## 7 SGP Case Study for Aerosol-Cloud Interaction  
To explore the aerosol-cloud interaction and illustrate the need for a complete measurement testbed we focus on an interesting case from those considered in [PERSON] _et al._[18]. In their study, while in most cases considered, a weak correlation was observed, the case of 13 April 2000 tends to show small anti-correlation. In their analysis, the surface extinction was obtained from a ground based Nephelometer, which seemed to show a slight increase in cloud droplet radius with increase in extinction in opposition to the assumed aerosol cloud trend. One possible explanation advanced in their study was that the surface aerosols were strongly decoupled from aerosols at the cloud base on this day. To assess this issue, we plot, in Figure 10, the Raman lidar derived extinction profile was shown for 13 April 2000. In particular, the period from 1500 to 1800 UTC time shows excellent horizontal homogeneity making the surface retrievals most reliable. However, although the aerosol convective mixing has begun to occur during this period, the Raman lidar extinction profiles seem to indicate that the vertical distribution of extinction is not uniform with smaller extinction at the surface, which may in part be due to aerosol extinction enhancement due to hygroscopic growth. In making this assessment, it is useful to point out that the SGP Raman Lidar has developed a sophisticated approach to extend the aerosol extinction measurement below the low channel overlap threshold greatly minimizing overlap issues [58]. In particular, analysis of near extinction data against Nephelometer measurements illustrates the near surface Raman Lidar returns to be realistic and errors should be significantly less than the dramatic differences observed between the surface and aloft aerosols. Figure 10: Raman Lidar derived extinction profile and marked cloud boundary for 13 April 2000 at ARM SGP site. In Figure 11, we show on log scale the relationship between the effective radius and aerosol extinction where we consider both the surface extinction (lidar ground bin = 30 m) as well as the extinction 150 m below the cloud base. In this case, we selected only those points whose LWP was constrained between \(90<\mathrm{LWP}<120\). This resulted in COD ranges \(20<\mathrm{COD}<30\). Here the cloud base was determined using a simple threshold of 1 m\({}^{-1}\) and is displayed as black circles in Figure 10.  
In this case, we see that the surface results are at best uncorrelated with no trend but a general trend is seen when considering the aerosol 150 m below cloud base (_i.e._, zb-150 m layer) although the statistical significance is not as high as we might expect. It is clear that the aloft extinction is on average significantly larger than the near surface which should be attributed to hygroscopic enhancement due to high RH in the vicinity of the cloud. However, local inhomogeneous concentration effects must also be a factor since the overall data clusters shape has been substantially modified. Overall, we believe that the most robust approach to account for the aerosol loading for ACI estimation is to make extinction measurements nearer to cloud base than to rely on surface measurements. In summary, we see that the surface measurements can be less than optimal in looking for aerosol-cloud interactions and that profile measurements near the cloud base can be used to better probe these interactions under suitable conditions. Based on the regression slope, an IE exponent (or ACI) of 0.23 is obtained which is realistic if not on the high side in comparison to other cases seen [15]. [PERSON] and [PERSON] [8] summarized results from seven different _in situ_ airborne studies made for determining the ACI. In five of these studies observed values of ACI were comparable to ours  
Figure 11: Log-Log plot of Raman lidar extinction below cloud against the cloud droplet effective radius.