These pages provide information on our effort to analyze high cloud (cirrus) cover over as long a time period as possible through analysis of satellite data.

Background

The CO₂ slicing method has been used in operational processing of GOES (Geostationary Operational Environmental Satellite) and HIRS (High resolution Infrared Radiometer Sounder) data, and has been found to have accuracies of approximately 50 hPa for clouds at heights above 3 km (approximately 700 hPa). This technique to infer cloud-top pressure and effective cloud amount (the cloud fraction multiplied by the emittance at 11 microns) has been discussed in detail by Menzel et al. (1983) and Wylie and Menzel (1999). Error analyses for the method are provided in Wielicki and Coakley (1981), Menzel et al. (1992), and Baum and Wielicki (1994). The method takes advantage of differing partial CO₂ absorption in several of the HIRS infrared bands located within the 15-micron CO₂ band, with each band being sensitive to a different level in the atmosphere. The bands located closer to the center of the CO₂ band at 15 microns are sensitive to high clouds only, while the bands away from the CO₂ band center are sensitive also to the presence of midlevel clouds.

Retrievals are derived from ratios of radiance differences between cloudy and clear-sky regions at two nearby wavelengths using the upwelling radiative transfer equation. The strength of this technique is its ability to detect thin cirrus clouds with visible optical depths > 0.1. However the height derived for thin cirrus is below the cloud top by up to 60 hPa for the most transmissive clouds. Due to signal-to-noise issues and the altitudes which the CO₂ channels a sensitive, cloud heights inferred from method must be in the range from approximately 700hPa to the tropopause. If no valid retrieval is found in this pressure range, the 11-micron band is used to infer cloud pressure assuming the cloud is opaque.

In HIRS operational processing, cloud top pressures are calculated for the following ratio pairs:

• 14.2/13.9 microns (band 4/band 5);
• 13.9/13.6 microns (band 5/band 6),
• 13.6/13.3 microns (band 6/band 7), and
• 13.9/13.3 microns (band 5/band 7).

Four channel pairings are used but only one solution is reported. The most representative cloud pressure is choosen that best satisifies the calculation of upwelling radiance for all four channels. The final cloud pressure is chosen by minimizing the difference between the observed cloud signal (i.e., the difference between the clear-sky and measured radiance) and the simulated cloud signal calculated from a forward radiative transfer model (Menzel et al. 1983).

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HIRS Data Reprocessing Effort

Results

Cloud Frequency (high clouds and all clouds) Over Land (20^oN - 20^oS):

Cloud Frequency (high clouds and all clouds) Over Water (20^oN - 20^oS):

HIRS Tropical Cloud Climatology from 1978 to 2002:

The SOI (Southern Oscillation Index) is the barometric pressure difference between Tahiti and Darwin, Australia. It's used as an index with ElNino - ENSO. The SOI is an indicator of the strength of the trades in the west pacific.

Comparison of 3I, ISCCP, and HIRS:

Conclusions

• clouds were found in more than 70% of HIRS observations since 1978, with high clouds found in more than 30% of the observations
• 23-year trends in HRIS high cloud statistics reveal modest increase during last decade compared with previous decade
• volcanic eruptions and El Nino events disrupt trends
• orbit drift and satellite differences must be mitigated (differences in cloud statistics and their trends are consistent with changes in local solar times of orbits for satellites that drifted)
• the high cloud frequencies for NOAA 15 and 16 are lower because of changes in the spectral positions of two channels in the transition to the upgraded HIRS-3 sensor
• loop of monthly means shows latitudinal cloud cover follows the sun
• ISCCP finds 10% fewer high and all clouds in tropics
• 3I agrees with UW HIRS on global high clouds
• VTPR will extend the data set back to 1973
• MODIS will extend the data set into the future

References

Baum, B. A. and B. A. Wielicki: Cirrus cloud retrieval using infrared sounding data: Multilevel cloud errors. J. Appl. Meteor., 33, No. 1, 107-117, 1994.

Menzel, W. P., W. L. Smith, and T. R. Stewart: Improved cloud motion wind vector and altitude assignment using VAS. J. Appl. Meteorol., 22, 377-384, 1983.

Menzel, W. P., D. P. Wylie, and K. I. Strabala: Seasonal and diurnal changes in cirrus clouds as seen in four years of observations with the VAS. J.. Appl. Meteorol., 31, 370-385, 1992.

Wylie, D. P., and H. Woolf: The Diurnal Cycle of Upper Tropospheric Clouds Measured by GOES-VAS and the ISCCP, Monthly Wea. Rev., 130, 171-179, 2002.

Wylie, D.P., W. P. Menzel, H. M. Woolf, K. I. Strabala, 1994: Four Years of Global Cirrus Cloud Statistics Using HIRS., J. Climate, 7, 1972-1986.

Wylie, D. P. and W. P. Menzel: Eight years of global high cloud statistics using HIRS. J. Climate ,12 , 170-184, 1999.

Wielicki, B. A. and J. A. Coakley, Jr. Cloud retrieval using infared sounder data: Error analysis. J. Appl. Meteorol., 20, 157-169, 1981.