Remote Sensing in Hydrology
Remote Sensing in Hydrology EES 5093
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Lectures 3 4 precipitation remote sensing 1 What is precipitation Precipitation types When cloud particles become too heavy to remain suspended in the air they fall to the earth as precipitation Precipitation occurs in a variety of forms hail rain freezing rain sleet snow Rain liquid precipitation gt05 mm in diameter Hail ice precipitation gt 5 mm in diameter Snow an aggregate of ice crystals that form onto akes Snow forms at temperatures below freezing For snow to reach the earth39s surface the entire temperature pro le in the troposphere needs to be at or below freezing If a layer is above freezing and the snow partially melts which is called sleet or ice pellets Freezing rain liquid precipitation that reaches the surface in the form of drops gt 05 mm in diameter The drops then freeze on the earth39s surface 2 Why is precipitation observation important Precipitation governs the daily life of the planet and is an important element for monitoring the climatic state of water in the earth stores Precipitation is the central component of the hydrological cycle and as such is of primary importance in hydrology Precipitation is arguably the most important component of the landatmospheric system accountable for most of the variability of terrestrial hydrology Precipitation has large and frequent spatial variations and usually exhibits rapid temporal variations Precipitation is the most important input to produce accurate simulations and forecasts for a suite of hydrological variables soil moisture stream ows and ood levels represents one of the most important goals of present day research efforts in distributed hydrological modeling and land data assimilation systems 3 How to measure precipitation Traditional method is using rain gauge problems are 1 point measurement which may miss rainfall just outside of the gauge location 2 if rainfall is too big tipping may not well functional 3 mechanical and data logging problems 4 out of battery Remote sensing based methods 1 passive microwave PM observations from earthorbiting satellite platforms TRMM microwave imager and others 2 proxy parameters cloudtop temperature and cloud particle size inferred from geostationary GOES observations of visible VIS and infrared IR radiances and 3 active microwave or radar observations NEXRAD TRMM precipitation radar and others Gaugebased point measurements suffer problems but what is the quality of remote sensingbased area measure It suffers a lot of problems as well Passive microwave satellite sensor can measure rainfall quite accurate over ocean while the satellitebased precipitation radar active microwave is better over land But both suffer from poor temporal sampling and the PM sensor even suffers from coarse spatial resolution IR based sensors provide better temporal resolution and moderate spatial resolution but determine precipitation indirectly by inference from cloudtop temperatures Tapiador et al 2004 A number of combined PM and IR algorithms data fusion have been developed to improve the estimates based on the assumption that PM algorithms can provide accurate estimates of instantaneous rain rates and that this information can be used to calibrate IR parameters to improve precipitation estimates from IR data that are available at high temporal frequency Groundbased radar also suffers a number of problems 4 Radar hydrology 41 What s radar hydrology The science of measurement of precipitation rainfall snowfall hail etc using radar technology and its application in hydrology surface runoff oods etc Physics of precipitation processes microphysics of clouds etc rain drop size distribution Propagation and interaction of electromagnetic waves through the atmosphere and with precipitation clouds and the ground scattering absorption transmission Engineering design of a complicated and sophisticated radar transmitting and receiving systems with both electronic and mechanical subsystems Signal processing and image analysis Optimal estimation and uncertainty control Radar rainfall snowfall products and validation NeXtRAD TRMM collocated rain gauge for validation Database organization and data visualization GIS Hydrologic application runoff ood etc 42 Physic of radar measurement of precipitation a Radar scattering of rain drop Radar equation backscattered power by target of active radar P063225 433R 139 P 0 transmitted power W Pr backscattered power W i G antenna gam engmeenng term to enlarge srgnal retum Radar scattenng ofram drop cross seenon m R dlstance between radar and target rn x wavelength of radar 3 Radar Crussrseclmn per mm area a lav HA 7 a radar Cmss aeennn nfn lth smgle rarnrlrnp as a scanner 1 