Satellite passive microwave detection of North America start of season Matthew O. Jones a, b, ⁎, John S. Kimball a, b , Lucas A. Jones a, b , Kyle C. McDonald c, d a The University of Montana Flathead Lake Biological Station, Polson, MT 59860, United States b Numerical Terradynamic Simulation Group, The University of Montana, Missoula, MT 59812, United States c CREST Institute and CUNY Crossroads Initiative, Department of Earth and Atmospheric Sciences, City College of New York, New York, NY 10031, United States d Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, United States abstract article info Article history: Received 30 November 2011 Received in revised form 6 March 2012 Accepted 24 March 2012 Available online 1 May 2012 Keywords: Phenology VOD AMSR-E MODIS NACP NDVI LAI TIMESAT Start of season The start of season (SOS) phenological metric indicates the seasonal onset of vegetation activity, including canopy growth, photosynthesis and associated increases in land–atmosphere water, energy and carbon (CO 2 ) exchanges influencing weather and climate variability. Satellite optical-infrared (IR) remote sensing is responsive to vegetation greenness and SOS, but measurement accuracy and global monitoring are con- strained by atmosphere cloud/aerosol contamination and seasonal decreases in solar illumination for many areas. The vegetation optical depth (VOD) parameter from satellite passive microwave remote sens- ing provides an alternative means for global phenology monitoring that is sensitive to vegetation canopy biomass and water content, and insensitive to atmosphere and solar illumination constraints. A global VOD record from the Advanced Microwave Scanning Radiometer for EOS (AMSR-E) was used to estimate North America SOS patterns and annual variability at the ecoregion scale. The SOS metrics were derived for a four year (2004–2007) record using TIMESAT and AMSR-E 10.7 GHz frequency VOD retrievals composited to 4-day median time series. The VOD SOS corresponded favorably with MODIS-for-NACP NDVI (0.73 b R b 0.81; p b 0.01) and LAI (0.66 b R b 0.89; p b 0.01) greenup dates, and stand level SOS esti- mates derived from flux tower gross primary production (r 2 = 0.61, p b 0.01) and ecosystem respiration (r 2 = 0.44, p b 0.01) estimates. The VOD SOS was temporally offset from MODIS greenup and tower SOS metrics by up to 4–7 weeks (RMSE) and the offset patterns coincided with the primary climate constraints (temperature and water) to vegetation growth. The VOD SOS generally preceded greenup in cold temper- ature constrained ecoregions and followed greenup in warmer, water limited ecoregions, with delays in- creasing for areas with greater woody vegetation cover. The AMSR-E VOD record captures canopy biomass changes independent of NDVI greenness or LAI measures, providing new and complementary phenological information for regional carbon, water and energy cycle studies. © 2012 Elsevier Inc. All rights reserved. 1. Introduction Phenology is the study of the timing of recurring biological events, the causes of their timing, their relationship to biotic and abiotic forces, and the inter-relations among phases of the same or different species (Lieth, 1974). The term land surface phenology (LSP) has been adopted by the Committee on Earth Observation Satellites Land Prod- uct Validation (CEOS-LPV) subgroup to represent satellite-based products that are derived by summarizing an annual land surface sig- nal. Vegetation LSP products, such as MODIS-for-NACP (Tan et al., 2010), describe seasonal changes in vegetation greenness and photo- synthetic leaf area, including canopy greenup date, peak date, brown- down date and season length. These LSP metrics are sensitive to the timing and duration of vegetation activity, including seasonal changes in land–atmosphere water, energy and carbon exchange, which also influence climate variability and global change (Morisette et al., 2009; Penuelas et al., 2009). Although the availability of satellite re- mote sensing based LSP products has grown, phenological patterns and trends based on spaceborne observations remain uncertain due to the variety of methods used to derive LSP metrics (White et al., 2009). This uncertainty is partially due to the challenge of equating satellite based LSP metrics to in situ phenological measurements (Schwartz & Hanes, 2009; Wang et al., 2005). Satellite LSP monitoring has also been largely limited to optical-infrared (IR) wavelengths, which are constrained by signal degradation from low solar illumina- tion, clouds, smoke and atmosphere aerosol contamination for many areas and periods. These constraints require spatial and temporal compositing of the data, which can create data gaps and reduce tem- poral precision, thereby degrading the LSP signal. Satellite microwave remote sensing at lower frequency wavelengths is sensitive to vegetation water content, canopy structure and biomass changes, and provides the potential for near daily global LSP monitoring that is insensitive to solar illumination and atmosphere contamination effects. Previous studies have applied satellite passive and active Remote Sensing of Environment 123 (2012) 324–333 ⁎ Corresponding author at: The University of Montana, CHCB 424, 32 Campus Dr., Missoula, MT 59812, United States. Tel.: +1 406 243 6318; fax: +1 406 243 4510. E-mail address: matt.jones@ntsg.umt.edu (M.O. Jones). 0034-4257/$ – see front matter © 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.rse.2012.03.025 Contents lists available at SciVerse ScienceDirect Remote Sensing of Environment journal homepage: www.elsevier.com/locate/rse