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GERB Data Monthly Averages Planning: Improving Solar Flux Estimates

This monthly data report outlines the process of obtaining and averaging GERB footprint scale fluxes for clear and cloudy skies, with plans to enhance the accuracy of solar flux estimates, SW averaging methods, and filling missing data gaps.

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GERB Data Monthly Averages Planning: Improving Solar Flux Estimates

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  1. Future product status: Monthly averages J E Russell (Imperial)

  2. Planned products • All sky and clear sky monthly time-step averages • (averages for each 15 minute time step over all days in the month, providing and average diurnal signal) • Traditional complete monthly average for all and clear sky • Obtained by averaging the 15 minute time step averages

  3. GERB footprint scale fluxes GERB DATA Cloud mask at GERB footprint scale SEVIRI DATA Clear sky flux for totally clear GERB footprints identify clear regions Intermediate scale flux estimates + Clear sky flux for partially cloudy GERB footprints Cloud mask at intermediate scale Cloud free intermediate scale fluxes identify clear regions grid to GERB scale + Filled GERB footprint scale clear sky fluxes Clear sky flux for overcast GERB footprints GERB footprint scale clear sky fluxes fit diurnal model on all days with sufficient clear sky observations average data at each time-step Monthly time-step mean clear sky flux Monthly mean clear sky flux average

  4. Required additions Clear sky flux estimates (LW and SW) added to BARG • Requires 24 hour cloud flag, (which would preferably be also be added to BARG as a ‘clear fraction’ field), and estimate derived from ARCH scale data which may be better done with different interpolation of correction factors • ISW field added to BARG • Values already calculated by RMIB • ISW field added to BARG • Increase in the accuracy of the solar zenith angle field to 0.1 (could be achieved by quantisation factor)

  5. Questions: Missing data Generally our all sky average will be ‘complete’ and require no additions or interpolation but: What do we do when we have missing data? • Odd bit of missing data should have insignificant effect , so just average that available • However for more than 4 months a year we have no measurements from about 20.30 – 02:00. In these instances we won’t make a full (all time-steps) monthly average but will still make time-step monthly averages for the available time-steps. • Longer term we could fill these missing time-steps with GERB-like data adjusted by an offline correction factor

  6. Questions: SW averages With a ‘complete’ observation of the all-sky shortwave flux we could just average the fluxes, to calculate a true average shortwave flux. • However this isn’t what has been done before, typically fluxes are converted to albedo, the albedos are averaged and converted back to flux by multiplying by the average incoming solar for that month. • If we average flux then in the case of missing data which was clustered towards the beginning or the end of the month we would need to make an adjustment or not calculate an average.

  7. Questions: clear sky filling & averaging • Diurnal interpolation in SW • CERES directional models could be used but they don’t look like our data currently (need to determine the source of this difference first) • Accuracy? – fit on daily or monthly mean basis? • SW averaging • How deal with days with no clear observations? • ISW correction (ERBE/CERES)? • Fill from previous day using directional models?

  8. _ α’(t)=1/n∑dα(t,d) _ _ α’=∑tα’(t) 3 4 _ _ _ _ α’→F=α’. Itrue 2 3 4 4 _ α(t)=∑dF(t,d)/∑dI(t,d) _ _ _ F(t)=α(t) . I(t)true _ _ F=1/n∑tF(t) SW – fill mean diurnal cycle? Albedo α(t,d) Time Day observation Interpolated value

  9. Convert back to flux, and average as for all sky? _ F=1/n∑t,dF(t,d) _ α’(t)=1/n∑t,dα(t,d) _ α(t)=∑t,dF(t,d)/∑t,d(I(t,d) _ _ _ _ α→F=α . Itrue SW – fill on each day? Albedo α(t,d) Time Day observation Interpolated value Filled value

  10. Summary • Calculation of monthly averages requires RMIB to make the following additions to the BARG product: • ‘clear fraction’ determined 24 hours a day • Ideally this would be a new cloud flag but could be a combination of the MPEF and GERB scene ID flags • clear sky flux estimate for LW and SW • Ideally would need interpolation of correction factors specific for clear sky, but could just use GERB-ARCH as is. • incoming shortwave • Increase to the accuracy of the solar zenith angle to 0.1

  11. Summary • Open issues include: • Treatment of missing data • SW averaging method (albedo’s or fluxes) • Filling and interpolation of ‘missing’ shortwave clear sky • A brief note summarizing these plans and intentions will be distributed to the GIST for comment.

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