Measurements of snow using a portable high resolution radar for testing remote sensingresolution radar for testing remote sensing retrieval algorithms and snow hydrological modelsHans-Peter MarshallCenter for Geoph sical In estigation of the Shallo S bs rface1Center for Geophysical Investigation of the Shallow Subsurface, Boise State UniversityU.S. Army Cold Regions Research and Engineering LaboratoryOutlineMti tihi df ?•Motivation - why microwave radar for snow?• Radar development and calibration• Spatial snow stratigraphy/depth measurements on alpine snow, polar firn, sea ice pp• Snow depth measurements with highly portable system in 1st order basinportable system in 1st order basin • Preliminary CLPX-II resultsCli•Conclusions• Future Work2Snowmelt dominated regions3• Annual snowfall / runoff. Red lines indicate regions where runoff is snowmelt-dominated [Barnett et. al., 2005]100-year projected change in annual runoff42007 IPCC projections for changes in annual runoff (water availability %) for the period 2090-2999 relative to 1990-1999Large scale observations•Global surface•Global surface temperature and sea level estimates have increased in accuracy• Uncertainties in N. HihHemisphere snow cover as large as 1925• Snow-covered area estimates much more hSWEaccurate than SWE[IPCC synthesis report, 2007]5report, 2007][IPCC, 2007]Snow Water Equivalent (SWE)S iti li t h i di t•Sensitive climate change indicator• Vital component of water cycle• Effects global energy balance•Estimates needed for water resourceEstimates needed for water resource management, flood forecasting, prediction of available hydropower energyof available hydropower energy=?6?But very difficult to estimate SWE i l lSWE at regional scales…• Can vary by more than 50% in 10 meters [Shook and Gray, 1996]•Standard deviations of•Standard deviations of 65-80% of basin mean are typical [Marchand and Killingtveit, 2005]• Manual measurements extremely timeextremely time consuming (~1 hr for standard snowpit)7Less than 3 SNOTEL sites itthdin most watersheds8Microwave Remote SensingMicrowave Remote SensingUfftdb ld•Unaffected by clouds• Penetrate snowpack, •Sensitivity to SWESe s t v ty to SW• Passive footprints are prohibitively large (~25km) for mountainsS iti it t i t t•Sensitivity to snow microstructure, snow layering, underlying soil9Interpretation of remote sensing ilmeasurements is complex• Large sub-footprint variability –gpyground truth measurements difficultbd•Narrowband measurements –integrate over entire depth of snowpacksnowpack• Sensitivity to grain-size and stratigraphy• Currently can map snowcover extent, but no operational SWE lith itf ti d10algorithms exist for active radarMotivationGdtthf t i•Ground truth for remote sensing• Rapidly characterize spatial distribution and structure of snow• Link manual point measurements• Provide framework for accurate estimates of total SWE at basin scaleestimates of total SWE at basin scale and remote sensing footprints•11GlaciologyHi h t iffi•High res. measurements in near-surface firn • spatial variability of annual accumulation•link shallow cores/ neutron probe/snowpits•link shallow cores/ neutron probe/snowpits• Remote sensing ground-truth (CryoSat,NASA IceBridge)g)• Scaled laboratory measurements to study melt-water pathways in glaciers12Snow avalanche• layer thicknessy• spatial variability of stratigraphy/storm snowfallsnowfall• geometry of a slab13Slope stability controlled by snow microstructure (μm-cm) and spatialmicrostructure (μmcm) and spatial variability (m-km)14FMCW radar for snow studies• Instead of 1-10 MHz for ice, GHz frequencies are optimal for near surface (13) f i til lti (1 )(1-3m) snow, for maximum vertical resolution (1 cm)• Microwave (GHz) frequencies allow small, lightweight, directional antennas • All active remote sensing measurements in GHz rangeMi d t di i i 1980’•Microwave radar studies in snow since 1980’s• [e.g. Ellerbruch and Boyne, 1980; Gubler and Hiller, 1984; Fujino et al., 1985; Holmgren et al., 1998; Kanagaratnam et al., 2001; Gogineni et al, 2003; Yankielun et al 2004]15Yankielun et al., 2004]• See [Marshall and Koh, CRST, 2008] for a reviewNASA Cold Lands Processes Experiment (CLPX)Colorado, 2002-2003,Gdbdii d iid•Ground based active microwave radar, coincident with aircraft, satellite, and intensive manual measurementsmeasurements• 4 different snowpacks• Wide range of sensor parameters (frequency, 16gp(qy,incidence angle, bandwidth, etc)Measuring stratigraphy:2-6 GHzsnow surfacelayer interfacesground17Measuring stratigraphy:14-18 GHzsnow surfacelayer interfacesground18[Marshall et al., Hydrol. Proc., 2004]But are rapidly attenuated if snowpack is wetp19But are rapidly attenuated if snowpack is wetp20Locate causes of major reflections•Metal reflectors placed at known depths to determine21Metal reflectors placed at known depths, to determine cause of reflections in original signalMetal reflector experiment22Metal reflector experiment23Metal reflector experiment24Metal reflector experiment25Accuracy of using mean dielectric ti t ti t l it < 2%properties to estimate velocity: < 2%26Comparing FMCW signal to in-situ electrical measurements• radar => in-situ dielectric i(Fiih fk)properties (Finish snowfork)[e.g. Harper and Bradford, 03]Iit ti > h i l•In-situ properties => physical properties(e g Sihvola et al 1985;(e.g. Sihvola et al, 1985; Schneebeli et al, 1998; Matzler, 1996)27In-situ Reflectivity1212εεεε+−=−situinR28Radar Snow Water Equivalent Estimates from Travel-timermsSWE(TWT,ρ)≈2%rmsSWE(TWT,d) ≈ 9%rmsSWE(TWT,ρ= 250kg / m3) ≈ 10%()szSWE z dzρ≡∫()/sgTWT z z cε=−2912ερ=+()gzSWE z dzρ≡∫()sgDevelopment of a Portable FMCW RadarOld radar•350 lb 1 h + t ti•~350 lbs, 1 hr+ setup time• Stationary, sweep ~2 m arc• Extensive post-processing required, little information in fieldinformation in fieldNew radar• <20 lbs, 10 min setup time•Improved signal processing sensitivityImproved signal processing, sensitivity, resolution• User-friendly MATLAB-based data acquisition software, real-time processing30q,pg• 20 complete radar profiles/sec• Easily operated by 2 skiersPortable radar allows rapid measurements of snow31Accuracy of radar estimated snow depths32NASA Energy and Water cycle Sd(NEWS)jtSponsored (NEWS) project• 3 new FMCW radars, 418 GH d l4-18 GHz dual