dA rllt39rerennal gen 1 Au memcal area cflhe H ralmlmp m oml area nfa radar scnswnesulunml elemem plxel m39 e dennlee llae smusucn mange Relmlou between 7 and o a UH 0 Radar crass uef clem eeenen per leolum c Radar speural eenuenng A39 r Y r L1 cross secnon nfa 14h slngle mmde as n senrrerer m2 dliT m1 geomemcal volume ofule llh mmdl op Inf 39esolurlon Valium plxrl nf Rela39lon berween a and m Radar cross semen onhe rm surgle mmdrop ofa cloud 27 7 E a me relam e balsam cor5mm of scatterer ram drop or xce Crysml diameter uframdmp vavelengd Radar cross secnon per mm x olume 5 For a preclp able cloud mm a mxmlmprsme dxslnbunon RSD N D a D mum a 1 N 1310MB 31 i ND be number cfmmdmps per ml Valulne ar diameter berween D and DdD D Ianges from 0 m m mty 43 Radar Reflectivity are duecdy relatedto radarre ecnvxty z m umt ofmmEm3 44 Precipitztinn rate Pmcxp anau me R mm bquot 1 sun nu ramimp n vD e mmdmp fan speed mm 1H Fm gnslance speedtm 03 mm C 17 76 m squot cm In x 1 Cm Fonhxs case a have 1117 57 Dmymym s D Factors mm affect the Iamdmp szze dummy RSD lbs spacernme mummy afRSD is typmany due a aVanery nfphysnczd processes such as 1 a apmnon culhslonrcoalescence 3 malmmnalbre p 4 mhomogenemes maqu by turbulcm nclunuous amp somng em 45 ZVR ralz nnship er relancnslnp Ramfall ammmls ram unenst 0 ram rare R m mmh or Iamfa accumulanon RA 111 mm x5 emulated using a 2R elatmnshxp 1 2 A lankarp table 3 A neural wmk General appmaches m select the ZrRrelaucn dmprsxze drsmburrorr RDS approach 241 rexauorrs are aerrred Em 1 se 39alm Opumlznuon apymacb radar re ecmuy measured 111 me annosphex by radar ls ralarad w smfacc absenzuons ofmmfall rate typxcally Er r mm gauge nemork 0 ZVR relarmr used an NexrxAD 1 1 300R Defnuh equaucn 230 R1 a For Impxcal ramfan mints mi R m mm bquot Uulenlmep R J A precxpnatmn as rufarra Atmosphenc Sciences 6010 112071233 2003 Exam Smith M Stems The mlcmphyslcal su ucmre of exueme from oundrbased amde specrra Journal of Lb mm R A y P n ach Dr an Mallet and r O39Baunun ma WSRV 88D ramfall algomhm Wemher andFMecasung 13 3777395 1993 5 Groundbased active microwave radar such as NEXRDA also called WSR88D Groundbased radar offer areal measurements of precipitation from a single location over a large area in near realtime Both single and multipolarization radars have been used over a range of wavelengths mostly 3 cm Xband 5 cm cband and 10 cm sband 51 NEXRDA 10 cm Details from Lectures and reading materials Here just about the NEXRDA anomalies In addition to mechanical failures calibration and alignment problems 1 range dependent the limitations imposed by the Earth s curvature and terrain induced blockage create a lack of coverage at low altitudes far away from the radar that can prevent the observation of boundary layer phenomena such as tornadoes and limit the accuracy of precipitation estimates near the ground 2 Ground clutter is the radar return from nonmeteorological targest that bias the re ectivity and velocity estimates Ground clutter has a significant impact on the accuracy of radar parameters 3 Virga effect 4 Range degradation 5 Beam blockage 6 Bright band contamination 7 52 CASA x band radar 3 cm The Center for Collaborative Adaptive Sensing of the Atmosphere CASA radar is developed to track tornadoes with high spatial and temporal resolution as well as mapping severe weather events in the lowest 2 km of the troposphere And is targeting for shortrange weather observations The rst generation of an automated network of four lowpower shortrange Xband polarimetric and Doppler radars CASA radar known as NetRad was deployed in cental Oklahoma in spring 2006 NetRad provides advantage such as reduction of transmit power smaller antenna and increase in reliabilityredundancy However there are new challenges severe attenuation due to precipitation limitation rangevelocity ambiguities and increased clutter effects CASA39s radar project consists of three test beds the rst in Oklahoma s tornado alley the second in Houston to monitor and predict oods more accurately and the third in Puerto Rico to improve monitoring of oods produced by thunderstorms and hurricanes over the island The second test bed led by Colorado State also will strive to improve the monitoring of air pollution and air transport of chemicals 6 Satellitebased active microwave TRMM PR radar The Instrument The Precipitation Radar PR actively emits radar pulses toward the ground at frequencies of 13796 and 13802 GHz with horizontal polarization and measures the strength of the backscatter quotechoquot or return signal Precipitation Radar has a horizontal resolution at the ground of about 25 miles four kilometers and a swath width of 137 miles 220 kilometers One of its most important features will be its ability to provide vertical