L. J. B. McArthur

Long‐term variations of UV‐B irradiance over Canada estimated from Brewer observations and derived from ozone and pyranometer measurements

Routine uniform spectral UV-B measurements with Brewer spectrophotometers in the Canadian network began in 1989. This relatively short duration of UV measurements militates against reliable detection of long-term changes in UV. A statistical model has been developed to extend the record of UV back to the early 1960s. It estimates UV values (at individual wavelengths and spectrally integrated) from global solar radiation, total ozone, dew point temperature, and snow cover. The model results are demonstrated to be in good agreement with the measurements. For example, the standard deviation of the difference between monthly values of measured and derived erythemally weighted UV irradiation is 3.3% for summer months. The major source of error in the model estimates is probably linked to rare occurrences of absorbing aerosols in the atmosphere. Long records of reliable measurements of total ozone, global solar radiation, and other parameters made it possible to derive UV-B values at three Canadian stations from the mid-1960s. Trends in derived erythemally weighted UV at two stations (Toronto and Edmonton) are similar to those expected from total ozone trends although the estimated error of the UV trends is more than 2 times larger. However, the increase in annual UV at Churchill (59°N) in 1979–1997 was found to be more than twice that expected from the ozone decline. This is a result of longterm changes in snow cover and clouds.

UV index climatology over the United States and Canada from ground‐based and satellite estimates

[1] Long-term monthly mean UV index values for Canada and the United States were calculated using information from two sources: from noon erythemal UV estimated from Total Ozone Mapping Spectrometer (TOMS) total ozone and reflectivity data and from UV index values derived from observations of global solar radiation, total ozone, dew point, and snow cover. The results are presented as monthly maps of mean noon UV index values. Mean UV index values in summer range from 1.5 in the Arctic to 11.5 over southern Texas. Both climatologies were validated against spectral UV irradiance measurements made by Brewer spectrophotometers. With snow on the ground the TOMSbased data underestimate UV by up to 60% with respect to Brewer measurements and UV derived from global solar radiation and other parameters. In summer, TOMS UV index climatology values are from 10 to 30% higher than those derived from global solar radiation and other parameters. The difference is probably related to aerosol absorption and pollution effects in the lower troposphere that are not currently detected from space. For 21 of 28 midlatitude Brewer sites, long-term mean summer UV measured values and UV derived from global solar radiation and other parameters agree to within +5 to 7%. The remaining seven sites are located in ‘‘clean’’ environments where TOMS estimates agree with Brewer measurements while UV derived from global solar radiation and other parameters is 10–13% lower. Brewer data also demonstrate that clean and ‘‘typical’’ sites can be as little as 70–120 km apart. INDEX TERMS: 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); KEYWORDS: UV index, Brewer, TOMS, pyranometer, climatology, ozone

Multisensor analysis of integrated atmospheric water vapor over Canada and Alaska

[1] Atmospheric water vapor is a key parameter for the analysis of climatic systems (greenhouse gas effect), in particular over high latitudes where water vapor displays an important seasonal variability. The sparse spatial and temporal sampling of atmospheric water vapor observations across Canada needs to be improved. A series of instruments and methods including a 940-nm solar absorption band radiometer (R) and radiosonde (S) analysis from a numerical weather prediction model and a ground-based bi-frequency Global Positioning System (GPS) were used to evaluate the integrated atmospheric water vapor (IWV) at various sites in Canada and Alaska from a multiyear database. The IWV-R measurements were collected within the framework of the North American Sun Radiometry network (AERONET/AEROCAN). Intercomparisons between [IWV-GPS and IWV-S], [IWV-R and IWV-GPS], and [IWV-R and IWV-S] show root mean square (RMS) differences of 1.8, 1.9, and 2.2 kg m � 2 , respectively. GPS meteorology appears to be the easiest approach to calibrate the solar radiometer water vapor band owing to its flexibility, and it allows us to overcome the Sun radiometry limitation in high-latitude areas like the Arctic. The sensitivity of the GPS retrieval to various parameters like GPS satellite constellation and meteorological data are discussed. The classical linear relationship between the surface temperature and the integrated weighted mean temperature profile needed for IWV-GPS retrieval may be significantly different for Arctic air masses compared with midlatitude air masses in the case of tropospheric temperature profile inversion. An ever-expanding multiyear (1994–2001) North American summer water vapor climatology, derived from AERONET/Canadian Sun Radiometer Network, is presented and analyzed, showing a mean value of 19.8 ± 6.1 kg m � 2 and variations from 17 kg m � 2 in Alaska to 23 kg m � 2 in southeastern Canada. The results in Bonanza Creek, Alaska, show significant interannual variations with a peak in 1997, which may

