Total Column Ozone Amount Research Articles

Effects of local meteorology and aerosols on ozone and nitrogen dioxide retrievals from OMI and pandora spectrometers in Maryland, USA during DISCOVER-AQ 2011.

An analysis is presented for both ground- and satellite-based retrievals of total column ozone and nitrogen dioxide levels from the Washington, D.C., and Baltimore, Maryland, metropolitan area during the NASA-sponsored July 2011 campaign of Deriving Information on Surface COnditions from Column and VERtically Resolved Observations Relevant to Air Quality (DISCOVER-AQ). Satellite retrievals of total column ozone and nitrogen dioxide from the Ozone Monitoring Instrument (OMI) on the Aura satellite are used, while Pandora spectrometers provide total column ozone and nitrogen dioxide amounts from the ground. We found that OMI and Pandora agree well (residuals within ±25 % for nitrogen dioxide, and ±4.5 % for ozone) for a majority of coincident observations during July 2011. Comparisons with surface nitrogen dioxide from a Teledyne API 200 EU NOx Analyzer showed nitrogen dioxide diurnal variability that was consistent with measurements by Pandora. However, the wide OMI field of view, clouds, and aerosols affected retrievals on certain days, resulting in differences between Pandora and OMI of up to ±65 % for total column nitrogen dioxide, and ±23 % for total column ozone. As expected, significant cloud cover (cloud fraction >0.2) was the most important parameter affecting comparisons of ozone retrievals; however, small, passing cumulus clouds that do not coincide with a high (>0.2) cloud fraction, or low aerosol layers which cause significant backscatter near the ground affected the comparisons of total column nitrogen dioxide retrievals. Our results will impact post-processing satellite retrieval algorithms and quality control procedures.

High precision, absolute total column ozone measurements from the Pandora spectrometer system: Comparisons with data from a Brewer double monochromator and Aura OMI

We present new, high precision, high temporal resolution measurements of total column ozone (TCO) amounts derived from ground‐based direct‐sun irradiance measurements using our recently deployed Pandora single‐grating spectrometers. Pandora's small size and portability allow deployment at multiple sites within an urban air‐shed and development of a ground‐based monitoring network for studying small‐scale atmospheric dynamics, spatial heterogeneities in trace gas distribution, local pollution conditions, photochemical processes and interdependencies of ozone and its major precursors. Results are shown for four mid‐ to high‐latitude sites where different Pandora instruments were used. Comparisons with a well calibrated double‐grating Brewer spectrometer over a period of more than a year in Greenbelt MD showed excellent agreement and a small bias of approximately 2 DU (or, 0.6%). This was constant with slant column ozone amount over the full range of observed solar zenith angles (15–80°), indicating adequate Pandora stray light correction. A small (1–2%) seasonal difference was found, consistent with sensitivity studies showing that the Pandora spectral fitting TCO retrieval has a temperature dependence of 1% per 3°K, with an underestimation in temperature (e.g., during summer) resulting in an underestimation of TCO. Pandora agreed well with Aura‐OMI (Ozone Measuring Instrument) satellite data, with average residuals of <1% at the different sites when the OMI view was within 50 km from the Pandora location and OMI‐measured cloud fraction was <0.2. The frequent and continuous measurements by Pandora revealed significant short‐term (hourly) temporal changes in TCO, not possible to capture by sun‐synchronous satellites, such as OMI, alone.

Anomalously low ozone of 1997 and 2011 Arctic spring: Monitoring results and analysis

Total ozone observations from the Total Ozone Unit (TOU) aboard the Chinese second generation polar orbiting meteorological satellite, Fengyun-3/A (FY-3/A), revealed that total column ozone over the Arctic declined rapidly from the beginning of March 2011. An extensive region of low column amount formed around mid March; monthly mean total column ozone in March 2011 was about 30% lower than the average observed during 1979—2010. Daily total column density of ozone near the center of low ozone area in mid March was less than 240 Dobson units, about half the total column ozone amount observed during the same period of the prior 10 years. We analyzed total column ozone data from different satellites during 1979—2011. Results show that the Arctic depletion of ozone in spring 2011 was initiated by the cold polar vortex in the lower stratosphere. The March mean total ozone over the Arctic has shown a decreasing trend over the past 32 years, and its variation is strongly correlated with the polar vortex. A similar low ozone process of spring 1997 was compared to that of 2011, but daily variations of total ozone in March over the Northern Hemisphere in 1997 and 2011 have different patterns.