pro les of the rain and snow from the surface up to a height of about 12 miles 20 kilometers The Precipitation Radar will be able to detect fairly light rain rates down to about 027 inches 07 millimeters per hour At intense rain rates where the attenuation effects can be strong new methods of data processing have been developed that help correct for this effect The Precipitation Radar is able to separate out rain echoes for vertical sample sizes of about 820 feet 250 meters when looking straight down It will carry out all these measurements while using only 224 watts of electric power the power of just a few household light bulbs The Precipitation Radar was built by the National Space Development Agency NASDA of Japan The PR measures the echo backscattered from rain because the strength of the echo is roughly proportional to the square of the volume of falling water the PR produces very accurate estimates of rain profiles The PR will determine the vertical distribution of precipitation by measuring the quotradar re ectivityquot of the cloud systems and the weakening of a signal as it passes through the precipitation Thus it will measure the 3D rainfall distribution over both land and ocean More specifically this instrument will define the layer depth of the precipitation and provide information about the rainfall reaching the surface the key to determining the latent heat input to the atmosphere Summary of Features of the TRMM PR sensor 1 uses radar frequencies of 13796 and 13802 GHz with horizontal polarization 2 horizontal resolution 43 km at nadir 3 obtains data in 220 km swaths 4 can perceive rain through clouds 5 makes quantitative measurements of rain mmh over land and ocean with a sensitivity better than 05 mmh 6 measures rain from the ground to an altitude of 15 km with a vertical quotrangequot resolution of 250 m Range resolution is the ability of the radar equipment to separate two re ecting objects on a similar bearing but at different ranges from the antenna The ability is determined primarily by the pulse length in use 7 provides 3dimensional rainfall distribution Data Products Available for the PR Algorithms have been developed for PR data to provide estimates of rainfall rate and vertical rainfall pro les Below is a brief description of the level 2 data product produced by these algorithms TRMM 2A25 precipitation radar PR pro le produces an estimate of the vertical rainfall rate profile for each radar beam The rainfall rate estimate is given at each resolution cell of the PR radar T 0 compare with groundbased radar data the attenuation corrected Z factor pro le is also given The average rainfall rate between the two predefrned altitudes is calculated for each beam position Other output data include parameters of ZR relationships integrated rain rate of each beam range bin numbers of rain layer boundaries and many intermediate parameters The objective of 2A25 is to correct for the rain attenuation in measured radar re ectivity and to estimate the instantaneous threedimensional distribution of rain from the TRMM Precipitation Radar PR data The estimates of attenuationcorrected radar re ectivity factor and rainfall rate are given at each resolution cell of the PR The estimated nearsurface rainfall rate and average rainfall rate between the two predefrned altitudes 2 and 4 km are also calculated for each beam position 7 Satellitebased passive microwave TRMM Microwave Imager TMI 0r PMW measurements Another set of techniques rely on passive microwave PM sensors onboard lowEarth orbit LEO satellites operated by government agencies such as the US Department of DefenseSpecial Sensor Microwave Imager SSMI on Defense Meteorological Satellite Program DMSP platforms Ferraro 1997 the National Oceanic and Atmospheric Administration NOAAAdvanced Microwave Sounding Unit AMSUB Ferraro et al 2000 and the National Aeronautics and Space Administration NASAAdvanced Microwave Scanning Radiometer AMSRE on the Earth Observing Satellite EOS Aqua Wilheit et al 2003 and the TRMM Microwave Imager TMI Microwave instruments respond in a more physically direct way than infrared sensors to the presence of precipitationsize water andor ice particles within clouds while remaining relatively insensitive to nonprecipitating clouds Atmospheric transmittance windows below 20 GHz from 30 to 40 GHz and at 90 GHz are used for rainfall monitoring Below 20 GHz rainfall absorption and emission are predominant and ocean surfaces are warmer than the background radiation Above 60 GHz evidence of rainfall is primarily from scattering where areas of heavy rainfall are colder than their backgrounds Between 20760 GHz a combination of absorption and scattering is present Instrument The TRMM Microwave Imager TMI is a passive multichannel