Multiscale periodicities in aerosol optical depth over India

Abstract

Aerosols exhibit periodic or cyclic variations depending on natural and\nanthropogenic sources over a region, which can get modulated by synoptic\nmeteorological parameters such as winds, rainfall and relative humidity, and\nlong-range transport. Information on periodicity and phase in aerosol\nproperties assumes significance in prediction as well as to examine the\nradiative and climate effects of aerosols including its association with\nchanges in cloud properties and rainfall. Periodicity in aerosol optical depth,\nwhich is a columnar measure of aerosol distribution, is determined using\ncontinuous wavelet transform over 35 locations in India. Continuous wavelet\ntransform is used in the study because it is better suited to extract the\nperiodic and local modulations present at various frequency ranges, as these\nfeatures are invisible in conventional methods such as Fourier Transform.\nMonthly mean aerosol optical depths (AODs) from MODerate Resolution Imaging\nSpectroradiometer (MODIS) on board the Terra satellite from January 2001 to\nDecember 2012 are used. Annual and quasi-biennial oscillations (QBO) in AOD are\nevident in addition to the weak semi-annual and quasi-triennial. The\nsemi-annual and annual oscillations are consistent with the seasonal and yearly\ncycle of variations in AODs. The 40-month periodicity indicates the presence of\nlong term correlations in AOD. The observed periodicities in MODIS Terra AODs\nare also evident in the ground-based AOD measurements made over Kanpur in the\nIndo-Gangetic Plain. The phase of the periodicity in AOD is stable in the\nmid-frequency range, while local disturbances in the high-frequency range and\nlong term changes in the atmospheric composition give rise to unstable phases\nin low-frequency range. That modulations in AOD over one location/region can\ninfluence the other is revealed by the presence of phase relation among\ndifferent locations.\n