Sudden increase in the total ozone density due to secondary ozone peaks and its effect on total ozone trends over Korea

Abstract Total column ozone (TCO) amounts observed by the Dobson spectrophotometer from 1985 to 2008 in Seoul and vertical ozone profiles obtained with the ozonesonde from 1995 to 2007 in Pohang were used to investigate the relationship between the occurrence of Secondary Ozone Peak (SOP) in the Upper Troposphere/Lower Stratosphere (UT/LS) layer and the enhancement in TCO over the Korean Peninsula. Based on Hybrid Single-Particle, Lagrangian Integrated Trajectory (HYSPLIT) simulations, the advection of a northern mid-latitudinal ozone-rich airmass from the northwest is closely related to SOP occurrences, which consequently lead to enhancements in the amount of ozone in both the UT/LS and the total column at stations in Korea. In addition, both the frequency of the northwesterly advection and the northern mid-latitudinal ozone amount are revealed to affect the amount of ozone in the UT/LS and total column by up to 7 DU. The relationship between the SOP occurrence frequency and the long-term TCO trend was investigated with observed data collected in all seasons. The UT/LS ozone enhancements, which are largely affected by SOP occurrences, are considered to be positively related to the TCO trend from 1985 to 2008 over the Korean Peninsula.

Near-Real-Time Ocean Color Data Processing Using Ancillary Data From the Global Forecast System Model

This paper investigates the improvements to the quality of ocean color data using an appropriate choice of ancillary data for National Oceanic and Atmospheric Administration (NOAA) operational ocean color data processing, which requires routine ocean color product production in near real time. The ancillary data, such as the total column ozone amount, sea surface wind speed, atmospheric pressure, and total column water-vapor amount, are required for satellite ocean color data processing for deriving ocean color products, e.g., normalized water-leaving radiance spectra data, chlorophyll-a concentration, water diffuse attenuation coefficient, etc. Currently, NOAA's CoastWatch program uses the climatology ancillary data for the near-real-time ocean color data processing. Alternative ancillary data sets that can replace the climatology data for the near-real-time ocean color data processing have been investigated and studied. The studies were carried out from four selected NOAA CoastWatch regions covering the U.S. coastal and Hawaii regions and for four months (January, April, July, and October), representing the four seasons. Based on the evaluation results, we propose to use the ancillary data produced from the Global Forecast System (GFS) model for the NOAA operational ocean color data processing, as well as for any other near-real-time data processing that requires ancillary data inputs. The effects of using the GFS model data on the accuracy of the derived ocean color products are also investigated and discussed.

Enhancement of Ozone and Carbon Monoxide Associated with Upper Cut-off Low during Springtime in East Asia

In order to verify the enhancement of ozone and carbon monoxide (CO) during springtime in East Asia, we investigated weather conditions and data from remote sensors, air quality models, and air quality monitors. These include the geopotential height archived from the final (FNL) meteorological field, the potential vorticity and the wind velocity simulated by the Meteorological Mesoscale Model 5 (MM5), the back trajectory estimated by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, the total column amount of ozone and the aerosol index retrieved from the Total Ozone Mapping Spectrometer (TOMS), the total column density of CO retrieved from the Measurement of Pollution in the Troposphere (MOPITT), and the concentration of ozone and CO simulated by the Model for Ozone and Related Chemical Tracers (MOZART). In particular, the total column density of CO, which mightoriginate from the combustion of fossil fuels and the burning of biomass in China, increased in East Asia during spring 2000. In addition, the enhancement of total column amounts of ozone and CO appeared to be associated with both the upper cut-off low near 500 hPa and the frontogenesis of a surface cyclone during a weak Asian dust event. At the same time, high concentrations of ozone and CO on the Earth’s surface were shown at the Seoul air quality monitoring site, located at the surface frontogenesis in Korea. It was clear that the ozone was invaded by the downward stretched vortex anomalies, which included the ozone-rich airflow, during movement and development of the cut-off low, and then there was the catalytic photochemical reaction of ozone precursors on the Earth’s surface during the day. In addition, air pollutants such as CO and aerosol were tracked along both the cyclone vortex and the strong westerly as shown at the back trajectory in Seoul and Busan, respectively. Consequently, the maxima of ozone and CO between the two areas showed up differently because of the time lag between those gases, including their catalytic photochemical reactions together with the invasion from the upper troposphere, as well as the path of their transport from China during the weak Asian dust event.

