Estimating evaporation duct heights from radar sea echo

Abstract

Abstract. The evaporation duct is a downward refracting layer that results from the rapid decrease in humidity with respect to altitude occurring in the atmospheric surface layer above bodies of water. The evaporation duct affects radar detection ranges at frequencies of approximately 1 GHz and above. Models based on Monin-Obukhov similarity theory are usually used to calculate evaporation duct refractivity profiles from bulk measurements of air temperature, humidity, wind speed, and the sea surface temperature. Modeling results by Pappert et al. [1992] indicated that the falloff of radar sea echo as a function of range was an increasing function of the evaporation duct height. On the basis of those results, the authors proposed inferring the evaporation duct height by a slope fit to modeled clutter power, a nonlinear least squares inversion procedure. Data for testing the inversion procedure were obtained using the S band Space Range Radar at Wallops Island, Virginia. Evaporation duct heights were inferred from the radar data on the basis of the assumption of a range-independent evaporation duct height and sea clutter radar cross section (o-ø). Validation data consist of buoy and boat in situ bulk measurements. The result of comparing the radar-inferred evaporation duct heights and those calculated from bulk measurements indicates that the radar-inferred duct heights are strongly correlated with those from the in situ measurements, but there is some uncertainty as to whether they are biased or unbiased. That uncertainty arises from the assumed dependence of o -ø on the grazing angle ½. That ½ dependence is currently a matter of debate in the open literature, with the lower and upper ends of modeling results being o.O • ½0 and o -ø • ½4, respectively. We show results for both dependencies and note that the o -ø • ½0 provides the best agreement with our measurements. It should be noted that inferring the evaporation duct height from radar sea echo is a problem that stresses the modeling of low-grazing-angle backscatter. IntroductionEvaporation ducts result from the rapid change in humidity and temperature that occurs within the surface layer of the atmosphere over bodies of water. The resulting vertical profiles of temperature and humidity change the profile of the index of refraction n and the derivative quantities of radio refractivity N and modified refractivity M so as to form a duct that affects electromagnetic (EM) propagation at frequencies of 1 GHz and above. Evaporation duct refractivity profiles are most often calculated using Monin-Obukhov similarity-theory-based models of Jeske [1971, 1973] surements of the sea surface temperature Ts, air temperature Ta, relative humidity RH, and wind speed U at a reference height above the sea surface. These are commonly referred to as "bulk measurements." The Jeske profile under neutral conditions (T a = Ts) is uniquely defined by the evaporation duct height 3, which is the height at which the refractivity profile changes from downward to upward refracting. The J...

Evaporation Duct Retrieval Using Changes in Radar Sea Clutter Power Versus Receiving Height

“…6 are generated on the basis of the radar sea clutters received at heights ranging from 10 to 25 m. For each radar frequency, these results are compared with four results of the CPM approach generated at four different antenna heights (10,15,20, and 25 m). The visual inspection shows that under a 3 GHz radar system, the CCPM method performs well for all the actual EDHs according to the small RMS errors (Fig.…”

“…Various traditional methods have been used to determine the atmospheric refractivity profile; these approaches include "bulk" models with in situ measurements as input, microwave refractometers [7], LIDAR techniques [8], and many numerical weather prediction models [9]. In recent years, a technique referred to as refractivity from clutter (RFC) has been widely used in retrieving the refractivity profile in maritime environments [10][11][12][13][14]. This method can easily be performed without additional instruments apart from the radars installed on ships.…”

“…The RFC technique has been successfully applied to the inversion of evaporation duct profiles (i.e., EDHs). To estimate EDH from radar sea clutter observations, Rogers et al [10] used a nonlinear least squares inversion procedure, which was tested with data from an experiment conducted at Wallops Island, Virginia, in 1998. Yardim et al [15] tracked the temporal and spatial evolution of evaporation duct parameters from radar sea clutter using an extended Kalman filter.…”

“…In retrieving the modified refractivity profile for an evaporation duct, this technique is generally performed on the basis of the clutter power received at a fixed antenna height, as in Rogers et al [10], Yardim et al [17], and Wang et al [19]. That is, only one observed clutter power pattern is used as input into an RFC inversion algorithm.…”

Retrieving Evaporation Duct Heights From Power of Ground-Based GPS Occultation Signal

“…These models are sensitive to input bulk measurements, especially under stable condition. Therefore, research on alternative means for remotely sensing evaporation duct heights continues [7][8][9]. Considering that the effects of evaporation ducts are manifested in radiowave propagation, it might be feasible to infer the structure of duct from the propagation measurements.…”