GROUND-BASED MEASUREMENT OF GRADIENTS IN THE WET RADIO REFRACTIVITY OF AIR

We have used a ground-based microwave radiometer, known as a water vap or radiometer, to investigate the local spatial and temporal variation of the wet propagation delay for a site on the west coast of Sweden. The data were obtained from a wide range of azimuths and from elevatio n angles greater than 23.6-degrees (air mass 2.5). Visual inspection o f the data suggested a simple ''cosine azimuth'' variation, implying t hat a first-order gradient model was required. This model was adequate for short time spans up to approximately 15 min, but significant temp oral variations in the gradient suggested to us that we include gradie nt rate terms. The resulting six-parameter model has proven adequate ( rms delay residual approximately 1 mm) for up to 30 min of data. Assum ing a simple exponential profile for the wet refractivity gradient, th e estimated gradient parameters imply average surface wet-refractivity horizontal gradients of order of 0.1-1 N km-1. These gradients are la rger, by 1-2 orders of magnitude, than gradients determined by others by averaging over long (approximately 100-km) distances. This result i mplies that for applications that are sensitive to local gradients, su ch as wet propagation-delay models for radio-interferometric geodetic studies, the use of meteorological data from widely spread stations ma y be inadequate. The gradient model presented here is inadequate for t imes longer than about 30 min. even if no gradients are present, becau se of the complicated stochastic like temporal behavior of the wet atm osphere. When gradients are present, they can change magnitude by appr oximately 50% over 10-15 min. Nevertheless, our ability to fit the rad iometer data implies that on timescales <30 min and for elevation angl es >23.6-degrees, the local structure of the wet atmosphere can be des cribed with a simple model. (The model is not limited to this range of elevation angles in principle.) The estimated gradient and gradient r ate vectors have preferred directions, which indicates a prevailing st ructure in the three-dimensional temperature and humidity fields, poss ibly related to systematic behavior in large-scale weather systems and /or the local air-land-sea interaction at this site.