1. European Geosciences Union General Assembly 2006 Vienna, Austria, 02 – 07 April 2006 Paper’s objectives: 1. Contribute to the validation of MODIS aerosol products over south-east Italy and investigate the correlation dependence on spatial resolution and identify regional biases of Lecce’s AERONET data 2. Can MODIS help us to understand to what extent the Lecce’s AERONET site can be considered representative of a larger area and hence, locally-derived aerosol parameters can be of use in General Circulation and Chemical Transport Models? AERONET versus MODIS retrievals at different spatial resolutions over south-east AERONET versus MODIS retrievals at different spatial resolutions over south-east Italy M. Santese, F. De Tomasi and M. R. Perrone Physics Department, University of Lecce, Italy (monica.santese@le.infn.it / Fax: +39 0832 – 297505) AERONET versus MODIS AOTs Over OceanOver land-ocean 50x50 km² 100x100 km² Fig. 2: Scatter plots of MODIS AOT referring to the 50x50, 100x100, and 300x300 km² window size centered on Lecce versus AERONET AOT mean values collocated in time. Solid red and black lines represent the linear regression lines and the 1:1 lines, respectively, dashed lines are MODIS pre launch expected uncertanties. AERONET versus MODIS temporal evolution Over OceanOver land-ocean 50x50 km² 100x100 km² 300x300 km² Fig.2 : Temporal evolution of MODIS (blue dots) and collocated in time AERONET (red dots) AOTs referring to the 50x50, 100x100, and 300x300 km². Open blue and red dots represent monthly averaged values of MODIS and AERONET AOTs collocated in time, respectively. Geographic location of the AERONET monitoring site 300x300 km² 100x100 km² 50x50 km² AERONET and MODIS-ocean fine fraction parameters Fig.3(a),(b) Temporal plots (red dots) of MODIS-ocean fine fraction η M (c) temporal plot (red dots) of the AERONET fine fraction parameter η A. Black full dots and error bars represent monthly average values and corresponding standard deviations. Blue boxes show on each panel the monthly distribution of data points. 300x300 km² Aerosol optical thicknesses AOTs and Fine Fraction parameters η retrieved by AERONET measurements from March 2003 to September 2004 at Lecce’s University, are compared to similar MODIS_Terra data retrieved over ocean and land-ocean at 550 nm and at different spatial resolutions (50x50, 100x100, and 300x300km²) co-located in space and time. Results Comments: The correlation factors R of linear regressions span the 0.88-0.83 range. MODIS AOTs meet expected uncertainties: Over ocean 70%, 67%, and 70% of data points of the 50x50 km², 100x100 km², and 300x300 km² window size, respectively is within expected uncertainties; Over land-ocean 85%, 88%, and 82% of AOT values retrieved at 50x50 km², 100x100 km², and 300x300 km² window size, respectively meets pre-specified accuracy conditions. MODIS overestimates AOTs at low aerosol loadings. This result can be due to the fact that the two algorithms understimate the ground surface reflectance. The slopes of the over land-ocean regression lines is closer to unity: the land- ocean MODIS AOT values can better represent the aerosol properties over south- east Italy. Comments: MODIS AOTs follow the temporal evolution of AERONET AOTs at all tested window sizes and are characterized by a significant seasonal dependence. The temporal evolutions of ocean and land-ocean mean AOTs are not dependent on window size. Comments: The η M temporal evolution is not affected by the window size: monthly average values of the 50x50 km² window size are rather similar to those of 300x300 km² window size. η M monthly means depend on seasons and take values in the 0.7- 0.8 and 0.4- 0.6 range in spring-summer and autumn-winter, respectively. η A monthly means span the 0.7- 0.8 range during all year and are not significantly affected by seasons. It is possible that the marked seasonal evolution of η M is mostly due to the MODIS-ocean algorithm that underestimates the fine fraction contribution on autumn-winter months. 50x50 km² 300x300 km² Despite previous investigations on the validation of MODIS retrievals, the results of this study refer to a single site on south-east Italy where different aerosol types may converge during the year and many aerosol types can superimpose mainly in summer as a consequence of the weather stability. Then, the area can be well suited to test the performance of MODIS retrieval algorithms. Conclusions ° MODIS AOT meet expected uncertainties; ° Regression lines fitting ocean- and land-ocean-MODIS AOT values indicate that MODIS overestimates AOTs at low aerosol loadings; ° The slope of the regression lines fitting the scatterplots with land- ocean-MODIS AOTs is closer to unity: the land-ocean-MODIS AOTs better represent the aerosol properties over south-east Italy. The temporal evolution of the MODIS fine fraction η M (fig. 3) depends on seasons, while the AERONET fine fraction η A doesn’t vary during all year: MODIS-ocean algorithm underestimates the fine fraction contribution on autumn-winter ? ° All these results can allow inferring that AERONET AOTs retrieved at Lecce can be considered representative at least of a 300x300 km² area centered on Lecce. ° Hence locally-derived aerosol parameters can be of use in General Circulation and Chemical Transport Models.