The Oceanic Warm Cloud Detections Of The Cloud Profiling Radar As Inferred From MODIS And CALIOP Measurements

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 one-year 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 oceanic single-layer clouds.The spatial distribution and macrophysical/microphysical properties of oceanic warm clouds that are missed by CPR are systematically analyzed and compared comprehensively with those that are captured by CPR.The biases of cloud properties induced by the severely incomplete cloud sampling are also quantized in detail.On this basis,diverse external and internal factors that have effects on CPR cloud detections are further discussed,and the potential relationship between cloud microphysics and CPR cloud detections are clarified.Main findings are as follows:(1)Global spatial distribution features and regional discrepancy of oceanic warm clouds missed by CPRIn this study,based on the filtered CPR-missed and CPR-captured cloud sample,we have investigated the underestimation of global oceanic warm clouds by CPR.The cloud amounts,CTHs and microphysical properties of CPR-missed clouds have been examined.The main conclusions are summarized as follows:(1)CPR-missed warm clouds mainly arise over the relatively cold waters of the eastern ocean.The warm-cloud miss rate of CPR is negatively correlated with the zonal average sea surface temperature.The miss rate in the central ocean is approximately 0.1-0.3 and increases gradually with closer proximity to the continents.The miss rate of inshore areas exceeds 0.7.(2)The CTH of the CPR-missed clouds is primarily located within 0.6-1.2 km and abundant CPR-missed clouds are located under 1 km.Conversely,most of the CPR-captured clouds are located within 1.0?2.5 km.In the central Pacific,Atlantic,and Indian oceans,where CPR-captured clouds play a dominant role,the difference in the CTH between the CPR-captured clouds and total warm clouds is generally smaller than 0.4 km,with an overestimation of about 20%.Nearer to the continents,CPR-missed clouds become gradually more dominant and the CPR overestimation becomes even more severe,with a difference even larger than 0.8 km(50%).(3)The DER and COT of the CPR-missed clouds are generally smaller.The corresponding CWP is lower than 50 g/m2.Most of the CPR-captured clouds are thicker and have CWP larger than 100 g/m2,more likely to form precipitation.It is also found that,in the lower atmosphere,besides ground clutter effects,CPR tends to miss clouds with rather small COT and DER.(2)CPR cloud detecting efficiency of different warm cloud types and the potential biases caused by missed detecitonsThis study gives an evaluation of CPR cloud detections for four typical warm cloud types(Ac,St,Sc,and Cu),and quantitively estimate the statistical biases of cloud micropysics due to CPR missed detections.The findings are as follows:(1)The fraction of CPR failure in cloud detection is highly different among the four cloud types.St has the maximal miss rate(0.61)while the minimum(0.23)occurs in Cu.The miss rate of Sc and Ac is moderate,approximately at 0.4 and 0.52.Given a cloud type,the global distribution pattern of the CPR-captured clouds is approximately consistent with that of the total ones.(2)Surface clutter effects induce absolute loss of clouds below 1 km,resulting in that 1 km level forms the lower boundary for valid CPR cloud detections on the global scale.Cloud detection failures of CPR also arise frequently above this level.For each cloud type,the miss rate decreases monotonically with height,leading to definite positive biases on cloud top height.It is only for oceanic warm clouds residing between 2.5 and 3.0 km altitude that the overall miss rate keeps around 0.1.(3)There is a strong negative correlation between the miss rate and CWP for all the cloud types.It is only for CWP exceeding 200 g/m2 that miss rate keeps below 0.1.Due to such uneven loss of cloud samples with respect to the microphysical properties,the global average of DER and COD of the four cloud types are overestimated by about 10?24%and 24?36%,respectively.(4)The induced overestimation of cloud microphysics is significant for all cloud types.Accordingly non-precipitating warm clouds are much more severely missed compared with those precipitating ones,suggesting a possible overestimation of precipitation frequency for oceanic warm clouds.Given the same CWP,St(Cu)is more(less)likely to be missed,which is attributed to their DER differences.The globally averaged overestimation on CWP is revealed to be 36.6 g/m2(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%.(3)Multiple factors explaining the deficiency of the Cloud Profiling Radar on detecting oceanic warm cloudsThe CPR detecting efficiency on low-level clouds is affected by many situations,including surface clutter,spatial resolution,and radar sensitivity.These underlying factors can uniquely or jointly result in the missed detections of oceanic single-layer warm clouds,which are classified into six types and evaluated by using independent measurements from CALIOP and MODIS.The main findings are as follows:(1)More than 70%of single-layer warm clouds over global oceans are detected by both CALIOP and MODIS but missed by CPR,leading to an overall miss rate at 0.73.Nearly half of missed detections occur below 1 km.Above 1 km,overcast CPR-missed clouds contributes up to 70%of missed detections,while the rest suffer from the non-overcast effect.Range resolution is the major cause for missed clouds under overcast situation accounting for 74%.By the elimination of external factors,there are still 18.4%of missed detections above 1 km which are uniquely attributed to their special cloud microphysics.In addition,missing type IV that is considered to be possibly affected by multiple factors accounts for 43%of total missed detections.In particular,the CPR performance is closely related to specific cloud coverage above]km and the miss rate falls to 0.25 for warm clouds with spatial extent.(2)Over global oceans,CPR-missed clouds induced by different factors have prominently different geographical distributions,although their height dependence is negligible.Most of CPR-missed clouds in west coasts of the continents,the Intertropical Convergence Zone and high latitudes are located below 1 km or even 0.5 km,suffering from the surface clutter.Range resolution is the main cause of missed detections in several typical stratocumulus regions while the non-overcast effect primarily emerges in remote subtropical oceans.Given the significant regional difference,all factors should be taken into consideration especially in regional analyses of oceanic warm clouds utilizing CPR cloud detections.(3)As for warm clouds with sufficient spatial structure above 1 km,CPR cloud detection is clearly concerned with the specific DER and COT.It is found that the detecting limit of DER stands around 10 ?m while it is only for larger DER that the effects from COT turns to arise.The limit of COT decreases from 12 to 8 as DER increases.As for moderate CWP,even for the same CWP,the miss rate ranges extensively from 0.2 to 0.9 with varying DER and COT.Given the same CWP,the one with large DER and small COT is more likely to generate sufficient reflectivity and be detected,relative to the one with small DER and large COT.(4)Microphysical properties are revealed to be underlying factors that modify the CPR cloud detection.In particular,the opposite outcomes of CPR cloud identification frequently arise for the same DER and COT,due to the potentially different droplet size distributions.The wide range of variable reflectivity that crosses the detecting sensitivity leads to the uncertain cloud detections.Warm clouds with larger thickness seems to be more likely to generate sufficient Zemax and captured by CPR than those geometrically thin ones.It suggests the enhancement of vertical inhomogeneity inside CPR-captured clouds.Besides,Zemax at-23 dBZ shows to be a conservative threshold for CPR cloud detections.(5)The CWP of the CPR-missed clouds is much smaller than that of CPR-captured clouds consistently for all types,with a difference ranging from 5.9 to 79.8 g/m2.The weighted average of CWP for all CPR-missed clouds is about 37.2 g/m2,leading to a significant bias as large as 98%.Therefore the loss rate of cloud water mass is approximately 0.42,smaller than the miss rate of cloud occurrence frequency at 0.73.The CRE of CPR-missed clouds in all types is smaller than that of CPR-captured clouds at both LW and SW band.These missed detections caused by different factors all would lead to significant biases in cloud radiation estimates.Especially at SW band,a significant bias between CPR-captured and total cloud sample reaches up to about 160%at both BOA and TOA.