Association between ENSO and extremes in total ozone content over northern India

Using daily station total ozone column (TOC) data from the Total Ozone Mapping Spectrometer (TOMS) onboard the Nimbus-7 satellite, an association between the El Niño–Southern Oscillation (ENSO) and extremes in TOC content has been revealed during the period 1979–1993 over northern India in the winter season. From lag-simultaneous correlations of extremes in ozone with Niño 3.4 sea surface temperatures (SSTs), it is seen that, during this season, the highest TOC values show a strong positive relationship at the beginning of the preceding year with the occurrence of the highest values for all the stations. A weak relationship is observed up to the month of July and its sign is then reversed. The negative but weak relationship continues until the occurrence of the event, becoming positive again afterwards. On the contrary, the occurrence of the lowest values shows opposite features. The analysis indicates that the increase in SSTs during the first half of the preceding year is favourable for an increase in the highest values occurring over different stations while the increase in SSTs during the latter half of the preceding year is favourable for an increase in the lowest values of ozone. The lag-simultaneous correlations of the low/high ozone days and the mean TOC values occurring during the winter season also suggest a significant positive relationship for the frequency of the high ozone days at the beginning of the preceding year, becoming weaker as time progresses. Although both features show that the relationship is statistically significant for only a few months of the preceding year, it gives a broad indication of the association between ENSO and the extremes in the TOC amount in addition to local/geographical factors.

Analysis of Ozone (O3) and Erythemal UV (EUV) measured by TOMS in the equatorial African belt

We presented time series of total ozone column amounts (TOCAs) and erythemal UV (EUV) doses derived from measurements by TOMS (Total Ozone Mapping Spectrometer) instruments on board the Nimbus-7 (N7) and the Earth Probe (EP) satellites for three locations within the equatorial African belt for the period 1979 to 2000. The locations were Dar-es-Salaam (6.8° S, 39.26° E) in Tanzania, Kampala (0.19° N, 32.34° E) in Uganda, and Serrekunda (13.28° N, 16.34° W) in Gambia. Equatorial Africa has high levels of UV radiation, and because ozone shields UV radiation from reaching the Earth’s surface, there is a need to monitor TOCAs and EUV doses. In this paper we investigated the trend of TOCAs and EUV doses, the effects of annual and solar cycles on TOCAs, as well as the link between lightning and ozone production in the equatorial African belt. We also compared clear-sky simulated EUV doses with the corresponding EUV doses derived from TOMS measurements. The TOCAs were found to vary in the ranges 243 DU − 289 DU, 231 DU − 286 DU, and 236 DU − 296 DU, with mean values of 266.9 DU, 260.9 DU, and 267.8 DU for Dar-es-Salaam, Kampala and Serrekunda, respectively. Daily TOCA time series indicated that Kampala had the lowest TOCA values, which we attributed to the altitude effect. There were two annual ozone peaks in Dar-es-Salaam and Kampala, and one annual ozone peak in Serrekunda. The yearly TOCA averages showed an oscillation within a five-year period. We also found that the EUV doses were stable at all three locations for the period 1979−2000, and that Kampala and Dar-es-Salaam were mostly cloudy throughout the year, whereas Serrekunda was mostly free from clouds. It was also found that clouds were among the major factors determining the level of EUV reaching the Earth´s surface. Finally, we noted that during rainy seasons, horizontal advection effects augmented by lightning activity may be responsible for enhanced ozone production in the tropics.

On least squares estimation for long-memory lattice processes

A flexible class of anisotropic stationary lattice processes with long memory can be defined in terms of a two-way fractional ARIMA (FARIMA) representation. We consider parameter estimation based on minimizing an approximate residual sum of squares. The method can be applied to sampling areas that are not necessarily rectangular. A central limit theorem is derived under general conditions. The method is illustrated by an analysis of satellite data consisting of total column ozone amounts in Europe and the Atlantic respectively.

