On the Generation of an Optimized Fractional Cloudiness

On the Generation of an Optimized Fractional Cloudiness Time Series using a Multi-Sensor Approach Wiel Wauben*, Marijn de Haij Reinout Boers, Henk Klein Baltink, Bert van Ulft, Mark Savenije *R&D Information and Observation Technology, Climate Observations Dept, Regional Climate Dept, Weather Research and Development Dept TECO-2010, Helsinki | 31

Contents • Introduction • Instruments • Combination algorithm • Cabauw Fractional Cloudiness • Conclusions and outlook TECO-2010, Helsinki | 31

CESAR Cabauw Experimental Site for Atmospheric Research • Five remote sensing techniques for cloud observations • Active and passive • Column and hemispheric (integrated and resolved/scanning) • 1 year data sets of 10 -minute cloud data (15 May 2008 - 14 May 2009, total cloud cover & base) • Generation of optimized & continuous cloudiness time series • Evaluation of different techniques 3 TECO-2010, Helsinki | 31 August

Instruments 35 GHz cloud radar & CT 75 K • Sensitive to detect high cirrus • CLOUDNET procedure Ceilometer (operational SYNOP/METAR cloud product) • column techniques • including cloud base height 4 TECO-2010, Helsinki | 31 August

Instruments Pyrgeometer (BSRN) • long-wave downward radiation • integrated hemispheric • APCADA algorithm Nubi. Scope • thermal infrared • scanning • cloud mask Total sky imager (TSI) • visual digital camera • cloud mask • day-time only 5 TECO-2010, Helsinki | 31 August

Combination Goal: construction of optimized & continuous cloudiness time series Manual approach • • • strong / weak points situation dependent subjective no reference! complex algorithm not generic Hence • “simple” weighted average based on experiences • checked with climatology of manual observations (1970 -2000) 6 TECO-2010, Helsinki | 31 August

Combination “Reference” algorithm • • • Rj is the reference cloudiness (in percentage) at time j Wi, j is the weighting value at time j for the i-th instrument Hi, j=1 when the i-th instrument has a valid output at time j, else =0 Ci, j is the cloudiness (in percentage) measured by the i-th instrument at time j • WNUB, j = WTSI, j = 1 • for APCADA, CLOUDNET, LD 40 • DCLOUDNET, j is the observed minimum CLOUDNET cloud base height in the 10 -minute period at time j • uncertainty for all 7 TECO-2010, Helsinki | 31 August

Cabauw fractional cloudiness Cloud cover histogram • Column n=0, 8 high n=2 -7 low • CLOUDNET 60% n=8 mainly due to cirrus • “reference” is good compromise • low n=2 -6 (higher during day time) 8 TECO-2010, Helsinki | 31 August

Cabauw fractional cloudiness Cloudiness versus cloud base height • Nubi. Scope & TSI generally best agreement • ACPADA & LD 40 lower • CLOUDNET too high 9 TECO-2010, Helsinki | 31 August

Cabauw fractional cloudiness Contingency matrix LD 40 ( ) versus Reference ( ) 0 1 2 3 4 5 6 7 8 % # 0 10. 5 6. 1 1. 2 0. 8 0. 4 0. 3 0. 4 0. 2 0. 1 20. 1 10309 1 2. 1 3. 8 1. 1 0. 7 0. 4 0. 3 0. 1 9. 3 4769 2 0. 3 0. 8 0. 5 0. 3 0. 2 0. 3 0. 0 3. 3 1700 3 0. 1 0. 4 0. 6 0. 3 0. 2 0. 3 0. 0 2. 8 1445 4 0. 1 0. 2 0. 3 0. 8 0. 6 0. 4 0. 5 0. 0 3. 3 1706 5 0. 0 0. 1 0. 3 0. 7 0. 6 0. 5 0. 7 0. 0 3. 1 1574 6 0. 0 0. 1 0. 4 0. 7 1. 2 0. 0 3. 3 1712 7 0. 0 0. 1 0. 3 0. 8 1. 8 6. 0 1. 8 10. 9 5575 8 0. 0 0. 1 0. 3 0. 9 10. 6 32. 0 44. 0 22592 % 13. 2 11. 6 4. 3 3. 9 3. 5 3. 8 5. 4 20. 2 34. 0 100. 0 # 6777 5981 2201 2024 1812 1972 2790 10377 17448 Band 0: 55. 6% Band 1: 85. 6% Band 2: 92. 4% Over: 2. 0% Under 5. 5% : 51382 Δ: -0. 1 |Δ|: 0. 7 • 8 % with differences > 2 okta; fraction clear sky & overcast 10 TECO-2010, Helsinki | 31 August

Cabauw fractional cloudiness Reference data set cloudiness • 98% availability 10 -minute cloudiness • e. g. daily with uncertainty 11 TECO-2010, Helsinki | 31 August

Conclusions & Outlook Conclusions • Reference is weighted combination of individual instruments • Not a true reference, but general and robust approach that produces useful results • Compromise whereby the Nubi. Scope and TSI are considered to be a higher quality product (weight 1) than the others (height dependent weight) • Uncertainty range of reference cloudiness determined from the negative and positive differences between the reference and the cloudiness reported by each instrument over the time period under consideration • Findings for instruments see paper • OBS also has limitations so 100% similarity not expected • Automated cloudiness using ceilometer introduced changes in climatological cloud observations records 12 TECO-2010, Helsinki | 31 August

Conclusions & Outlook • APCADA and TSI are being / have been optimised as a result of this study • Physical definition of cloud/cloudiness, threshold possibly dependent on application • Usage of hemispheric method to overcome changes in climatological cloud observations records should be considered • Towards scanning reference system? 13 TECO-2010, Helsinki | 31 August

Thank you for your attention! Lookup conference paper for more information Boers, R. , M. J. de Haij, W. M. F. Wauben, H. Klein Baltink, L. H. van Ulft, M. Savenije and C. N. Long (2010), Optimized Fractional Cloudiness Determination from Five Ground based Remote Sensing Techniques, submitted to J. Geophys. Res. 14 TECO-2010, Helsinki | 31 August