Effective Path Length for Estimating Rain Attenuation Over an Earth-Space Path Using Features of Stratiform and Convective Precipitation at a Tropical Location

The effective path length (L-E) for the rain attenuation estimation over the Earth-space path depends on rain cell size, rain rate, and rain height which in turn are determined by the types of rain. The present study examines the variation of L-E based on the experimental measurements of Ku-band attenuation and rain rate at a tropical location Kolkata (22 degrees 34N, 88 degrees 22E) in comparison with the ITU-R. The study also utilizes measurements from a 24-GHz micro rain Doppler radar to differentiate the precipitation types as stratiform and convective rain on the basis of the presence and absence of a melting layer. Power-law models of L-E in terms of rain rate are proposed for convective and stratiform types of rain. The investigation reveals the physical phenomena behind the variability of L-E at low and high rain rates which are associated with stratiform and convective rain respectively. The proposed models provide a significantly better agreement with the experimental data compared to the ITU-R model and also account for distinguishing features of two types of rain, thus giving a physical basis of the proposed model which is not reflected in the ITU-R formulations. Plain Language Summary Estimations of effective path length (L-E) for an Earth-space path from Ku-band rain attenuation and rain rate measurements at a tropical location Kolkata reveal that the variation of L-E is controlled by the type of rain namely, stratiform and convective precipitation. Low rain rate events mostly associated with stratiform rain are identified by the presence of a melting layer as observed by micro-rain Doppler radar. There is, however, no melting layer signature in convective precipitation which is characterized by high rain rates. For low rain rates measured at the receiver site, it is expected that higher rain rates will prevail at other points along the Earth-space path. However, for high rain rates at the receiver end, comparatively lower rain rates are likely to occur along the satellite link. The present study shows that the ITU-R model formulations for effective path length are inadequate to account for the larger variability of L-E at low rain rates, underestimating rain attenuation for stratiform rain. Whereas for convective rain, the ITU-R model overestimates L-E and rain attenuation at higher rain rates. Proposed separate power-law models for L-E for the two types of rain provide rain attenuation estimations that agree well with actual experimental measurements.