Abstract
Knowledge of the spatial and temporal variability of near-surface water vapor is of great importance to successfully model reliable radio communications systems and forecast atmospheric phenomena such as convective initiation and boundary layer processes. However, most current methods to measure atmospheric moisture variations hardly provide the temporal and spatial resolutions required for detection of such atmospheric processes. Recently, considering the high correlation between refractivity variations and water vapor pressure variations at warm temperatures, and the good temporal and spatial resolution that weather radars provide, the measurement of the refractivity with radar became of interest. Firstly, it was proposed to estimate refractivity variations from radar phase measurements of ground-based stationary targets returns. For that, it was considered that the backscattering from ground targets is stationary and the vertical gradient of the refractivity could be neglected. Initial experiments showed good results over flat terrain when the radar and target heights are similar. However, the need to consider the non-zero vertical gradient of the refractivity over hilly terrain is clear. Up to date, the methods proposed consider previous estimation of the refractivity gradient in order to correct the measured phases before the refractivity estimation. In this paper, joint estimation of the refractivity variations at the radar height and the refractivity vertical gradient variations using scan-to-scan phase measurement variations is proposed. To reduce the noisiness of the estimates, a least squares method is used. Importantly, to apply this algorithm, it is not necessary to modify the radar scanning mode. For the purpose of this study, radar data obtained during the Refractivity Experiment for H 2 O Research and Collaborative Operational Technology Transfer (REFRACTT_2006), held in northeastern Colorado (USA), are used. The refractivity estimates obtained show a good performance of the algorithm proposed compared to the refractivity derived from two automatic weather stations located close to the radar, demonstrating the possibility of radar based refractivity estimation in hilly terrain and non-homogeneous atmosphere with high spatial resolution.
Highlights
Knowledge of the spatial and temporal variability of near-surface water vapor is of great importance to forecast convective initiation and boundary layer atmospheric processes [1]
Radar measurements of refractivity may help to improve the temporal resolution of water vapor pressure data over small areas since, at warm temperatures, the atmospheric refractivity is a good proxy for water vapor pressure
It is worth noting that the assumptions considered previously in the analysis of the contributions to the total phase are consistent with the maximum variations of refractivity and vertical gradient derived, respectively, from Figure 4d–e considering 5 min time instants (±5 N-units and ±14 N-units km−1)
Summary
Knowledge of the spatial and temporal variability of near-surface water vapor is of great importance to forecast convective initiation and boundary layer atmospheric processes [1]. It has been shown that small variations in temperature (≈1 ◦C) and moisture (≈1 g·kg−1) can make the difference between the initiation or not of a convective process [3,4]. The lack of an adequate method to observe moisture variations with such accuracy and spatial resolution is one of the main limitations for the development of accurate numerical weather prediction methods [6]. At 18 ◦C, a change of 1 ◦C in temperature or a much smaller change of 0.2 g·kg−1 in water vapor results in refractivity changes of 1 N-unit. Refractivity variations are mainly linked to water vapor variations at warm temperatures [7,8]
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