An Improved Soil Moisture Retrieval Algorithm for ERS and METOP Scatterometer Observations

Abstract

The scatterometers onboard the European Remote Sensing satellites (ERS-1 & ERS-2) and the METeorological OPerational satellite (METOP) have been shown to be useful for surface soil moisture retrieval using the so-called TU-Wien change detection method. This paper presents an improved soil moisture retrieval algorithm based on the existing TU-Wien method but with new parameterization as well as a series of modifications. The new algorithm, WAter Retrieval Package 5 (WARP5), copes with some limitations identified in the earlier method WARP4 and provides the possibility of migrating soil moisture retrieval from ERS-SCAT to METOP-ASCAT data. The WARP5 algorithm results in a more robust and spatially uniform soil moisture product, thanks to its new processing elements, including a method for the correction of azimuthal anisotropy of backscatter, a comprehensive noise model, and new techniques for calculation of the model parameters. Cross-comparisons of WARP4 and WARP5 data sets with the Oklahoma Mesonet in situ observations and also with European Centre of Medium Range Weather Forcast (ECMWF) ReAnalysis (ERA-Interim) global modeled data show that the new algorithm has a better performance and effectively corrects retrieval errors in certain areas.

Intercomparison of Soil Moisture Retrievals From In Situ, ASAR, and ECV SM Data Sets Over Different European Sites

“…The Essential Climate Variable Soil Moisture (ECV SM) product (Liu et al, 2011;Liu et al, 2012;Wagner et al, 2012) (Wagner et al, 1999a;Wagner et al, 1999b;Naeimi et al, 2009) is used to convert backscatter measurements to soil moisture values and the Land Parameter Retrieval Model (LPRM) developed jointly by VU University Amsterdam and the NASA Goddard Space Flight Center (Owe et al, 2001;De Jeu and Owe, 2003;Owe et al, 2008) is used to convert brightness temperatures to soil moisture respectively. The ECV SM product has been validated across different regions using in-situ, model and SAR-derived soil moisture datasets in previous studies (e.g.…”

“…The spatial and temporal coverage attainable by spaceborne remote sensing has demonstrated the capability to monitor soil moisture over large areas at regular time intervals, and several approaches for soil moisture retrieval have been developed using optical, thermal infrared (TIR), and microwave (MW) sensors over the last three decades (Barrett and Petropoulos, 2013;Petropoulos et al, 2015). Since the late 1970s, coarse resolution (25 -50km) soil moisture products derived from past and present microwave radiometers (Advanced Microwave Scanning Radiometer (AMSR-E) (Njoku et al, 2003) and WindSat (Li et al, 2010)) and scatterometers (European Remote Sensing satellites (ERS) scatterometer (SCAT) (Wagner a (Bartalis et al, 2007;Naeimi et al, 2009)) have been available on an operational basis. Data from the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) (Kerr et al, 2012;Mecklenburg et al, 2012) and the National Aeronautics and Space Administration (NASA) Soil Moisture Active Passive (SMAP) (Entekhabi et al, 2010) dedicated soil moisture missions are strengthening this record of observations and further facilitating the study of long term soil moisture behaviour (please see Petropoulos et al (2015) and Zeng et al (2015) for further details of available satellite derived soil moisture products).…”

Üçlü Eşleştirme Yöntemi ile Uydu ve Hidrolojik Model Kaynaklı Toprak Nemi Değerlerinin Hata Oranlarının Bulunması

“…Bu gözlemler VUA tarafından uygulanan tek tabakalı modele dayanan radyasyon transfer denklemleri ile 0,25° mekansal ve 1 günlük zamansal çözünürlükteki LPRM toprak nemi değerlerine dönüştürülmüştür [8][9]. Bu verilerin analizleri şu ana kadar birçok çalışmada yapılmıştır [20][21][22]. Bu çalışmada kullanılan alçalan C-ve X-bandı pasif mikrodalga ölçümlerine dayanan gözlemlerden elde edilen LPRM verileri, VUA tarafından sağlanmıştır.…”

“…METOP uydusuna bağlı olan ASCAT sensörünün aktif uzaktan algılama yöntemiyle elde ettiği uydu kaynaklı ASCAT toprak nemi ürünü, LPRM verisinden sonra en çok kullanılan uydu kaynaklı veriler arasındadır [21]. Suya doyma oranı temelli olan toprak nemi ürünü %0 ile %100 arasında değişmektedir.…”

“…ASCAT kaynaklı toprak nemi değerlerinin diğer toprak nemi değerleri ile karşılaştırılması şu ana kadar birçok çalışmada yapılmıştır [18,19]. Bu ürün için ayrıntılı bilgi [20,21] çalışmalarında mevcuttur. …”