radiometer whose signals in combination can measure rainfall quite accurately over oceans and somewhat less accurately over the land TMI is not a new instrument It is based on the design of the highly successful Special Sensor MicrowaveImager SSMI which has been ying continuously on Defense Meteorological Satellites since 1987 The TMI measures the intensity of radiation at five separate frequencies 107 194 213 37 855 GHz These frequencies are similar to those ofthe SSMI except that TMI has the additional 107 GHz channel designed to provide a more linear response for the high rainfall rates common in tropical rainfall The other main improvement that is expected from TMI is due to the improved ground resolution which will result from the lower altitude of TRMM 218 miles 350 kilometers compared to 537 miles 860 kilometers of SSMI TMI has a 487 mile 780 kilometer wide swath on the surface The higher resolution of TMI on TRMM as well as the additional 107 GHz frequency will make TMI a better instrument than its predecessors Summary of Features of the TMI instrument 1 uses a scan angle of 65 degrees 2 collects data over a swath width of 760 km 3 can perceive rain through clouds 4 makes quantitative measurement of rain intensity mmh as integrated column precipitation content and areal distribution How the TMI measures Rainfall with Microwaves The TMI measures the microwave radiation emitted by Earth39s surface and by cloud and rain drops Calculating rainfall rates from TMI requires some fairly complicated calculations The basis of these calculations is in Planck s radiation law which describes how much energy a body radiates given its temperature Water surfaces such as oceans and lakes have an additional property which is very important The water surfaces emit only about one half the microwave energy specified by Planck s law and therefore appear to have only about half the real temperature of the land surface Water surfaces therefore look very quotcoldquot to a passive microwave radiometer Raindrops on the other hand appear to have a temperature that equals their real temperature They appear warm to a passive microwave radiometer and therefore offer a contrast against quotcoldquot water surfaces The more raindrops the warmer the whole scene appears and research over the last three decades now make it possible to obtain fairly accurate rainfall rates based on the temperature of the microwave scene Land is very different from oceans in terms of the emitted microwave radiation appearing to have about 90 percent of its real temperature In this case there is little contrast to observe the quotwarmquot raindrops Certain properties of rainfall however still can be inferred The high frequency microwaves 855 GHz measured by TMI are strongly scattered by ice present in many raining clouds This reduces the microwave signal at the satellite and offers a contrast against the warm land background Because large ice particles often present in upper cloud regions tend to scatter this emitted radiation from the land surface and rain the remaining radiation that reaches the PM sensor is interpreted as a colder brightness temperature The TMI uses its various channels along with cloud models to discriminate between these processes and quantify the rain and ice responsible for the observed microwave signatures Data Products Available for the TMI The TRMM 2A12 hydrometeor profile provides the level 2 data product for the TMI The profiling techniques of the algorithm use the Goddard Cumulus Ensemble Model and generate vertical hydrometeor profiles on a pixel by pixel basis For each pixel cloud liquid water precipitation water cloud ice water precipitation ice and latent heating are given at 14 vertical layers based upon the five channels of the TRMM microwave imager TMI The top of each layer is given at 05 10 15 20 25 30 35 40 50 60 80 100 140 and 180 km above the surface The surface rainfall and the associated con dence indicator is also calculated Each data granule is one orbit plus 50 scan lines of preorbit overlap and 50 scan lines of postorbit overlap Each data granule consists of two parts metadata and swath data 8 Satellitebased infrared precipitation The first set of techniques for deriving precipitation are based on using visible and infrared VISIR imagery to discriminate the brightness of the cloud in the visible spectrum andor the low temperature of the cloud top as seen in the thermal spectrum eg Arkin amp Meisner 1987 Barrett amp Martin 1981 Lovejoy amp Austin 1979 Other VISIR techniques focus on adding sophisticated criteria such as cloud area extent time history and textural features eg Adler amp Negri 1988 Scofield 1987 Wu et al 1985 The VISIR techniques however are inherently indirect depending only in