A new technique for retrieval of tropospheric and stratospheric ozone profiles using sky radiance measurements at multiple view angles: Application to a Brewer spectrometer

This paper describes and applies a new technique for retrieving diurnal variability in tropospheric ozone vertical distribution using ground‐based measurements of ultraviolet sky radiances. The measured radiances are obtained by a polarization‐insensitive modified Brewer double spectrometer located at Goddard Space Flight Center, in Greenbelt, Maryland, USA. Results demonstrate that the Brewer angular (0–72° viewing zenith angle) and spectral (303–320 nm) measurements of sky radiance in the solar principal plane provide sufficient information to derive tropospheric ozone diurnal variability. In addition, the Brewer measurements provide stratospheric ozone vertical distributions at least twice per day near sunrise and sunset. Frequent measurements of total column ozone amounts from direct‐sun observations are used as constraints in the retrieval. The vertical ozone profile resolution is shown in terms of averaging kernels to yield at least four points in the troposphere–low stratosphere, including good information in Umkehr layer 0 (0–5 km). The focus of this paper is on the derivation of stratospheric and tropospheric ozone profiles using both simulated and measured radiances. We briefly discuss the necessary modifications of the Brewer spectrometer that were used to eliminate instrumental polarization sensitivity so that accurate sky radiances can be obtained in the presence of strong Rayleigh scattering and aerosols. The results demonstrate that including a site‐specific and time‐dependent aerosol correction, based on Brewer direct‐sun observations of aerosol optical thickness, is critical to minimize the sky radiance residuals as a function of observing angle in the optimal estimation inversion algorithm and improve the accuracy of the retrieved ozone profile.

UV measurements in the 3000–5000 m altitude region in Tibet

We present measurements of solar UV radiation performed with multichannel moderate‐bandwidth NILU‐UV filter instruments during winter and summer in 2003 in the altitude region from 3000 m to 5000 m at 29°N in the Lhasa region in Tibet. During summer the UV index was found frequently to exceed 15 on clear days and occasionally to exceed 20 on partially cloudy days. High altitudes, low ozone column amounts, clean atmospheres, and relatively low latitudes are factors that contribute to the high UV levels on the Tibetan plateau. UV index values of 12 were measured in late winter for a solar zenith angle of 40° at a snow‐covered 5000 m altitude site. This is a 35% increase compared to a corresponding snow‐free surface. Our measurements show that the solar UV radiation increases with altitude. For clear‐sky and snow‐free conditions the altitude increase is 7–8% per km for erythemal UV dose rates and 3% per km at a wavelength of 340 nm. Results from clear‐sky calculations using a multiple‐scattering radiative transfer model were found to agree within 5% with clear‐sky UV measurements. Radiative transfer calculations combined with measurements were used to estimate the influence of clouds on the UV radiation at the surface. On the average the variable cloud cover in Lhasa reduced the daily integrated erythemal UV dose by 25%. The NILU‐UV instruments also provide total ozone column amounts. The mean difference between daily total ozone column amounts derived from NILU‐UV measurements and from Earth Probe TOMS data was −1.4% ± 3.2% (1σ).

Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2004 tropical ozone climatology: 3. Instrumentation, station‐to‐station variability, and evaluation with simulated flight profiles

The Southern Hemisphere Additional Ozonesondes (SHADOZ) project has collected more than 3000 ozone profiles from 14 tropical and subtropical sites using balloon‐borne electrochemical concentration cell (ECC) ozonesondes flown with standard radiosondes. Published analysis of ozonesonde precision from SHADOZ profiles measured in 1998–2000 revealed that variations in ozonesonde technique might cause small station‐to‐station biases in the total ozone measurement. We present further evaluation of imprecisions and accuracy based on 1998–2004 SHADOZ data, results from laboratory simulations of ozonesonde flights, and a revised back‐scattered ultraviolet algorithm, the TOMS version 8, 2004 release. The laboratory studies used standard ECC ozonesondes in a flight simulation chamber to evaluate techniques used at SHADOZ stations. Ozone deviations from a standard ozone instrument in the chamber tests resemble those of SHADOZ station data relative to a SHADOZ‐defined climatological reference. Certain systematic variations in SHADOZ ozone profiles are accounted for by differences in solution composition, data processing, and instrument manufacturer. When SHADOZ total ozone column amounts are compared to the new TOMS algorithm, discrepancies between sonde and satellite decline 1–2 percentage points on average, compared to version 7 TOMS.

