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Trends & Variability of Liquid Water Clouds from Microwave Satellite Data

This presentation discusses 18 years of microwave satellite data to analyze trends and variability in liquid water clouds. Topics covered include motivation, sensors, climatology comparison, diurnal cycle, and long-term trends. The study aims to establish a benchmark for global climate models by providing a robust measure of liquid cloud properties. The results show promising regional trends, particularly in northern high latitudes.

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Trends & Variability of Liquid Water Clouds from Microwave Satellite Data

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  1. Trends & Variability of Liquid Water Clouds from Eighteen Years of Microwave Satellite Data: Initial Results 6 July 2006 Chris O’Dell & Ralf Bennartz University of Wisconsin-Madison

  2. Talk Outline • Motivation • Description of sensors & retrieval product • Mean climatology & comparison with ERA40, ISCCP • Diurnal cycle • Long-term trends?

  3. Motivation for a cloud liquid water path (LWP) climatology • Anthropogenic trends in cloud properties are possible, due both to global warming and aerosol effects. • A robust LWP climatology can serve as a benchmark for global climate models. • The 18-year passive microwave record contains a robust and independent measure of liquid clouds.

  4. Previous successes from passive microwaves: • Increases in lower tropospheric temperature, decreases in stratospheric temperatures from Microwave Sounding Unit (Mears et al. 2003, Christy et al. 2003, Vinnikov & Grody 2003) • Increases in global (especially northern hemisphere) water vapor path (Trenberth et al., 2005)

  5. Microwave sensors measuring LWP All instruments are conical scanners, with footprints ~ 40 km

  6. Calibration/Retrieval • All satellites have been intercalibrated – the radiances are consistent from one satellite to the next (RSS, unpublished!) • All satellites use the same, modern retrieval algorithm to simultaneously retrieve LWP, water vapor path, and surface wind speed. • Probably better than older algorithms which often used 2 channels to retrieve a given quantity, algorithms tended to be regression-based, and tended to retrieve different quantities independently.

  7. No global trend with simple average! LWP Agreement between sensors is good

  8. Processing Scheme At Remote Sensing Systems • Retrieved LWP binned daily onto a 0.25º grid (1440x720) for both morning & evening overpasses • Even pixels with heavy rain retrieve LWP (but not water vapor or surface winds) At Wisconsin • Quantities further binned to 2.5º grid, monthly average for each sensor & local overpass time. • Monthly diurnal cycle fits made for each pixel (average of all years). • Diurnally-corrected monthly means calculated for each pixel. • Seasonal & annual LWP trends calculated for each pixel.

  9. MAM Mean LWP

  10. Does microwave LWP agree with ERA40?

  11. Problem in Sc regions! Does microwave LWP agree with ERA40?

  12. Problems at higher latitudes Ice? Does microwave LWP agree with ISCCP*? * ISCCP D3 water path (WP)

  13. Diurnal Cycle Fitting • Goal is to make a diurnally-corrected LWP climatology • Previous work with TRMM only retrieved diurnal cycle for tropics. Possible midlatitude diurnal cycle? • For each 2.5º pixel & month, fit local time versus LWP to this function: • ( corresponds to 24 hours) • Use resultant fits to correct each monthly binned observation.

  14. F11 F13 F15 F14 SSM/I LWP Diurnal Cycle Strength TRMM-TMI Wood et al., Geophysical Research Letters, 29 (23), 2002

  15. Normalized Diurnal Amplitude Liquid Water Path [kg/m2] Local Time [hours]

  16. Liquid Water Path [kg/m2] Local Time [hours]

  17. Liquid Water Path [kg/m2] Local Time [hours]

  18. Liquid Water Path [kg/m2] Local Time [hours]

  19. Liquid Water Path [kg/m2] Local Time [hours]

  20. Liquid Water Path [kg/m2] Local Time [hours]

  21. Robust Regional Trends in LWP?

  22. Local trends – tropical western pacific

  23. Local trends – northern midlatitudes

  24. Local trends – southern mid-high latitudes

  25. Local trends – Arctic ocean

  26. Conclusions • Existing passive microwave observatinos appear to provide a stable, long-term record for climate studies of liquid clouds. • ERA40’s cloud parameterization seems to poorly characterize LWP seasonal and interannual variability in the subtropical high stratocumulus regions. • The diurnal cycle of LWP has been well-characterized in most ocean locations, and is generally in agreement with previous studies. • Initial studies of LWP trends are promising, with hints of regional trends (especially in the northern high latitudes), but no significant long-term global trend.

  27. “To Do List” • Principle component analysis - may reveal interesting patterns of variability or problems with the data set. • Further investigation of the derived diurnal cycles – how constant are they from year-to-year? How well do they compare with CA & precip diurnal cycles? • More sophisticated statistical analyses of LWP trends…(hint to audience for guidance) • Make the complete LWP climatology available on the web.

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