Oceanic single‐layer warm clouds missed by the Cloud Profiling Radar as inferred from MODIS and CALIOP measurements

Abstract

Attributed to its unique advantage of cloud vertical resolving, Cloud Profiling Radar (CPR) measurements have been used as the primary component in synthetic cloud data for relevant studies. However, due to surface clutter and sensitivity limitation, considerable warm clouds over global oceans are missed by CPR, which causes severe sampling biases and problematic statistics of cloud properties. By using independent cloud mask data jointly from Moderate Resolution Imaging Spectroradiometer and Cloud‐Aerosol Lidar with Orthogonal Polarization measurements, this study gives an evaluation on these issues and focuses on single‐layer clouds. By excluding effects from CPR's spatial resolution, the CPR detection failures are found to be frequent over global oceans, with an overall miss rate at ~0.39. For each cloud type, altocumulus, stratus, stratocumulus, and cumulus, its miss rate decreases monotonically with height, and it is only at 2.5–3.0 km altitude that the miss rate is negligible for the ensemble of oceanic single‐layer warm clouds. The miss rates are different among cloud types, which are attributed to their distinct microphysical properties. Clouds with droplet effective radius below 12 µm or cloud optical depth below 4 are very likely to be missed by CPR, resulting the globally averaged overestimation of 10–24% and 24–36%, respectively. The miss rate has a strong negative correlation with cloud water path (CWP) and decreases below 0.1 only for CWP exceeding 200 g m−2. The resulting overestimation on globally averaged CWP is 36.6 g m−2 (44.3%). Throughout the globe, the biases are mostly positive and have notable regional variations. Especially in the typical oceans that have abundant warm clouds, the CWP is overestimated by 20%–80%.

Multiple Factors Explaining the Deficiency of Cloud Profiling Radar on Detecting Oceanic Warm Clouds

“…Warm cloud pixels derived in this way actually include some ones that are embedded in cold cloud systems, which have distinct dynamics and cloud properties compared with real warm clouds (Schumacher & Houze, ; Liu & Zipser, ; Liu et al, ). To exclude these inauthentic samples, a spatial continuity test is used to identify the warm cloud cluster that consists exclusively of warm cloud pixels (Liu et al, ).…”

“…In addition, CPR has practical difficulties in detecting optically thin clouds with optical thickness <0.3, which are mostly thin cirrus (Marchand et al, ; Stephens et al, ). Low‐level stratiform clouds that significantly contribute to the planetary albedo are also great challenges for CPR, which is attributed to their very low altitude that is possibly within the radar blind zone (Bennartz, ; Burns et al, ; Liu et al, ; Sassen & Wang, ). Since the climatology of shallow cloud systems and their interactions with aerosols have been extensively concerned by applying spaceborne radar measurements (Rosenfeld et al, ; Turner et al, ; Winker et al, ), it is essential to clarify the performance of CPR cloud detection, especially in lower atmosphere.…”

“…These missed detections are mostly due to too weak backscattering echoes with respect to the CPR sensitivity, which are inevitably caused by special cloud microphysics. By using collocated CALIOP and MODIS cloud mask data, Liu et al () paid attention to the overcast warm clouds and revealed a global miss rate of about 0.39. Such a high ratio cannot be fully explained by surface clutter.…”

“…But due to the diversity of microphysical properties for oceanic warm clouds and their impact on radar reflectivity, the actual missed detections of CPR are strongly dependent on cloud types (Marchand et al, ). For instance, it was revealed that for warm clouds with equivalent cloud water path, those with larger optical thickness but smaller droplet size are more likely to be missed by CPR (Liu et al, ). This means that given the columnar cloud water, distinct configurations of cloud microphysics could even lead to totally opposite outcome of CPR cloud detection.…”

Characteristics of Oceanic Warm Cloud Layers within Multilevel Cloud Systems Derived by Satellite Measurements

“…There are exceptionally low values over the narrow regions adjacent to west coast of continents. These should be attributed to the deficiency of CPR for detecting clouds at very low levels, which are mostly coastal stratus and they occur very frequently in these regions [24,28,29]. Furthermore, Figure 2c shows the ratios of profiles with only warm cloud layers to those with warm cloud layers.…”