Applications of passive remote sensing using emission: Remote sensing of sea surface temperature (SST)

Transcription

1 Lecture 5 Applications of passive remote sensing using emission: Remote sensing of sea face temperature SS Objectives:. SS retrievals from passive infrared remote sensing.. Microwave vs. R SS retrievals. Required reading: G: 7. Additional/advanced reading: Electronic lecture on SS: arton.j. Satellite-derived sea face temperatures: Current status. Journal of Geophysical Research 00: Emery et al. Estimating sea face temperature from infrared satellite and in situ temperature data. ulletin of the American Meteorological Society J.Vazquez-Cuervo and R. Sumagaysay A comparison between sea face temperature as derived from the European Remote Sensing Along-rack Scanning Radiometer and the NOAA/NASA AVHRR Oceans Pathfinder Dataset. ulletin of the American Meteorological Society SS retrievals from passive infrared remote sensing Principles: meae R radiances in the ospheric window and correct for contribution from clear sky by using multiple channels called split-window technique Using Eq.[4.0] we can write R radiance at OA: 0; exp exp d 0 [5.]

2 Let s re-write this equation using the transmission function exp and that ] [ 0; [5.] where is an effective blackbody temperature which gives the ospheric emission d exp ] [ 0 [5.3] We want to eliminate the term with in Eq.[5.]. Suppose we can meae R radiances and at two at the adjacent wavelengths and ] [ [5.4] ] [ [5.5] NOE: two wavelengths need to be close to neglect the variation in Let s apply the aylor s expansion to at temperature [5.6] Using this expansion for both wavelengths we have [5.7] [5.8] and thus eliminating - we have ] [ / / [5.9] Let s introduce brightness temperatures for these two channels b and b

3 b and b and apply [5.9] to b and to and / [ b ] [5.0] / b / [ ] [5.] / Let s substitute the above expressions for b and to in Eq.[5.5] / [ b ] [5.] / / { [ ]} [ ] / where and are transmissions in the channels and. Eq.[5.] becomes b Using Eq.[5.4] we can eliminate [ ] [5.3] b γ [ ] [5.4] where γ ; and are transmissions in the channels and. Performing liberalization of Eq.[5.4] γ ] [5.5] b [ b b he principle of the SS retrieval algorithm: SS is retrieved based on the linear differences in brightness temperatures at two R channels. wo channels are used to eliminate the term involving and solve for. NOE: Clouds cause a serious problem in SS retrievals > need a reliable algorithm to detect and eliminate the clouds called a cloud mask. 3

4 One needs to distinguish the bulk sea face temperature and skin sea face temperature: ulk -5 m depth SS meaements: Ships uoys since the mid-970s: buoy SSs are much lees nosy that ship SSs Data from buoys are included in the SS retrieval algorithm Skin SS from infrared satellite sensors: SR Scanning Radiometer and VHRR Very High Resolution Radiometer both flown on NOAA polar orbiting satellites: since mid-970 AVHRR Advanced Very High Resolution Radiometer: since channels started on NOAA-6 since channels started on NOAA- able 5. AVHRR CHANNELS AVHRR Wavelength Channel m AVHRR MCSS Multi-Channel SS algorithm: SS a b4 γ b4 - b5 c [5.6] where a and c are constants. 4 γ 4 and 5 are transmission function at AVHRR channels 4 and

5 AVHRR NLSS Non-Linear SS operational algorithm Version 4.0: SS a b b4 c b4 - b5 SS guess d b4 - b5 [secθ sat -] [5.7] where SS guess if a first-guess SS; b4 and b5 are brightness temperature meaed by AVHRR channels 4 and 5; a b and c are coefficients that calculated for two different regimes of b4 - b5 : one set for b4 - b5 < or 0.7 and another set for b4 - b5 > 0.7 he coefficients a b and c are estimated from regression analyses using co-located in situ buoy and satellite meaements called matchups. Alternative approach used in the SS retrieval algorithm in ASR Along-rack Scanning Radiometer on ERS; ASR has 4 channels and m SS a 0 a i b i [5.8] Coefficients a i are calculated from a fit to a radiative transfer model instead of in situ observations as in the AVHRR algorithm. NOE: both algorithms work for cloud-free pixels > cloud mask is required Examples of SS retrieved from AVHRR. 5

6 El Nino 6

7 . Microwave vs. R SS retrievals Factor affecting nfrared radiometry Microwave radiometry radiometry Magnitude of emitted radiation from the sea face [] large [-] small Sensitivity of brightness to SS [] large [-] small is proportional to Emissivity [] ε about [-] ε about 0.5 Clouds [-] Not transparent [] Clouds largely transparent improvement at longer wavelengths Sea state e.g. [] ndependent [-] ε varies with sea state roughness Atmospheric interference [-] Requires complex correction [] Easily corrected with multichannel radiometer Spatial resolution [] A narrow beam can be focused. Diffraction is not a problem in achieving high spatial resolution with a small instrument [] Diffraction controls the beam at large wavelengths. Large antenna required for high spatial resolution Viewing direction on face [] Surface radiance largely independent of viewing direction [-] ε varies with viewing direction Absolute calibration [] Readily achieved using heated onboard target [-] Absolute calibration target not readily achieved Presently achievable 0. degree K.5 degree K sensitivity Presently achievable absolute accuracy 0.6 degree K degree K 7