Memo Review

Memo: 415 - Phase Correction using Submillimeter Atmospheric Continuum Emission
      Matsushita, Matsuo, Wiedner, and Pardo

Reviewer: Jose Cernicharo

Date Received: 2002Nov14


I have been reading memo 415 and I have serious concerns relative to the
data, to their interpretation and to the implications for ALMA 

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.