Abstract
Abstract. X-band Synthetic Aperture Radars (X-SARs), able to image the Earth's surface at metric resolution, may provide a unique opportunity to measure rainfall over land with spatial resolution of about few hundred meters, due to the atmospheric moving-target degradation effects. This capability has become very appealing due to the recent launch of several X-SAR satellites, even though several remote sensing issues are still open. This work is devoted to: (i) explore the potential of X-band high-resolution detection and retrieval of rainfall fields from space using X-SAR signal backscattering amplitude and interferometric phase; (ii) evaluate the effects of spatial resolution degradation by precipitation and inhomogeneous beam filling when comparing to other satellite-based sensors. Our X-SAR analysis of precipitation effects has been carried out using both a TerraSAR-X (TSX) case study of Hurricane "Gustav" in 2008 over Mississippi (USA) and a COSMO-SkyMed (CSK) X-SAR case study of orographic rainfall over Central Italy in 2009. For the TSX case study the near-surface rain rate has been retrieved from the normalized radar cross section by means of a modified regression empirical algorithm (MREA). A relatively simple method to account for the geometric effect of X-SAR observation on estimated rainfall rate and first-order volumetric effects has been developed and applied. The TSX-retrieved rain fields have been compared to those estimated from the Next Generation Weather Radar (NEXRAD) in Mobile (AL, USA). The rainfall detection capability of X-SAR has been tested on the CSK case study using the repeat-pass coherence response and qualitatively comparing its signature with ground-based Mt. Midia C-band radar in central Italy. A numerical simulator to represent the effect of the spatial resolution and the antenna pattern of TRMM satellite Precipitation Radar (PR) and Microwave Imager (TMI), using high-resolution TSX-retrieved rain images, has been also set up in order to evaluate the rainfall beam filling phenomenon. As expected, the spatial average can modify the statistics of the high-resolution precipitation fields, strongly reducing its dynamics in a way non-linearly dependent on the rain rate local average value.
Highlights
Nowadays global climate change is of increasing importance both for public opinion, the scientific community, and for its implications on the global economy and life of Earth’s inhabitants
It is worth stressing that these results have been obtained by a sensor simulator where a Gaussian filter has been applied to the pixel area without considering the antenna side-lobe effects
An analysis of two case studies, related to Hurricane “Gustav” on 2008 over Mississippi using co-registered Weather Radars (WRs) and TSX X-band Synthetic Aperture Radars (X-synthetic aperture radars (SARs)) data and nearly contemporary Tropical Rainfall Measurement Mission (TRMM) ones, and to a Central Italy storm event observed by CSK in 2009, has shown: (i) a correlation between WR reflectivity measurements and X-SAR measured Normalized Radar Cross Section (NRCS); (ii) a correlation between the X-SAR temporal coherence reduction and WR reflectivity field
Summary
Nowadays global climate change is of increasing importance both for public opinion, the scientific community, and for its implications on the global economy and life of Earth’s inhabitants. Spaceborne passive radiometers provides global spatial and temporal coverage of precipitation, but they are limited to a spatial resolution of tens of kilometers which tends to smooth convective rain cell and cluster signatures (e.g., Marzano et al, 2002; Tapiador et al, 2004). Their sensitivity to shallow stratiform rainfall over land is relatively low at lower microwave frequencies, whereas at higher.
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