Analysis of HATS data showed that the spatial resolution of
finite-difference estimates of horizontal and vertical gradients can
be matched by making the vertical spacing of the two horizontal arrays
approximately equal to the sonic spacing within those arrays. For OHATS,
WHOI engineers working on the array support structure (Mike Purcell and
Megan Carroll) have chosen the sonic spacing to be 58 cm.
The closest vertical spacing used for HATS was 1 m. Comparison of wind
speed, TKE, and u* between the horizontal arrays and a nearby reference
tower do not show any evidence of flow distortion. However it is possible
that the closer vertical spacing for the OHATS arrary may start to show
some evidence of flow distortion. Peter Sullivan, John Wyngaard, and
I have discussed this issue and will address it from two approaches.
Peter found a potential flow solution for two parallel cylinders,
surrogates for the two horizontal masts, in Morse and Feshbach, Volume 2.
I have made preliminary steps to applying the technique of Wyngaards's
1981 investigation of flow distortion of a single cylinder to the two
cylinder problem, using Mathematica. In the meantime, however, we have
also erected a subset of the planned OHATS array at Marshall to directly
measure flow distortion.
The Marshall array, or OHATS ops0, consists of two horizontal ASTER
masts at nominal heights of 4.75 m and 5.27 m. (Because of interference
between the cross-bracing of the vertical ASTER masts and the attachment
hardware, we could not separate the horizontal masts by the desired 58 cm
and had to settle for 52 cm.) The horizontal masts are oriented E-W so
that winds from the south are perpendicular to them. About 6 m to the W
we erected a reference tower. Nine sonic yokes were placed on each of
the two horizontal masts, with a spacing of 58 cm, and CSAT electronics
were mounted on the back of the masts in the intervening spaces. Sonics
were mounted at the center three locations on both horizontal masts and
at the same two heights on the reference tower.
The sonics are designated as, e.g. u3, l2, etc, with u and l referring
to the upper and lower arrays and the ordinal number increasing from west
to east. Thus u1 and l1 are the reference sonics and u3 and l3 are the
center sonics in the horizontal arrays. The specific sonics are:
Loc S/N Channel
l1 855 200
l2 246 201
l3 247 202
l4 800 203
u1 856 204
u2 367 205
u3 369 206
u4 853 207
Sonics 853, 855, and 856 (u4, l1, u1) have the new CSI modification to
enhance their ability to operate in rain. However, one of the nylon
rain socks is missing on an upper transducer of SN 853.