Comment on “Kinematic and dynamic estimates from electromagnetic current meter data” by David G. Aubrey and John H. Trowbridge

Aubrey et al. [1984] and Aubrey and Trowbridge [1985] (henceforth AST and AT, respectively) report the results of laboratory calibrations of electromagnetic current meters with spherical probes manufactured by Marsh-McBirney, Inc. (henceforth MMI). AT (p. 9137) remark, The present study was motivated by an examination of the NSTS [Nearshore Sediment Transport Study] experimental data from Santa Barbara, California, held in February 1980, in which the higher-order velocity moments from current meter measurements showed a time variability not obviously related to time scales of change of the forcing conditions (e.g., wave groupiness), and by field studies that have shown a persistent offshore near-bottom flow that is not satisfactorily explained by nearshore circulation theories. On the basis of their calibrations, AT (p. 9144) suggest that alternative sensors are required for many surf zone applications and state, "If sensor performance is as dependent on turbulent properties as suggested here, then the spherical [MMIs] have limited utility in surf zone studies." Poor MMI performance in the surf zone would have significant consequences because several major field studies (Nearshore Sediment Transport Study, Canadian Coastal Sediment Study, Duck 82, Superduck and others) and the ensuing published results have relied significantly on these instruments. In section 2 the offshore directed mean flows and velocity moments which motivated the AT study are discussed. Published comparisons of MMI sensors with colocated sea surface elevation and/or pressure sensors suggest that MMI field performance may be substantially better than implied by AT (section 3). Finally, in sections 4-6 several technical shortcomings and limitations of the AT analysis procedure and data set are discussed. In their reply to this comment, Aubrey and Trowbridge [this issue] refer several times to my "misinterpretations" and state that "at least a significant part of the concerns raised are based on inferences drawn from AST and AT rather than on direct statements." To avoid confusing the statements of AT with my own inferences, I will indicate in some detail precisely which remarks by AT and/or AST raised each item discussed.