## 1 Introduction
In particular, a number of studies have made the attempt to explore Aerosol-Cloud Interaction at the SGP site using the available instrument suites. For example, the MMCR/MWR technique was applied [15] to explore a number of potential cases together with a Raman lidar, which was used to estimate aerosol CCN below the cloud. This study did in fact find a number of cases showing reasonable Aerosol Indirect Effects. In particular, the Aerosol Cloud Index (ACI) was defined [16; 17] as  
\[ACI=-\left(\frac{d(\log(R_{\text{eff}}))}{d(\log(\alpha))}\right) \tag{1}\]  
which represents the relative change in mean cloud droplet effective radius (R\({}_{\text{eff}}\)) for a relative change in aerosol extinction (\(\alpha\)) for clouds having the same LWP. Values of ACI \(\sim\)0.1 were found for multiple cases over the multiyear datasets which are reasonable for considering the upper theoretical limit of ACI \(<\) 1/3. However, one of the main purposes is to avoid the use of the MMCR and explore the potential of the MFRSR/MWR retrieval. For example, [PERSON] _et al._[18] used this approach to explore potential aerosol-cloud modifications. Unlike the [PERSON] study however, they did not make use of the lidar extinction being content to use surface Nephelometer measurements instead. This approach seems to be sufficient if the cloud base is sufficiently low to the surface, which in fact holds for most examples considered. However, it was noted in their study that some examples did not satisfy this requirement and that the poor results on these days are inferred to decoupling between the surface and aloft aerosols. Due to lack of current long-term data at the NYC CCNY site, we limit our present study to testing and demonstrating this approach using the multi-year measurements performed at the SGP site of the Atmospheric Radiation Measurement (ARM) Program. In addition, we make use of the filtering accomplished by previous studies to identify optimal cases. The present study is organized as follows: In Section 2, we provide a detailed assessment of satellite based retrievals in comparison to existing ground retrievals pertaining to overcast clouds and highlight the need for utilizing ground based remote sensing for effective cloud retrievals. In Section 3, we describe the details of ground-based instruments used for cloud property and provide assessments when possible to illustrate the performance of these instruments. In Section 4, we describe our retrieval algorithm and make quantitative comparisons with the other algorithms. Section 5 mainly focuses on validating our retrieval approach with the comparison of algorithms described in the previous section. In Section 6, a thorough sensitivity analysis is performed focusing on cloud droplet effective radius retrieval errors resulting from both uncertainties in the ambient environment (including surface albedo) as well as those from the measurements themselves. In Section 7, we re-examine a previously studied aerosol-cloud interaction case at SGP site to highlight the importance of using upper atmosphere aerosol measurements. Finally, section 8 provides a thorough discussion followed by our conclusions in Section 9.