Abstract
Weather radar is used by forecasters for identifying storms and estimating its corresponding precipitation. Anomalous propagation of the radar beam may lead to misinterpretation of the weather events and associated errors in precipitation estimates. As the weather radar transmits electromagnetic waves, it is affected by the refractive index of the atmosphere which depends on the temperature, pressure and water vapor content. It is important to understand the refractive index of the atmosphere and how it affects the beam propagation of the radar to interpret the echoes better. Meteorological conditions causing anomalous propagation is well described in literature by Battan (1973), Doviak and Zrnik (2006) and Rinehart (2001). The vertical refractivity gradient (VRG) affects the propagation of radio waves in the atmosphere (Gossard, 1977). These anomalous propagation cause clutter to be displayed in the radar images. The intensity of the clutter was differentiated into various groups by the amount of clutter present in the radar image. Refractivity parameters at various heights and the height of the temperature inversion layer were calculated using radiosonde observational data at the Visakhapatnam (VSK) station. The observed values from the radiosonde data were compared with the intensity groups and it was found that three parameters were influential in determining the intensity of the clutter which is the presence of the temperature inversion layer above the radar, the VRG of the temperature inversion layer above the radar and the VRG from the radar to a height of 1 km from sea level.
Highlights
Weather radar is widely used by the forecasters for now-casting and estimation of Rainfall
As the Refractivity is known for each height level, the vertical refractivity gradient observed in the layer of temperature inversion Vertical Refractivity Gradient (VRG) (Htir) was calculated as per below formula : Value of N at Htir − Value of N at Radar eigt
It can be inferred that in the 10 observations, the clutter may be contributed by air borne clutters and partial beam blockage correction due to super-refractivity level VRG (Htir) and in remaining 26, the bending of radio waves was closer to normal propagation due to super-refractivity level VRG (1 km) being skewed towards normal propagation causing only the partial beam blockage correction to be the major contributor of clutter
Summary
Weather radar is widely used by the forecasters for now-casting and estimation of Rainfall. The refractive index changes occur due to a decrease in humidity with height which is caused by Temperature inversion wherein a warm dry air lies over a cool moist layer. Weather radars are equipped with post-processing algorithms which compensate for the partial beam blockage occurring due the complex orographic features in the surrounding mountains These algorithms work on the assumption of normal propagation, but in superrefractive conditions where the beam is bending towards the earth surface thereby lowering the height of the beam, a totally blocked beam may be considered as partial blocked leading to incorrect corrections in all the pixels of the particular radar ray (Bech et al, 2007)
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