Aapo Tanskanen

Aerosols and surface UV products from Ozone Monitoring Instrument observations: An overview

We present an overview of the theoretical and algorithmic aspects of the Ozone Monitoring Instrument (OMI) aerosol and surface UV algorithms. Aerosol properties are derived from two independent algorithms. The nearUV algorithm makes use of OMI observations in the 350-390 nm spectral region to retrieve information on the absorption capacity of tropospheric aerosols. OMI-derived information on aerosol absorption includes the UV Aerosol Index and absorption optical depth at 388 nm. The other algorithm makes use of the full UV-to-visible OMI spectral coverage to derive spectral aerosol extinction optical depth. OMI surface UV products include erythemally weighted daily dose as well as erythemal dose rate and spectral UV irradiances calculated for local solar noon conditions. The advantages and limitations of the current algorithms are discussed, and a brief summary of several validation and evaluation analysis carried out to assess the current level of uncertainty of these products is presented. Copyright 2007 by the American Geophysical Union. U7 - Export Date: 2 August 2010 U7 - Source: Scopus U7 - Art. No.: D24S47

Validation of daily erythemal doses from Ozone Monitoring Instrument with ground‐based UV measurement data

[1] The Dutch-Finnish Ozone Monitoring Instrument (OMI) on board the NASA EOS Aura spacecraft is a nadir viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600 km wide viewing swath and it is capable of daily, global contiguous mapping. The Finnish Meteorological Institute and NASA Goddard Space Flight Center have developed a surface ultraviolet irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths, noontime erythemal dose rate (UV index), and the erythemal daily dose. The overpass erythemal daily doses derived from OMI data were compared with the daily doses calculated from the ground-based spectral UV measurements from 18 reference instruments. Two alternative methods for the OMI UV algorithm cloud correction were compared: the plane-parallel cloud model method and the method based on Lambertian equivalent reflectivity. The validation results for the two methods showed some differences, but the results do not imply that one method is categorically superior to the other. For flat, snow-free regions with modest loadings of absorbing aerosols or trace gases, the OMI-derived daily erythemal doses have a median overestimation of 0–10%, and some 60 to 80% of the doses are within ±20% from the ground reference. For sites significantly affected by absorbing aerosols or trace gases one expects, and observes, bigger positive bias up to 50%. For high-latitude sites the satellite-derived doses are occasionally up to 50% too small because of unrealistically small climatological surface albedo.

Surface ultraviolet irradiance from OMI

The Ozone Monitoring Instrument (OMI) onboard the NASA Earth Observing System (EOS) Aura spacecraft is a nadir-viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument has a 2600-km-wide viewing swath, and it is capable of daily, global contiguous mapping. We developed and implemented a surface ultraviolet (UV) irradiance algorithm for OMI that produces noontime surface spectral UV irradiance estimates at four wavelengths (305, 310, 324, and 380 nm). Additionally, noontime erythemal dose rate and the erythemal daily dose are estimated. The OMI surface UV algorithm inherits from the surface UV algorithm developed by NASA Goddard Space Flight Center for the Total Ozone Mapping Spectrometer (TOMS). The OMI surface UV irradiance products are produced and archived in HDF5-EOS format by Finnish Meteorological Institute. The accuracy of the surface UV estimates depend on UV wavelength and atmospheric and other geolocation specific conditions ranging from 7% to 30%. A postprocessing aerosol correction can be applied at sites with additional ground-based measurements of the aerosol absorption optical thickness. The current OMI surface UV product validation plan is presented.

Use of the moving time-window technique to determine surface albedo from TOMS reflectivity data

The seasonal variation of the surface albedo, due to snow or ice, complicates satellite estimation of the high-latitude surface UV irradiance. The TOMS instrument, that measures the backscattered radiances from the Earths atmosphere and surface, does not distinguish cloud backscattering from surface backscattering. When the TOMS UV algorithm is used, false interpretation of the measured high reflectivity as thick cloudiness leads to substantial underestimation of the surface UV irradiance. While the largest UV irradiance is usually received during the summer, the spring exposure to UV radiation is the main concern in high-latitudes since the sensitivity of some biological organisms to UV radiation is more pronounced at low temperatures, and snowcover enhances the surface UV irradiance. This paper presents a new method for estimation of the surface reflectivity. The method is based on analysis of the TOMS Lambertian equivalent reflectivity data using the moving time-window technique. The new method treats the measured reflectivity data as samples from a distribution whose lower tail corresponds to surface albedo. The basic method assumes that the distribution is homogeneous, i.e. the surface albedo is constant within the window. Adequate statistics is achieved only by using a wide time-window which, unfortunately, leads to underestimation of the surface albedo during spring and autumn transitions. Therefore, the method was developed further to account for transitions. The feasibility of the new method has been studied globally for high-latitude regions, and it is expected to improve springtime UV irradiance estimates of polar regions.

