Abstract
Abstract. The precision of the two-layer cloud height fields derived from the Atmospheric Infrared Sounder (AIRS) is explored and quantified for a five-day set of observations. Coincident profiles of vertical cloud structure by CloudSat, a 94 GHz profiling radar, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), are compared to AIRS for a wide range of cloud types. Bias and variability in cloud height differences are shown to have dependence on cloud type, height, and amount, as well as whether CloudSat or CALIPSO is used as the comparison standard. The CloudSat-AIRS biases and variability range from −4.3 to 0.5±1.2–3.6 km for all cloud types. Likewise, the CALIPSO-AIRS biases range from 0.6–3.0±1.2–3.6 km (−5.8 to −0.2±0.5–2.7 km) for clouds ≥7 km (<7 km). The upper layer of AIRS has the greatest sensitivity to Altocumulus, Altostratus, Cirrus, Cumulonimbus, and Nimbostratus, whereas the lower layer has the greatest sensitivity to Cumulus and Stratocumulus. Although the bias and variability generally decrease with increasing cloud amount, the ability of AIRS to constrain cloud occurrence, height, and amount is demonstrated across all cloud types for many geophysical conditions. In particular, skill is demonstrated for thin Cirrus, as well as some Cumulus and Stratocumulus, cloud types infrared sounders typically struggle to quantify. Furthermore, some improvements in the AIRS Version 5 operational retrieval algorithm are demonstrated. However, limitations in AIRS cloud retrievals are also revealed, including the existence of spurious Cirrus near the tropopause and low cloud layers within Cumulonimbus and Nimbostratus clouds. Likely causes of spurious clouds are identified and the potential for further improvement is discussed.
Highlights
Improving the realism of cloud fields within general circulation models (GCMs) is necessary to increase certainty in prognoses of future climate (Houghton et al, 2001)
The CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) data products have 1 and 5 km ground resolution, respectively, the majority of the difference is due to the relative sensitivity of each instrument to hydrometeors that was discussed in Sect
That Atmospheric Infrared Sounder (AIRS) detects more clouds than CALIPSO is an indication of (1) some false cloud detections by AIRS, (2) missed clouds by CALIPSO, or (3) increases in field of view (FOV) size lead to increases in perceived cloud frequency within some spatially heterogeneous cloud fields
Summary
Improving the realism of cloud fields within general circulation models (GCMs) is necessary to increase certainty in prognoses of future climate (Houghton et al, 2001). Active sensors provide relatively direct observations of cloud vertical structure compared to passive IR sounders, which derive cloud vertical structure using combinations of radiative transfer modeling and a priori assumptions about the surface and atmospheric state. CloudSat penetrates through clouds well beyond the sensitivity limit of IR sounders, but is insensitive to small hydrometeors and will often miss tenuous cloud condensate at the tops of some clouds or clouds composed only of small liquid water droplets In this comparison, a subset of publicly released products is used: cloud top height (ZA) and effective cloud fraction (fA) from AIRS, the radar-only cloud confidence and cloud classification masks from CloudSat, and the 5 km cloud feature mask from CALIPSO. For further detail about AIRS cloud retrievals, cloud validation efforts, and cross-comparisons with the Moderate Resolution Imaging Spectroradiometer (MODIS) and Microwave Limb Sounder (MLS), please refer to Susskind et al (2006), Kahn et al (2007a, b), Weisz et al (2007), and references therein
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