HIRS Observations of a Decline in High Clouds since 1995 February 2002 W. Paul Menzel NOAA/NESDIS Madison, Wisconsin 53706 USA Donald P. Wylie Cooperative Institute for Meteorological Satellite Studies University of Wisconsin-Madison 12 year trends Effects of orbit drift and ancillary Tsfc Separating diurnal signal Comparison with CERES February 2002

The average plot, COMBINED The average plot, COMBINED.GIF, shows mostly constant cloud cover with time except for a drop in the northern hemisphere, 30-60 N, starting with the winter of 98 which had about 5% less cloud cover than the previous winters. Note the annual cycle is largest in the northern hemisphere.

Cloud cover over oceans, OCEANAVE Cloud cover over oceans, OCEANAVE.GIF, remained nearly constant with time. There is a slight drop in 2000-01, probably because of NOAA 16. NOAA 15 has not been used because we believe its statistics are low. We haven't found the cause of this problem.NOAA 11 is included in both COMBINED.GIF and OCEANAVE.GIF.

Cloud Properties from CO2 Slicing RTE for cloudy conditions indicates dependence of cloud forcing (observed minus clear sky radiance) on cloud amount () and cloud top pressure (pc) pc (I - Iclr) =    dB . ps Higher colder cloud or greater cloud amount produces greater cloud forcing; dense low cloud can be confused for high thin cloud. Two unknowns require two equations. pc can be inferred from radiance measurements in two spectral bands where cloud emissivity is the same.  is derived from the infrared window, once pc is known.

reveal cloud properties at different levels hi - 14.2/13.9 Different ratios reveal cloud properties at different levels hi - 14.2/13.9 mid - 13.9/13.6 low - 13.6/13.3 Meas Calc pc (I1-I1clr) 1  1 dB1 ps ----------- = ---------------- (I2-I2clr) 2  2 dB2

Determining Cloud Presence and Properties Use bands where (I - Iclr) > 1 mW/m2/ster/cm-1 in CO2 slicing estimation of pc Estimate IRW using IRW radiances If more than one pc is estimated, use RTE for all bands to select best one If no bands qualify, try IR window estimate for opaque cld If too low in atmosphere, declare FOV clear

Generating Clear Sky Radiances in Cloudy FOVs Use IR Window Moisture Corrected Brightness Temperature Test against a priori surface temperature to identify nearby clear sky FOVs BT11 + aPW * (BT11 - BT12) - Sfc Temp < 2 C aPW of 0.8 has been used Sfc Temp estimated from GDAS Estimate Iclr by interpolating nearby clear FOVs cld = x x x o o = clr x x x o o o x x o o o x x x o Attempt to derive CO2 cloud properties in x (note that CO2 cloud algorithm attempt on x can change FOV to o)

Global high cloud cover stable for TIRW(H2O corr) – Tsfc < 1 to 3

Examining the land data, LANDAVE Examining the land data, LANDAVE.GIF, shows decreases in cloud cover starting in the summer of 1995 in both the northern continents and the tropics. Land comprises 36% of the 30-60 north latitude belt and thus influenced the combined statistics.In the tropics, 20 south - 20 north, land is only 11% of the area so it has a smaller affect on the combined statistics.

The tropics show the same changes as 30-60 north over land - TROLND The tropics show the same changes as 30-60 north over land - TROLND.GIF.

GOES Sounder detecting diurnal change of cloud cover

Deviation from cloud mean 20 N-20S

Wielicki et al (2002) CERES deviation of reflected shortwave flux wrt 1985-89 mean for 20N-20S

NH (36% land) Tropics (11% land) SH (very little land)

Conclusions * 12 yr trends in UW HIRS cloud statistics reveal high clouds increased until 1993 and then gradually decreased below 1989 levels. * Cloud cover decreases since 1995 occur in all zones (NH, Tropics, SH); NH shows most. These decreases are mainly observed over land and especially in higher clouds; ocean cloud cover is mostly stable. * Differences in cloud statistics and their trends are consistent with changes in the local solar times of the orbits for satellites that drifted. Satellite that did not drift, NOAA 12, shows a very large cloud detection decrease. A decrease in tropical cloud cover in the UW HIRS statistics is similar to that reported by Weilicki et al (2002) using CERES data. Future Work * Extend CO2 analysis back to 1978 (working with Bates and Jackson)