Open AccessToward the characterization of upper tropospheric clouds using Atmospheric Infrared Sounder and Microwave Limb Sounder observations
Author(s) -
Kahn Brian H.,
Eldering Annmarie,
Braverman Amy J.,
Fetzer Eric J.,
Jiang Jonathan H.,
Fishbein Evan,
Wu Dong L.
Publication year - 2007
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2006jd007336
Subject(s) - microwave limb sounder , depth sounding , atmospheric infrared sounder , advanced microwave sounding unit , troposphere , altitude (triangle) , environmental science , lidar , cirrus , radar , standard deviation , microwave , atmospheric sounding , remote sensing , water vapor , atmospheric sciences , geology , meteorology , physics , telecommunications , oceanography , geometry , mathematics , quantum mechanics , computer science , statistics
We estimate the accuracy of cloud top altitude (Z) retrievals from the Atmospheric Infrared Sounder (AIRS) and Advanced Microwave Sounding Unit (AMSU) observing suite (Z A ) on board the Earth Observing System Aqua platform. We compare Z A with coincident measurements of Z derived from the micropulse lidar and millimeter wave cloud radar at the Atmospheric Radiation Measurement (ARM) program sites of Nauru and Manus islands (Z ARM ) and the inferred Z from vertically resolved Microwave Limb Sounder (MLS) ice water content (IWC) retrievals. The mean difference in Z A minus Z ARM plus or minus one standard deviation ranges from −2.2 to 1.6 km ± 1.0 to 4.2 km for all cases of AIRS effective cloud fraction ( f A ) > 0.15 at Manus Island using the cloud radar only. The range of mean values results from using different approaches to determine Z ARM , day/night differences, and the magnitude of f A ; the variation about the mean decreases for increasing values of f A . Analysis of Z ARM from the micropulse lidar at Nauru Island for cases restricted to 0.05 ≤ f A ≤ 0.15 indicates a statistically significant improvement in Z A − Z ARM over the cloud radar‐derived values at Manus Island. In these cases the Z A − Z ARM difference is −1.1 to 2.1 km ± 3.0 to 4.5 km. These results imply that the operational Z A is quantitatively useful for constraining cirrus altitude despite the nominal 45 km horizontal resolution. Mean differences of cloud top pressure (P CLD ) inferred from coincident AIRS and MLS ice water content (IWC) retrievals depend upon the method of defining AIRS P CLD (as with the ARM comparisons) over the MLS spatial scale, the peak altitude and maximum value of MLS IWC, and f A . AIRS and MLS yield similar vertical frequency distributions when comparisons are limited to f A > 0.1 and IWC > 1.0 mg m −3 . Therefore the agreement depends upon the opacity of the cloud, with decreased agreement for optically tenuous clouds. Further, the mean difference and standard deviation of AIRS and MLS P CLD are highly dependent on the MLS tangent altitude. For MLS tangent altitudes greater than 146 hPa, the strength of the limb technique, the disagreement becomes statistically significant. This implies that AIRS and MLS “agree” in a statistical sense at lower tangent altitudes and “disagree” at higher tangent altitudes. These results provide important insights on upper tropospheric cloudiness as observed by nadir‐viewing AIRS and limb‐viewing MLS.