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CLOUDS retrieval by broadband SW radiometers. Blind test – real cases

CLOUDS retrieval by broadband SW radiometers. Blind test – real cases. Giandomenico Pace ENEA , Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy COST Action ES0702: Working group meeting. Köln , 7 - 9 Feb 2012.

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CLOUDS retrieval by broadband SW radiometers. Blind test – real cases

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  1. CLOUDS retrieval by broadband SW radiometers. Blind test – real cases Giandomenico Pace ENEA, Italian National Agency for New Technologies, Energy and Sustainable Economic Development, Italy COST Action ES0702: Working group meeting. Köln, 7 - 9 Feb 2012

  2. Estimation of SW CloudOptical Depth (COD) Algorithm Based on the dependence of SW diffuse or total irradiance (diffuse = total for thick cloud) from COD COD= f(r) Two (similar) versions of the algorithm have been published: the first one empirical by Barnard and Long (2004) and the second semi-empirical by Barnard and Long, 2008. In both algorithms the variables f and r are chosen by minimizing the differences between COD values from an independent database and values of f(r). COD obtained using the Min and Harrison (1996) algorithms were used to this purpose. The COD of that database are calculated by LUT linking the COD of liquid water cloud to the diffuse irradiance at 415 nm; in case LWP is also knows Re is also estimated.

  3. Estimation of SW Cloudopticaldepth (COD)

  4. Estimation of SW Cloudopticaldepth (COD) Care have to be used to define the denominator (clear sky irradiance*cos(sza)^α), that has to be derived from real measurements of the piranometerfor removing possibly calibration errors! (NO modeling estimation!!) In a previous works concerning the selection of clear sky period Long and Ackerman (2000) presented a methodology for this scope. Normally this methodology requires a series of daily measurements for deriving the term in clear sky day in the same period of the studied overcast day. LIMITATIONS of the COD retrieval Applicable properly only to fully overcast skies with cloud fraction >0.99 (homogeneous clouds assumption) cos(sza)<0.15 Determinations of clear sky irradiances by the same instrument measurements

  5. Estimation of Sky Cover 8 Sep. 2007 FIRST STEP: Determination of cloud fraction by piranometer measurements (Long at al 2006) Methodology based on a relationship between the cloud cover amount and the downelling diffuse cloud effect, i.e diffuse SW irradiance – clear sky diffuse irradiance from Long and Ackermann 2000.

  6. Estimation of Sky Cover 11 June 2007 FIRST STEP: Determination of cloud fraction by piranometer measurements (Long at al 2006) NOT THE RIGHT DAY FOR THIS APPROACH (broken clouds?) Total SW irradiance values greater than SW clear sky irradiance are due to cloud enhancement

  7. Estimation of the effectiveRadiur (Re) SECOND STEP: Evaluation of average COD values using mean irradiances on 5 and 11 minutes, to be consistent with the ASSUMPTION of homogeneous clouds. This also implies the condition of sky cover =1 on 5 or 11 minutesrespectively. THIRD STEP: Estimation of the effective radius using the LWP (MWR) and the two formula LWP = 2/3 * cot * Reff *ro Vertically Uniform LWP = 5/9 * cot * Reff *ro Adiabatic Stratified

  8. Results: 08 September 2007 Re values for Adiabatic Stratified are presented. The lowest Re values (evidenced by 3 black circles) are mainly at the edge of the selected periods, i.e. less true the homogeneity assumption.

  9. Results: 11 June 2007 Re values for Adiabatic Stratified are presented. Black circle evidences a nice example of how is important the homogeneous assumption: Conditions at 11 minutes does not satisfied around 43000 s + very large variability in LWP  NO physical results I am wondering about 10 microns around 50000 s… What do you estimate?

  10. DATASET LIMITATIONS, i.e. could I havedonebetter/differently? Limitation of the database for this analysis. 1) This methodology relies also on the data continuity. Irradiances are at 1 min rate for all the day LWP are at higher resolution, but gap in the measurements are present  Re estimation is possible if a sufficient number of LWP measurements are done during each minute 2) Too few cases to determine a good clear sky estimation (1 half day), but data are generated by Long (I guess) and already have estimation of clear sky that are consistent with my few estimation. So I used the Long clear sky values available in the NC file.

  11. I am very sorry to not be there. Thanks to all for the attention and to Gianni for presenting these slides.

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