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ALMA Memo # 517
Turbulence simulations of dry and wet phase
fluctuations at Chajnantor.
Part I: The daytime convective boundary layer.
Alison Stirling, John Richer, Richard Hills, Adrian Lock
We have performed numerical simulations of the atmosphere for typical
daytime convective conditions at Chajnantor, and derived the resulting
wet and dry contributions to the atmospheric phase fluctuations. The
simulations show that:
- Dry phase fluctuations are concentrated in two layers --
near to the ground, and at the temperature inversion. The wet
fluctuations are concentrated at the inversion, while the total phase
fluctuations are more uniformly distributed within the convective layer.
This is because of significant positive and negative correlations
between the dry and wet refractive index fluctuations.
- The phase structure function is well described by a Kolmogorov
turbulence spectrum on small scales, with a turn over on a scale of
order the depth of the boundary layer.
- The variation of total r.m.s. phase with elevation shows a
dependence on the square root of air mass for the total phase,
but the dry component shows a linear variation with air mass, and
the wet component varies as air mass to the power 0.75.
A scaling analysis has been used to relate the r.m.s. wet and
dry phase fluctuations to the vertical profiles of temperature and
water vapour so that an estimate of the phase fluctuations at
Chajnantor can be obtained from radiosonde data.
- Using this approach, the r.m.s. dry fluctuations along a
single line of sight are found to be 100-200 um at the 25-75
percentiles respectively, and the equivalent wet fluctuations
are found to lie in the range 180-530 um. The total r.m.s. path
fluctuations were estimated to be 240-525 um, and we have
compared these estimates with independent measurements of the total
r.m.s. phase obtained from interferometric measurements
(Evans et al., 2003), and these show excellent agreement.
- The correlation coefficient between total and wet phase
fluctuations is estimated, and this is found to lie in the
range 0.75-0.97 at the 25-75 percentiles. This suggests that,
even under conditions where the dry phase fluctuations are
expected to be at their highest, water vapour radiometry is
expected to be able to remove a high percentage of phase
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Last modified: 2005-03-03