Derivation of UV‐A irradiance from pyranometer measurements

Two statistical models have been developed from 6 years of simultaneous measurements of global radiation by pyranometers, UV-A by a Brewer spectrophotometer and from total ozone, dew point temperature and snow cover data at Toronto. The models estimate instantaneous UV-A irradiance at 324 nm from pyranometer data with an uncertainty as low as 3.5% (1σ) for summer sunny conditions and between 6–10% for cloudy conditions. These uncertainties are reasonably small when compared with the uncertainty of UV-A and global solar irradiance measurements (2–3%). The uncertainty is reduced when daily and longer-term averages are considered. The major source of error in the models is likely linked to rare occurrences of absorbing aerosols in the atmosphere. The models were also tested on a 6-year, independent record from Edmonton. The uncertainties at Edmonton are 30–45% larger than at Toronto for the instantaneous data, approximately 20% larger for daily integrated values.

Estimating UV Index Climatology over Canada

Abstract Hourly UV index values at 45 sites in Canada were estimated using a statistical relationship between UV irradiance and global solar radiation, total ozone, and dewpoint temperature. The estimation method also takes into account the enhancement of UV irradiance by snow using an empirical correction derived from Brewer UV measurements. Different characteristics of UV index distribution over Canada were estimated from the derived UV irradiance for the period 1979–87 and then presented in the form of monthly maps. Direct comparisons of Brewer measurements at seven Canadian sites with derived UV irradiance show agreement within 2%–3% except for periods of melting snow when variations in snow albedo yield higher errors in the derived UV irradiance.

Relations between Optically Derived Aerosol Parameters, Humidity, and Air-Quality Data in an Urban Atmosphere

Abstract This paper deals with diurnal and mensual correlations between ground-based atmospheric observations of columnar and surface optical parameters, standard surface humidity parameters, and surface air-quality data. The implications of a significant portion of small, Rayleigh-free optical depths being attributable to continuum water vapor absorption are analyzed in terms of the impact on the computation of aerosol optical depth and Angstrom spectral coefficients in relatively clear atmospheres. Multiwavelength correlation analysis between aerosol optical depth and precipitable water indicators (surface vapor pressure or vertically integrated precpitable water) yielded a systematic, inverse-wavelength type of dependency in the extracted slopes (apparent attenuation coefficients) that was suggestive of a simple correlation between precipitable water and the accumulation-mode number density of the aerosols. On a diurnal basis, increasing trends in aerosol optical depth were negatively correlated with s...

Estimating UV index climatology over North America

Hourly UV Index values at 45 sites in Canada and 52 in the USA were estimated using a statistical relationship between UV irradiance and global solar radiation, total ozone, and dew point temperature. The estimation method also takes into account the enhancement of UV irradiance by snow using an empirical correction derived from Brewer UV measurements. Different characteristics of the UV Index distribution over North America were estimated from the derived UV irradiance for the period 1979-1987 and then presented in the form of monthly maps. Brewer UV measurements at 11 Canadian and 20 US sites and erythemal UV estimates from TOMS data were used for validation. Direct comparisons with Brewer measurements at 7 Canadian sites for the period in the 1990s when both pyranometer and spectral UV data were available demonstrated agreement within 2-3 percent except for periods of melting snow when variations in snow albedo yield higher errors in the derived UV irradiance.