The Power Spectrum of Atmospheric Path Fluctuations at the ALMA Site from Water Vapour Radiometer Observations
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ALMA Memo # 592
The Power Spectrum of Atmospheric Path Fluctuations at the ALMA Site from Water Vapour Radiometer Observations
R. Bolton, B. Nikolic, J. Richer
2011-09-14
This memo investigates the properties of atmospheric turbulence above the ALMA site using measurements from the Water Vapour Radiometers (WVRs), in particular testing the Kolmogorov turbulence model and searching for evidence of an outer length scale. We fit model power-spectral density (PSD) curves to the measured power spectra for a number of observations and extract the slope, the amplitude and (when convincingly found) the break frequency for a sloping component and also the amplitude (or limit) on a white noise (thermal) component.
We find that the majority of the power-spectra are consistent with a thick-screen Kolmogorov turbulence model, but find a transition from thick- to thin-screen behaviour in four of 18 observations. When we scale the frequency of these transitions to spatial scales using the measured (ground-level) wind speed, we find that these transitions occur at lengths in the range 40-400 m.
The WVRs are used to predict and correct for the line-of-sight path fluctuations caused by water vapour for each of the 54 12m ALMA antennas. When noise is present, applying the WVR correction on baselines where the path fluctuations are dominated by this noise will increase the phase fluctuations rather than reducing them. We use the recovered PSD of fluctuations to estimate the lower limits on time scales and baselines on which application of the WVR data might be expected to help.
By combining several adjacent observations, we have assembled WVR path estimates over 3 hours. We find no evidence of an outer scale in the turbulence (which would present itself in a turnover in the PSD at long timescales or spatial frequencies), which suggests that in this case the outer scale was longer than approximately 16 km.
View a pdf version of ALMA Memo #592.
