A simulation study was designed to investigate the error characteristics of retrieved local refractivity arising only from the assumption of spherical symmetry. The convergence of error estimation with respect to different horizontal resolutions was examined, and a resolution higher than 30 km was found to be reasonable for this simulation study.
Two cases, one in summer 1997 and the other in winter 1998, were used to study errors of retrieved local refractivity. The error can reach 10 units in the lower troposphere under the assumption of spherical symmetry. Error decreases with height linearly, until about 3 km and then dramatically above 3 km. A local maximum error occurs at approximately 6 km in the 1997 case because there were large refractivity disturbances on the lee side of the Rocky Mountains around that level.
The level of the maximum error over land (∼ 2-3 km) was higher than over ocean (∼ 1 km) due to complex topography, as well as a deeper boundary layer over land. As a result, the average root mean square error below 3 km presents a meandering feature over the entire domain of interest. Moreover, the error close to the surface over ocean was slightly higher than that over land. However, in the 2-4 km range the error was slightly lower over ocean than over land. Note that the altitude of maximum error over ocean can be shifted upward when strong disturbances, such as severe weather, exist over the region. The error during summer was larger than that during winter, and it was greater at lower latitudes than at higher latitudes, as reported in previous studies.
When modeling retrieved local refractivity, the observational error is a function of the model horizontal resolution. The error was reduced as model resolution was decreased but there was a lower limit for an optimal resolution. This optimal resolution in the lower atmosphere was higher (∼ 190 km) than that in the middle and upper atmosphere (∼ 250 km) in this study, due to a stronger vertical gradient of refractivity and higher vertical resolution in the lower troposphere.