Memo Review Memo: 415 - Phase Correction using Submillimeter Atmospheric Continuum Emission Matsushita, Matsuo, Wiedner, and Pardo Reviewer: Jose Cernicharo Date Received: 2002Nov14 Review: I have been reading memo 415 and I have serious concerns relative to the data, to their interpretation and to the implications for ALMA calibration. ATM calculations by J.R. Pardo indicate (see figure 2 of memo 415) that the opacity at 220 GHz is affected by a factor 8 when including liquid water with respect to the case with only water vapor, while the corresponding at 675 GHz is only affected by a factor 2. This seems to be, in a naive and quick interpretation, in agreement with the data shown in Figure 3. But in fact, that means that opacity at 220 GHz changes from 0.05 to 0.45 while the opacity at 675 GHz changes from 1 to 2. Figure 3b would match the model results if only liquid water was present in the atmosphere which seems a rather unlikely situation. My main question concerning figure 3b is what are the error bars for the 675 GHz opacities when opacity at 220 GHz is larger than 0.2 ? An additional argument against Figure 3b is that at Mauna Kea, at the CSO, there are two radiometers at 225 and 850 GHz. The data over the last two years indicate a standard ratio tau(850)/tau(225) of 22 for low opacities at 225 GHz and no sense opacities at 850 GHz when tau(225)>0.2-0.3 (note that the opacity at 850 GHz is more or less similar to the opacity at 675 GHz). Figure 3 of memo 415 seems to indicate that the atmosphere was completely dry and that all the opacity was almost totally due to liquid water. This does not make sense. The measurements with low opacity at 220 GHz could give some sensibility to the opacity measurements at 675 GHz. However, as soon as the opacity at 220 GHz is larger that 0.15-0.2 the sensibility to derive an opacity at 675 GHz, where the atmosphere is now extremely optically thick, is practically negligible. I consider that Figure 3a represents a realistic case for opacities at 675 GHz below 2.5-3.0. For larger opacities the error bar in the opacity is extremely large. Phase corrections when there is liquid water in the atmosphere will be impossible and I dont understand why we have to consider a case in which observations with ALMA will be useless. FTS observations could be very useful when atmospheric opacities at high frequencies are of the order, or below, 1-2. Larger opacities at 675 GHz mean no atmospheric transmission in the other atmospheric windows at higher frequencies. Juan Ramon supplied figure 2 for memo 415 and was not very enthusiastic with the data as he already commented to our Japanese colleagues. A different situation that J.R. Pardo and myself we are considering is the effect of small ices particles in high altitude cirruses. Preliminary calculations indicate that the effect at low frequency will be negligible (attenuation and phase) while at high frequency, the ice particles will introduce important scaterring, attenuation and polarization effects. These calculations will be shown in an ALMA memo within 3-4 months.