Observations of solar spectral irradiance change during cycle 22 from NOAA‐9 Solar Backscattered Ultraviolet Model 2 (SBUV/2)

The NOAA‐9 Solar Backscatter Ultraviolet Model 2 (SBUV/2) instrument is one of a series of instruments providing daily solar spectral irradiance measurements in the middle and near ultraviolet since 1978. The SBUV/2 instruments are primarily designed to measure the stratospheric profile and total column amount of ozone, using the directional albedo as the input to the ozone retrieval algorithm. Almost all optical components are common to both radiance and irradiance measurements, whose ratio forms the directional albedo, so that most response changes cancel out. As a result, the SBUV/2 instrument does not require onboard monitoring of time‐dependent sensitivity changes for production of ozone data. We use vicarious comparisons with coincident Shuttle SBUV (SSBUV) solar irradiance measurements during 1989–1996, combined with observed calibration drift during the solar activity minimum in 1985–1986, to determine the long‐term instrument characterization for NOAA‐9 SBUV/2. This approach allows us to derive more accurate solar spectral irradiances for the period from March 1985 to May 1997, spanning two solar cycle minima with a single instrument. The NOAA‐9 irradiance data show an amplitude of approximately 9.3% at 200–205 nm for solar cycle 22. This is consistent with the result of ΔF200–205 = 8.3% for cycle 21 from Nimbus‐7 SBUV and ΔF200–205 = 10% for cycle 23 from UARS SUSIM. NOAA‐9 data at 245–250 nm show a solar cycle amplitude of ∼5.7%. The observed irradiance change at 200–205 nm between the minima of solar cycles 21 and 22 is not consistent with the UV irradiance change expected from the total solar irradiance trend suggested by Willson and Mordvinov [2003]. NOAA‐9 SBUV/2 data can be combined with data from other instruments to create a 25‐year record of solar UV irradiance.

Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 2. Tropospheric variability and the zonal wave‐one

The first view of stratospheric and tropospheric ozone variability in the Southern Hemisphere tropics is provided by a 3‐year record of ozone soundings from the Southern Hemisphere Additional Ozonesondes (SHADOZ) network (http://croc.gsfc.nasa.gov/shadoz). Observations covering 1998–2000 were made over Ascension Island, Nairobi (Kenya), Irene (South Africa), Réunion Island, Watukosek (Java), Fiji, Tahiti, American Samoa, San Cristóbal (Galapagos), and Natal (Brazil). Total, stratospheric, and tropospheric column ozone amounts usually peak between August and November. Other features are a persistent zonal wave‐one pattern in total column ozone and signatures of the quasi‐biennial oscillation (QBO) in stratospheric ozone. The wave‐one is due to a greater concentration of free tropospheric ozone over the tropical Atlantic than the Pacific and appears to be associated with tropical general circulation and seasonal pollution from biomass burning. Tropospheric ozone over the Indian and Pacific Oceans displays influences of the waning 1997–1998 El Niño, seasonal convection, and pollution transport from Africa. The most distinctive feature of SHADOZ tropospheric ozone is variability in the data, e.g., a factor of 3 in column amount at 8 of 10 stations. Seasonal and monthly means may not be robust quantities because statistics are frequently not Gaussian even at sites that are always in tropical air. Models and satellite retrievals should be evaluated on their capability for reproducing tropospheric variability and fine structure. A 1999–2000 ozone record from Paramaribo, Surinam (6°N, 55°W) (also in SHADOZ) shows a marked contrast to southern tropical ozone because Surinam is often north of the Intertropical Convergence Zone (ITCZ). A more representative tropospheric ozone climatology for models and satellite retrievals requires additional Northern Hemisphere tropical data.

Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998–2000 tropical ozone climatology 1. Comparison with Total Ozone Mapping Spectrometer (TOMS) and ground‐based measurements

A network of 10 southern hemisphere tropical and subtropical stations, designated the Southern Hemisphere Additional Ozonesondes (SHADOZ) project and established from operational sites, provided over 1000 ozone profiles during the period 1998–2000. Balloon‐borne electrochemical concentration cell (ECC) ozonesondes, combined with standard radiosondes for pressure, temperature, and relative humidity measurements, collected profiles in the troposphere and lower to midstratosphere at: Ascension Island; Nairobi, Kenya; Irene, South Africa; Réunion Island; Watukosek, Java; Fiji; Tahiti; American Samoa; San Cristóbal, Galapagos; and Natal, Brazil. The archived data are available at: 〈http://croc.gsfc.nasa.gov/shadoz〉.1 In this paper, uncertainties and accuracies within the SHADOZ ozone data set are evaluated by analyzing: (1) imprecisions in profiles and in methods of extrapolating ozone above balloon burst; (2) comparisons of column‐integrated total ozone from sondes with total ozone from the Earth‐Probe/Total Ozone Mapping Spectrometer (TOMS) satellite and ground‐based instruments; and (3) possible biases from station to station due to variations in ozonesonde characteristics. The key results are the following: (1) Ozonesonde precision is 5%. (2) Integrated total ozone column amounts from the sondes are usually to within 5% of independent measurements from ground‐based instruments at five SHADOZ sites and overpass measurements from the TOMS satellite (version 7 data). (3) Systematic variations in TOMS‐sonde offsets and in ground‐based‐sonde offsets from station to station reflect biases in sonde technique as well as in satellite retrieval. Discrepancies are present in both stratospheric and tropospheric ozone. (4) There is evidence for a zonal wave‐one pattern in total and tropospheric ozone, but not in stratospheric ozone.

A study of impacts of tropospheric synoptic processes on the genesis and evolution of extreme total ozone anomalies over southern South America

During the austral autumn of 1997, a sequence of record low ozone events with anomalous behavior occurred at subtropical latitudes to midlatitudes over southern South America, also known as the South Cone (Chile, Argentina, Uruguay, and Southern Brazil). The extreme low ozone events took place in May and early June 1997, i.e., at a time of the year when according to the climatological studies for Northern Hemisphere ozone miniholes, the most significant events would not be expected (if such studies could be simply extrapolated to the Southern Hemisphere). The first and most prominent ozone minihole, with a quasi‐stationary evolution, reached values near 200 DU, i.e., a negative anomaly near 90 DU, close to 40°S. Simultaneously, extreme weather conditions were observed over the South Cone: three significant blocking events took place separated by short intervals. Both the behavior of the ozone layer over the region and the evolution of the blocking events were analyzed. As expected, the evolution of the total ozone column is shown to be linked to these perturbed weather conditions. Despite the apparently similar nature of the blocking events and of the ozone decrease during each of these events, the present study shows that the response of the UT/LS region is different in each case. The respective roles of the vertical displacement of the tropopause and the horizontal advection/divergence of ozone is discussed for the two most important events, showing how these two mechanisms combine in different ways to yield very low total ozone values. The vertical displacement of the tropopause, coupled to the size of the tropopause pressure gradient, appears to be the most efficient mechanism to modify the total ozone column amounts.

Retrieval of ozone column from global irradiance measurements and comparison with TOMS data. A year of data in the Alps

Spectral UV global irradiance has been measured regularly during the year 2000, in the French Alps, using two UV spectroradiometers. The total ozone column amount is retrieved from the ratio of irradiances around 305 and 340 nm, using look up tables based on modeling data. During cloudless days, ozone values from direct sun observations agree to within 5 DU. These ground based values compare reasonably well (to within about 4%, or 12 DU) with TOMS ozone data; this is consistent with the uncertainty of both methods which is estimated to 5 DU for TOMS, and 5–7 DU for clear sky ground based measurements. Generally the ground based ozone values seem to be slightly higher than the TOMS ozone values. For a few cases, the difference between TOMS and ground based values, can reach 10 to 15%. Different possible explanations of the discrepancies are considered.

Total ozone determinations from National Oceanic and Atmospheric Administration operational solar backscattered ultraviolet 2 instrument observations: An update