a statistical sense on the presence of rain below the cloud top Petty 1995 81 Visible Infrared Scanner V IRS 0f TRMM The VIRS measures radiance in five bandwidths from the visible through the infrared spectral regions T he VIRS is a 5 channel crosstrack scanning radiometer operating at 063 16 375 1080 and 120 microns It is similar to AVHRR The VIRS is intended to provide very high resolution information on cloud coverage type and cloud top temperatures and also serve as the link between these data and the long and virtually continuous coverage by the geosynchronous meteorological satellites Scientists will use the infrared IR data to make rough estimates of tropical precipitation The instrument with a swath width of 720 km will eventually provide cloud distributions by type and height and rain estimates from brightness temperatures at a horizontal resolution of 21 km nadir At this point in time there are no level 2 data products available for the VIRS A level 1 data product 1B01 Visible and Infrared Radiance is available which produces calibrated radiances for each of the five wavelengths measured by the instrument 82 GOES GOES satellites provide the kind of continuous monitoring necessary for intensive data analysis They circle the Earth in a geosynchronous orbit which means they orbit the equatorial plane of the Earth at a speed matching the Earth39s rotation This allows them to hover continuously over one position on the surface The geosynchronous plane is about 35800 km 22300 miles above the Earth high enough to allow the satellites a fulldisc view of the Earth Because they stay above a fixed spot on the surface they provide a constant vigil for the atmospheric quottriggersquot for severe weather conditions such as tornadoes ash oods hail storms and hurricanes When these conditions develop the GOES satellites are able to monitor storm development and track their movements GOES satellite imagery is also used to estimate rainfall during the thunderstorms and hurricanes for ash ood warnings as well as estimates snowfall accumulations and overall extent of snow cover Such data help meteorologists issue winter storm warnings and spring snow melt advisories Satellite sensors also detect ice fields and map the movements of sea and lake ice 9 Validation All products can be used for various applications such as hydrological modeling and climate modeling and many others But the quality of the products is very important I will introduce you some basics on how it can be done and what it is going on Statistics parameters HydroNEXRAD Community Resource for Use of RadarRainfall Data Presented by Witold F Krajewski A Kruger JA Smit R Luwrece R Goska P Domaszczynski C Gunyon BC Seo ML Baeck AA Bradley MK Ramamurthy WJ Weber SA Delgreco B Nelson S Ansari M Murthy D Dhutia M stelner AA Ntelekos G Villarini DQLEU 1255 What is Hyd roNEXRAD A Webbased prototype application that allows ordering customized radarrainfall maps products for hydrologic studies based on NEXRAD data Why to provide the hydrologic community with ready access to the vast archives and realtime information on precipitation collected by the national network of WSR88D weather radars K HydroNEXRAD NSF ITR Project 0 The University of Iowa Lead Conceptual design database development and maintenance metadata calculation algorithms software engineering Graphical User Interface user suppch Archival data future im lementation of the s stem 0 UCARUnidata Program Center Realtime data provider hydrologic product distribution map server maintenance Hyd roNEXRAD s Main Features 0 Basin centric USGS HUC2 through HUC8 Research Appeal Basin centric 0 Radar expertise not required Highly customizable products 0 Repeatability Let s transition to live demonstration back from live demonstration after receiving email notification ftp the products and open them in Arc View GIS Where Are We Now with NCD to transfer the missing data Working with NCDC to implement HydroNEXRAD metadata calculations at the NCDC Developed a prototype realtime version tested at the Clear Creek and Susquehanna WATERS observatories Working with Unidata to add realtime products to LDMbased distribution Conducting largescale tests and evaluation experiments Debugging enhancing testing checking debugging enhancing Limitations some No true real time 0 No quantification of uncertainty When It Is All Done Complete metadata database 0 Opening the system to the community this summer OFacing the question What s next quot quot quotquotquotquot 12836 radaryears Of metadata o crdWIcl s lJ executables 27 scripts 1000 HUCs and growing The Issue What s next NSF project ending soon Will take years to match HydroNEXRAD s functionality Needs Unidata All need hardware and software enhancements and maintenance