## 5 Inter-Comparison of Ground-Based Cloud Property Retrieval Approaches at the ARM SGP Site  
At SGP ARM site, the Microwave Water Radiometer (MWR, Model WVR-1100) measures column-integrated amounts of water vapor and liquid water. This MWR receives microwave radiation from the sky at dual frequencies (23.8 GHz and 31.4 GHz), which allow simultaneous determination of total water vapor and liquid water burdens along the selected path. In the present study, we have taken care to exclude LWPs recorded during precipitation from our analysis. Various studies have reported uncertainties in LWP retrievals to be around 20-30 g\(\cdot\)m\({}^{-2}\)[47, 48, 49, 50].  
Existing cloud optical properties from MFRSR are obtained using the MIN algorithm which are stored within the SGP Value Added Product (VAP) called MFRSRCLDOD [51]. To apply our algorithm, we use the atmospheric transmittance as a function of cosine of solar zenith angles for thick  
Figure 7: Scatter plot between retrieved instantaneous COD and REFF using RET approach compared to those of MIN (**a**,**b**) and PAR (**c**,**d**) retrievals.  
cloud cases along with instantaneous LWP measurements from MWR, which are available in the VAP. Cloud retrievals from our approach (RET) are compared with MIN retrievals and parameterization (PAR) scheme are shown as scatter plots in Figure 7 for the period from April to September 2008. COD retrievals from our approach show excellent correlation (r\({}^{2}\)\(\sim\) 0.99) and consistency with those obtained from both the methods and this is maintained over large datasets. However, we note that there is a spread or discrepancy in the retrieved R\({}_{\text{eff}}\) from RET approach which we will see is mainly due to the difference in LWP measurements used by the MIN algorithm.  
In understanding the R\({}_{\text{eff}}\) discrepancy, we note that the MIN retrievals are based on an internal estimation of 5 min averaged LWP and the details of that averaging are not accessible. Therefore, when we perform an averaging procedure, it may not be identical to the approach that MIN uses. To test this, we perform in Figure 8(a) a closure study where the MIN retrieval products are used to \"derive\" the LWP (MIN-DER) which are compared to the averaged LWP. The discrepancy is of the same order of magnitude of the errors in R\({}_{\text{eff}}\) observed illustrating that there is ambiguity in how the average was calculated. The sensitivity is further demonstrated where the instantaneous LWP is compared to the averaged LWP demonstrating comparably higher variability (Figure 8(b)). The point we wish to emphasize is that the effective radius errors, which are clearly driven by the LWP uncertainties, are not only due to instantaneous radiometric errors in LWP but also due to the in atmospheric variability of LWP within a given averaging window. However, in our study we do not plan on exploring this averaging ambiguity in assessing error since literature estimates of the LWP error were available and this value was the one that was ultimately used in our improved sensitivity analysis (Section 6). On the other hand, in our comparisons against PAR, maximum relative fractional differences are less than 5% and 8% for COD and R\({}_{\text{eff}}\) respectively were obtained when compared with the MIN retrievals at ARM SGP site. Here, no ambiguity in the actual LWP occurs since this parameter is identically provided in both algorithms.