The PROMOTE UV Record: Toward a Global Satellite-Based Climatology of Surface Ultraviolet Irradiance

This paper describes the PROMOTE UV Record, which aims to provide a global long-term record of the surface UV radiation. The algorithm developed takes as input cloud information from the International Satellite Cloud Climatology Project (ISCCP) and a recently developed multisensor assimilated record of the total ozone column. Aerosols and surface albedo are based on climatologies. Here, first validation results of the PROMOTE UV Record are presented through comparison against ground-based measurements of daily erythemal UV doses at eight European stations. The validation shows that the method is working reasonably, although there is a clear tendency toward overestimation. Typically, the median bias as compared to measurements is 3%-10% and 56%-68% of the daily doses are within plusmn20% from the ground-based reference. The prototype version of the PROMOTE UV Record included in this paper covers the period from July 2002 to June 2005. The time series will later be extended to start in 1983.

OMI very fast delivery and the Sodankyla/spl uml/ Satellite Data Centre

The Ozone Monitoring Instrument (OMI) operates onboard the National Aeronautics and Space Administrations Earth Observing System Aura satellite, which was launched in July 2004. Like its sister spacecraft Terra and Aqua, Auras capabilities include direct broadcast (DB), i.e., the ability to broadcast data at the same time as they are being measured and stored in the spacecrafts memory for later transmission to Earth. The Finnish Meteorological Institutes Satellite Data Centre at Sodankyla/spl uml/ in Finnish Lapland is exploiting this capability to receive OMI data while Aura is in sight of the receiver, which enables nearly immediate production of OMI data products for a region that includes a large part of Europe, stretching from the North Pole to the Italian Alps. The current OMI Very Fast Delivery (VFD) products include maps of surface UV-B, ozone columns, and cloud coverage.

Spectral solar UV monitoring: worth it?

Monitoring of the terrestrial solar ultraviolet irradiance by using a radiometer is often considered as expensive and laborious or the data collected as insufficient in spatial coverage and in some cases in its temporal resolution, too. Therefore, alternative methods, all relying on modelling in one way or the other, have been developed. They differ in which input they receive, either standard meteorological information, space-based radiance measurements or ground-based irradiances from broadband or multiband UV radiometer or from pyranometer. A comparison of performance is presented between three methods during a 15-month period. The ground reference instrument is the Brewer Mk-III #107 spectroradiometer of the Observatory of Jokioinen, Finland. Compared to the reference, the space-based method overestimates the UV irradiance at noon by 14.6% and the pyranometer-based by 0.9% with root-mean-square differences of 35.5% and 10.4%, respectively. Daily erythemal doses agree by 3.8% for the space-based and 0.4% for the pyranometer-based method with a scatter of 16.5% and 4.6%, respectively. Spectral irradiances generated by the pyranometerbased model agree within 0.4% on average with a standard deviation of 17%. A rough estimate on the cost of each approach suggests that none of them is clearly superior to the others and the actual nature of the data needed may be used in decision making concerning monitoring strategies.

Calibrating six years of multiband UV measurements at Ushuaia and Marambio for model and satellite comparisons

An Antarctic UV-monitoring network established in 1999 as a Spanish-Finnish-Argentinian co-operation consists of multiband filter radiometers located at Belgrano, Marambio, and Ushuaia. To provide with quality controlled and assured calibrated groundbased Antarctic UV data, bi-weekly lamp tests were used on every site and visits of travelling reference instruments on two of the sites. Along the six years of operation, the sensitivity in some of the instrument channels was found to drift up to 61%. In both stations, always the same channels showed the best stability or worst instability. The rigorous quality assurance programme ensured that reliable time series of solar data could be produced, however. The most recent Antarctic ozone depletion period of 2005/2006 was studied by comparing OMI satellite-based erythemally weighted daily doses with the measured polynomial corrected data for August 2005-March 2006 for Ushuaia and Marambio. The root mean square (RMS) of difference between the groundbased and satellite-retrieved daily doses was on monthly basis smaller for Ushuaia (19 - 28 %) than for Marambio (17-58 %), possibly due to e.g. bigger heterogeneity of the ground albedo, and variability of the cloudiness. Our final task of combining the polynomial corrected lamp calibration factors and the traveling reference calibration factors, to produce the final calibrated Antartic UV data, is discussed, too.

The UV service of the ESA-GSE Project PROMOTE

In Europe (EU25) about half a million skin cancer cases are occurring per year and this is strongly associated with personal habits in relation to sun exposure and its UV component. Within the frame of the European GMES-Program (GMES=Global Monitoring for Environment and Security) the ESA-GSE Project PROMOTE addresses this problem by developing and implementing a UV information service that aims to reach as many as possible citizens of Europe (EU25). The overall PROMOTE UV service contains forecast and monitoring products. The underlying methods, the use of satellite data, the various UV products including related user interfaces, as well as accuracy aspects are described. One central ambition of the PROMOTE project is the close interaction between providers and users. Experiences that have been made and will be made during the different stages of the PROMOTE project contribute significantly to the further up-grading of the services.