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Xingming Liang, Alexander Ignatov, Yong Han, and Hao Zhang

-- CRTM Daytime Performance --. -- CRTM Daytime Performance --. -- CRTM Daytime Performance --. Validation of CRTM v1.1 Solar Reflectance Model Using AVHRR IR Band 3.7 μm and Suggested Improvements. Xingming Liang, Alexander Ignatov, Yong Han, and Hao Zhang. -- CRTM Daytime Performance --.

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Xingming Liang, Alexander Ignatov, Yong Han, and Hao Zhang

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  1. -- CRTM Daytime Performance -- -- CRTM Daytime Performance -- -- CRTM Daytime Performance -- Validation of CRTM v1.1 Solar Reflectance Model Using AVHRR IR Band 3.7 μm and Suggested Improvements Xingming Liang, Alexander Ignatov, Yong Han, and Hao Zhang

  2. -- CRTM Daytime Performance -- Background • MICROS has been employed to validate and improve the CRTM since July 2008. • Nighttime M-O biases are small and within their expected ranges. • Daytime biases are much larger, show unexplained features, and remained anomalous during the full monitoring period, regardless of any ACSPO or CRTM version updates. • Daytime analyses reveal large negative M-O biases (~ -20K) in sun glint area and positive biases elsewhere (~ +5K). • The daytime anomalies should be fixed before CRTM is used for physical SST retrieval and ACSPO cloud mask during daytime.

  3. -- CRTM Daytime Performance -- Solar Reflectance Model in CRTM v1.1 • In CRTM v1.1, solar reflectance is defined as an integral, as one minus Wu-Smith emissivity. • Using the integrated co-emissivity as solar reflectance is not based on physical consideration, since the solar down-welling irradiance comes from only narrow range of solid angles around specular point. Solar reflected radiance CRTM v1.1 daytime RTE (omit multiple scattering): (1) (2) Wu-Smith emissivity: (3) CRTM v1.1 Solar Reflectance: Sky Reflected radiance Solar Reflected radiance Solar radiance Sky radiance

  4. n z θ θ0 Reflected Solar ray θn Incident Solar ray χ χ x y Δφ ω α -- CRTM Daytime Performance -- Instantaneous Specular Reflectance Model (ISRM) • An instantaneous specular reflectance based on Cox-Munk model was tested to improve daytime CRTM performance. (e.g., Gordon, 1997; Breon, 1993 ) • ISRM assumes the solar reflected radiance is only contributed by instantaneous facets in the interval Zx±½δZx’, Zy±½δZy’ with the probability of P(Zx,Zy)δZx’δZy’. • The (Zx,Zy), or (θn, α) is defined by solar incident direction and sensor measurement direction, and assumes the facet reflection is specular reflection. (4) (5) (6)

  5. -- CRTM Daytime Performance -- Facet PDF (1) The facet PDF is a function of facet slope angle and wind speed (7) Cox-Munk Model (measuring aerial photographs): (8) • The PDF distribution is constrained in a facet slope angle range of [-20°, 20°]. • The PDF distribution becomes peaked and narrow when wind speed is small.

  6. -- CRTM Daytime Performance -- Facet PDF (2) Several empirical models were proposed in addition to Cox-Munk (9) Breon-Henriot Model (ADEOS/POLDER): (10) Ebuchi-Kizu Model (GMS/VIRRS): • Preliminary analysis shows that CM and BH models are more accurate than EK

  7. -- CRTM Daytime Performance -- Evaluation of ISRM • CM in conjunction with JPL wind speed product is used. • Wind direction is not considered. • M-O biases in sun glint area decrease from approximately ~-20 K to ~-2 K. Biases elsewhere improve from about ~+5 K to ~-1 K. • ISRM is more accurate than reflectance model employed in CRTM v1.1, and based on a solid physical considerations.

  8. -- CRTM Daytime Performance -- Comparison of ISRM and the CRTM v1.1 solar reflectance ISRM CRTM CRTM v1.1 significantly underestimates solar reflectance in sun glint area, and overestimates elsewhere, resulting in unrealistic M-O biases.

  9. -- CRTM Daytime Performance -- Validation of ISRM Understand and minimize the remaining M-O biases. Dependencies on the following factors were analyzed: • Glint angle • Wind speed Objective:

  10. -- CRTM Daytime Performance -- Glint angle and Wind speed dependencies Glint angle Wind speed • Overall, M-O biases are negative during daytime. (This is expected because CRTM uses Reynolds SST, which does not resolve diurnal variability.) • M-O biases are largest at low winds due to diurnal cycle but disappear a wind increases • Negative biasesin the sun glint are likely due to underestimated reflectance in ISRM

  11. -- CRTM Daytime Performance -- Validation of the CM PDF An inversion algorithm used to evaluate CM PDF: Validate the Cox-Munk PDF by inverting daytime RTE Objective: Eveluate the need for improvements in Cox-Munk PDF

  12. -- CRTM Daytime Performance -- Inverted Algorithm The Observed Solar Reflected Radiance: (11) (12) (13)

  13. -- CRTM Daytime Performance -- Comparison of Cox-Munk and inverted PDFs • Inverted PDF well matches Cox-Munk PDF  the ISRM is physical & accurate solar reflectance model • Unlike the model PDF, the inverted PDF does not asymptotically approach zero away from sun glint. • There are large standard deviation near 0°of facet incident angle, may be due to small pixels in this area. • Facet distribution is asymmetric at the center of 0° of facet incident angle.

  14. -- CRTM Daytime Performance -- Comparison of PDFs wrt Wind Speed • The model PDF is overestimated for wind speed <2 m/s In the glint area (near 0°of facet slope angle),matches the inverted PDF for medium wind speeds (6-8 m/s) and becomes underestimated at higher wind speeds.

  15. -- CRTM Daytime Performance -- Conclusion • A formal reflectance estimated from Wu-Smith integrated co-emissivity used as solar reflectance in current CRTM result in much larger M-O biases for Ch3B in daytime. • ISRM used instead of current CRTM solar reflectance model, and it dramatically improve M-O biases: • In the sun glint area significantly decrease from approximately -20 K to -2 K; • The biases elsewhere improve from about +5 K to -1 K. • Glint angle, wind speed dependence’s analyses used to better understand and minimize the remain M-O bias in ISRM. • M-O biases are largest at low winds due to diurnal cycle but disappear a wind increases; • M-O biases are negative during daytime. This is expected because CRTM uses Reynolds SST, which does not resolve diurnal variability. • An inverted algorithm for the CM PDF validation was proposed. The result shows: • The model PDF matches the inverted PDF very well. it clearly proved that the ISRM can well present the physical solar reflectance; • Unlike the CM PDF, the inverted PDF does not asymptotically approach zero away from sun glint, due to unresolved diurnal cycle in Reynolds SST. • The CM PDF is overestimated for wind speed <2 m/s near 0°of facet slope angle, matches the inverted PDF for medium wind speeds (6-8 m/s), and becomes underestimated at higher wind speeds. • Adding aerosol in CRTM is current under way, and may slightly change current result.

  16. -- CRTM Daytime Performance -- Future • ISRM will be used in the next version of CRTM (v2.0). • include aerosol model in CRTM solar reflectance; • Fine tune Cox-Munk coefficient; • All functionalities will be used to validate the daytime performance for SEVIRI/MSG, ABI/GOES-R.

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