Comparison of Measured and Modeled Snow Brightness Temperature Using Various Field Techniques for Grain Size Measurement Edward KIM Michael DURAND Noah MOLOTCH Daniel F. BERISFORD Steven MARGULIS Zoe COURVILLE NASA Goddard Space Flight Center Ohio State University University of Colorado, Boulder Jet Propulsion Laboratory University of California Los Angeles Cold Regions Research and Engineering Laboratory Outline • Problem statement • Our approach • Description of snow field measurements • EM models • Microwave radiometer • Tb comparison results • Summary & Conclusions The Problem with Snow Remote Sensing and “Grain Size” • For microwave remote sensing of snow, accurate grain size numbers are extremely important to have because… • the microwave signature of snow is highly sensitive to grain size • Unfortunately “grain size” is not easy to quantify accurately, especially in the field • So what is the “best” way to measure grain size in the field for use in microwave snow emission models? How to determine a “best” grain size field measurement technique? • What do we really need? • What do we already have? • How do we compare (and will anyone else believe our results)? – What are we going to use as “truth”? • What is practical? Our Approach • Field Measurements – – – – Small campaign: 6 days, 1 snow pit per day Limited set of techniques; side-by-side Stereology chosen as “truth” Location: Storm Peak Lab, Steamboat, Colorado, USA • Brightness Temperature Comparison – – – – Try 2 EM models, both multi-layer Grain size & stratigraphy info fed into EM models Models output brightness temperatures (Tb) Compare model Tb vs. observed Tb Grain Size Measurement Techniques • Hand lens • NIR photography • Spectroscopy – Contact – New probe • Stereology Hand lens grain size measurements Pit A Pit B Pex & Dmax related as in Durand, Kim, Margulis, 2008 • Plenty of precedent (e.g., CLPX) • Non-repeatable (e.g., two pits one meter apart; different days, plus user variations; ‘B’ pit was in the radiometer FOV) NIR Camera grain size 1m Pack Depth ~1m • Repeatable (in theory) • Empirically-based (transferability?) Matzl and Schneebeli, 2006 Mätzler, 2002 Contact Spectroscopy • Measures reflectance across entire VIS/IR range • Reflectance varies with grain size (see plot at right) • Vertical resolution ~2cm • Requires commercial spectrometer ($$) • Requires dark tarp to block unwanted background light Spectral Profiler Probe Prototype • Sends an optical package into a slotted sleeve inserted into the snowpack to perform contact spectroscopy in-situ, w/o snowpit. • Black tarp not shown to block light at base. drive tube fiber optic to spectrometer probe carrier body optical camera spectral reflectance probe aluminum sleeve nylon brush Spectral Profiler Probe • Send optics down hole • Lateral reflectance spectra • Fiber optic sends signal to spectrometer on surface • No snowpit needed! • Prototype unit; 1st field trial, so analysis still in progress Stereology grain size (1/4) • • • • 3D cast of actual snow grain structure made with dimethyl phthalate Frozen in field with dry ice (stop metamorphism) Shipped to CRREL for processing in cold room Relatively well-known technique, but logistically intensive Perla & Davis, 1980’s & earlier references Matzl and Schneebeli, 2010 Red line = 1mm Stereology grain size (2/4) Time consuming laboratory work to cut and photograph: 20 slices / sample Stereology grain size (3/4) Red line = 1mm L=total length of cycloid lines I=# of intersections crossed v=ice volume fraction Do= optical equiv. grain size • Cycloids overlaid on image for estimating surface area • Time-consuming manual counting work • Yields SSA directly, but then need to convert to pex for MEMLS Matzl and Schneebeli, 2010 Stereology grain size (4/4) • Image is classified as air or ice • Draw line through image, compute autocorrelation, do for each vertical line • Exponential is fit to true autocorrelation function Wiesmann et al., 1998 EM models • MEMLS • Multi-layer HUT MEMLS sensitivity of Tb to grain size • Vertically averaged optical equivalent grain size from pit • Run MEMLS • Sensitivity is the tangent to the curve • For this grain size (vertical line), sensitivity is similar for 19 & 37 GHz • To achieve 5K Tb accuracy, need 10% grain size accuracy, so for typical grain size, this means Radiometric measurements • Brightness measured daily for three days at 19 and 37 GHz, v-pol Results & Discussion • Grain size & stratigraphy info fed into EM models • Models output brightness temperatures (Tb) • Compare model Tb vs. observed Tb MEMLS (pex) vs. observations • Bias and mean absolute error < 5 K • No empirical tuning factors required! • Based on laboratory work Multi-layer HUT results • • • • • • • 19v observed: 249 K 19v modeled: 230 K (similar for SPL5 and SPL6) 37v observed: 230 K 37v modeled: 153 K (similar for SPL5 and SPL6) The 19v is off by 20 K, and the 37v is off by 80 K. Averaged together, and you have 50 K. MEMLS results Multi-layer HUT results • Note: HUT is set up to use hand lens Dmax measurements, so we’ve compared that to MEMLS hand lens, and then to MEMLS stereology as a reference Summary & Conclusions • Used hand lens, spectroscopy, NIR, & stereology methods to measure grain size; stereology was our ‘truth’ • Used the grain size/correlation length to drive EM models (MEMLS, multi-layer HUT) • Compared EM model Tb’s vs. observed Tb’s (‘truth’) • Using lab-based methods, Tb errors are 5-8 K • Using field-based methods, Tb errors are ~10 K • For error <=5K, need grain size accurate to ~10% ==> 20-100um • Using pex directly from stereology, no tuning empirical factor is required • Using SSA, an empirical factor (0.74) is required to get the right Tb – attested to in literature (Mätzler, 2002) • Need further examination of multi-layer HUT to understand apparent cold bias