without such information it is impos- sible to quantify the causes of the observed discrepancy between mod- eled and measured UV irradiances and photolysis rates. We report re- sults of a 17-month aerosol column absorption monitoring experiment conducted in Greenbelt, Maryland, where the imaginary part of effective refractive index k was inferred from the measurements of direct and diffuse atmospheric transmittances by a UV-multifilter rotating shadow- band radiometer (UV-MFRSR, U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network). Colocated ancillary measure- ments of aerosol effective particle size distribution and refractive index in the visible wavelengths (by CIMEL sun-sky radiometers, National Aero- nautics and Space Administration (NASA) Aerosol Robotic Network (AERONET)), column ozone, surface pressure, and albedo constrain the forward radiative transfer model input, so that a unique solution for k is obtained independently in each UV-MFRSR spectral channel. Inferred values of k are systematically larger in the UV than in the visible wave- lengths. The inferred k values enable calculation of the single scattering albedo v, which is compared with AERONET inversions in the visible

Recent changes in total column ozone based on the SBUV Version 8.6 Merged Ozone Data Set

The Solar Backscatter Ultraviolet (SBUV) Merged Ozone Data Set (MOD) provides the longest available satellite-based time series of profile and total ozone from a single instrument type. The data span a 44 year period from 1970 to 2013 (except a 5 year gap in the 1970s). Data from nine independent SBUV-type instruments are included in the record, one of which is still operating. Although modifications in instrument design were made in the evolution from the Nimbus-4 Backscattered Ultraviolet instrument to the modern SBUV(/2) model, the basic principles of the measurement technique and retrieval algorithm remain the same, lending consistency to this record compared to those based on measurements using different instrument types. Nevertheless, each instrument has specific characteristics, and known anomalies must be incorporated in the MOD uncertainty estimates. In this study we describe the latest version of the MOD data set based on SBUV data processed using the Version 8.6 algorithm. We assess the measurement consistency across instruments and use this information to assign a drift uncertainty to the MOD. We then fit a multiple regression model to the MOD time series alternately using Equivalent Effective Stratospheric Chlorine (EESC) or linear trend fits over varying time series segments to analyze trends. Regression results indicate a statistically significant positive trend in total ozone outside the tropics based on the EESC proxy fit to the full record, but a linear trend fit to the last 13 years of data does not yield a statistically significant ozone increase.

Aerosol ultraviolet absorption experiment (2002 to 2004), part 1: ultraviolet multifilter rotating shadowband radiometer calibration and intercomparison with CIMEL sunphotometers

Radiative transfer calculations of UV irradiance from total ozone mapping spectrometer (TOMS) satellite data are frequently over- estimated compared to ground-based measurements because of the presence of undetected absorbing aerosols in the planetary boundary layer. To reduce these uncertainties, an aerosol UV absorption closure experiment has been conducted at the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) site in Greenbelt, Maryland, using 17 months of data from a shadowband radi- ometer (UV-multifilter rotating shadowband radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network) colocated with a group of three sun-sky CIMEL radiometers (rotating reference instruments of the NASA Aerosol Robotic Network (AERONET)). We describe an improved UV-MFRSR on-site calibration method augmented by AERONET-CIMEL measurements of aerosol ex- tinction optical thickness (t a) interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals. The estimated t a is used as input to a UV-MFRSR spectral-band model, along with indepen- dent column ozone and surface pressure measurements, to estimate zero air mass voltages V0 in three longer wavelength UV-MFRSR chan- nels (325, 332, 368 nm). Daily mean ^V0&, estimates and standard de- viations are obtained for cloud-free conditions and compared with the on-site UV-MFRSR Langley plot calibration method. By repeating the calibrations on clear days, relatively good stability (62% in ^V0& )i s found in summer, with larger relative changes in fall-winter seasons. The changes include systematic day-to-day ^V0& decline for extended peri- ods along with step jump changes after major precipitation periods (rain or snow) that affected the diffuser transmission. When daily ^V0& values are used to calculate t a for individual 3-min UV-MFRSR measurements on the same days, the results compare well with interpolated AERONET t a measurements (at 368 nm most daily 1s root mean square (rms) differences were within 0.01). When intercalibrated against an AERO- NET sunphotometer, the UV-MFRSR is proven reliable to retrieve t a , and hence can be used to retrieve aerosol column absorption in the UV. The advantage of the shadowband technique is that the calibration ob- tained for direct-sun voltage can then be applied to diffuse-radiance volt- age to obtain total and diffuse atmospheric transmittances. These trans- mittances, in combination with accurate t a data, provide the basis for estimating aerosol column absorption at many locations of the USDA UV-B Monitoring and Research network and for correction of satellite estimations of surface UV irradiance.