Total ozone amounts have been derived from observations of the Solar Backscattered Ultraviolet 2 (SBUV/2) instruments on National Oceanic and Atmospheric Administration (NOAA) operational satellites since 1985. The data from the NOAA‐9 instrument for the period 1985–1997 have been reprocessed using instrument characterizations based on in‐orbit operation and comparisons with Shuttle Solar Backscattered Ultraviolet (SSBUV) observations. The data from the NOAA‐11 satellite for the period 1989–1994 have also been reprocessed using mainly internal spacecraft information. The data from the NOAA‐14 satellite for the period 1995–1998 have been reprocessed using complete instrument characterization. The NOAA‐11 and NOAA‐14 data for 1998 to the present, briefly discussed herein, are preliminary. The calibration adjustments used in the reprocessing of NOAA‐9, NOAA‐11, and NOAA‐14 total ozone data are internal, with the exception of those derived from SSBUV analysis, and are independent of Dobson and Total Ozone Mapping Spectrometer (TOMS) total ozone measurements. The reprocessed data have been validated with ground‐based Dobson spectrophotometer observations, and are compared to other sources of total column ozone amounts. The mean bias between NOAA‐9 data and Dobson data is 1.9%±0.6% over the 12‐year record with a trend of 0.1% per year. The mean bias between NOAA‐11 and Dobson data is 0.8%±0.2% with a near‐zero trend. The mean bias between NOAA‐14 and Dobson data is 1.5%±0.3% with a trend of 0.2% per year; the NOAA‐14 calibration and characterization data for the period 1998 to the present are not yet fully analyzed. The methods used in the validations are discussed here, as are some reasons for the results. Finally, a recommendation is made for the use of a relatively continuous total ozone data set.

A method to generate surface UV radiation maps over Europe using GOME, Meteosat, and ancillary geophysical data

This paper presents a method for generating surface ultraviolet (UV) radiation maps over Europe, with a spatial resolution of 0.05°, and potentially on a half‐hour basis. The UV irradiance is obtained by interpolation in a look‐up table (LUT), the entries of which are solar zenith angle, total column ozone amount, cloud liquid water thickness, near‐surface horizontal visibility, surface elevation, and UV albedo. Both satellite (Meteosat, GOME) and nonsatellite (synoptic observations, meteorological model results, digital elevation model) data are exploited to assign values to the influencing factors. With the help of another LUT simulating the visible signal, Meteosat data are processed to retrieve the cloud liquid water thickness. The radiative transfer calculations are performed with the UVspec code. A preliminary step consists in generating an effective surface Meteosat albedo map from a series of 10 consecutive days. In this process the well‐known difficulty of distinguishing clouds from snow‐covered surfaces is encountered. An attempt is made to partially resolve the ambiguity by using the Meteosat infrared channel and modeled snow cover data. After additional empirical cloud filtering, the effective albedo map is used as a baseline to estimate the cloud liquid water thickness. The UV surface albedo is assigned uniform values for land and sea/ocean, except in the presence of snow. In this case it is given a value proportional to the Meteosat effective albedo. The total column ozone is extracted from the level 3 GOME products. The aerosol optical thickness is mapped by gridding the daily measurements performed by ∼1000 ground stations. The digital elevation model is the GTOPO30 data set from the U.S. Geological Survey. European wide UV dose rate maps are presented for one day in April 1997, and the influence of the various factors is illustrated. A daily integrated dose map was also generated using 27 Meteosat acquisitions at half‐hour intervals on the same day. The dose map produced in this way takes into account the evolution of the cloud field and is thought to be more accurate than if it were estimated from one data take, in particular at the relatively high spatial resolution of the product. Finally, a preliminary comparison of modeled dose rate and daily dose with measurements performed with a ground instrument is discussed.

A climatology of ozone mini-holes over the northern hemisphere

A comprehensive climatology of ozone mini-hole occurrence over mid-latitudes of the northern hemisphere is constructed, based on daily measurements of total column ozone from the Nimbus-7 TOMS (total ozone mapping spectrometer) satellite, using version-7 data, over a 14-year period. An ozone mini-hole is a synoptic-scale area of strongly reduced total column ozone amount, which undergoes a cycle of growth and decay, in direct association with tropospheric weather systems. A computer-based analytical method for searching for local minima in the daily TOMS ozone fields is employed to build up a catalogue of mini-hole events, tracking location and intensity. Geographical and seasonal variations in the resulting data are summarised and possible trends over the data period are assessed. Ozone mini-hole activity exhibits a strong annual cycle, peaking in the late winter. Mini-holes are shown to be generally more frequent over the N. Atlantic/European sector than the N. Pacific/N. American sector. An alternative method of defining a threshold is also discussed in which ozone minima are viewed as anomalies from local climatological mean ozone levels. Mini-hole activity is seen to exhibit a close spatial correlation to each of the storm-track regions of the hemisphere. © 1998 Royal Meteorological Society