Analysis of flow in complex terrain using multi-Doppler lidar retrievals

Tyler M Bell,Petra Klein,Norman Wildmann,Robert Menke

doi:10.5194/amt-13-1357-2020

Abstract

Abstract. Strategically placed Doppler lidars (DLs) offer insights into flow processes that are not observable with meteorological towers. For this study we use intersecting range height indicator (RHI) scans of scanning DLs to create four virtual towers. The measurements were performed during the Perdigão experiment, which set out to study atmospheric flows in complex terrain and to collect a high-quality dataset for the validation of meso- and microscale models. Here we focus on a period of 6 weeks from 1 May 2017 through 15 June 2017. During this Intensive Observation Period (IOP) data of six intersecting RHI scans are used to calculate wind speeds at four virtual towers located along the valley at Perdigão with a temporal resolution of 15 min. While meteorological towers were only up to 100 m tall, the virtual towers cover heights from 50 to 600 m above the valley floor. Thus, they give additional insights into the complex interactions between the flow inside the valley and higher up across the ridges. Along with the wind speed and direction, uncertainties of the virtual-tower retrieval were analyzed. A case study of a nighttime stable boundary layer flow with wave features in the valley is presented to illustrate the usefulness of the virtual towers in analyzing the spatially complex flow over the ridges during the Perdigão campaign. This study shows that, despite having uncoordinated scans, the retrieved virtual towers add value in observing flow in and above the valley. Additionally, the results show the virtual towers can more accurately capture the flow in areas where the assumptions for more traditional DL scan strategies break down.

Highlights

Scanning Doppler lidar (DL) systems have proven to be useful in many different sectors of atmospheric study
We focused the analysis on three cases: first we selected a case with quasi-2D flow and small vertical velocities, we chose a quasi-2D flow case with large vertical velocities, and last we selected a fully 3D case with complex flow interactions that are associated with strong variability of the flow along the valley
The virtual towers help fill the gap in wind speed measurements inside the valley above the height of the physical towers (100 m) and where more traditional DL scanning strategies may not be fully valid given the complexity of the site and in particular for this experiment provide nicely distributed measurements along the valley at heights which are not captured by any other instrument

Summary

Introduction

Scanning Doppler lidar (DL) systems have proven to be useful in many different sectors of atmospheric study. The simplest techniques to derive the wind speed and direction are the velocity azimuth display (VAD) technique (Browning and Wexler, 1968) and the Doppler beam swinging (DBS) technique (Strauch et al, 1984). These techniques make the assumption that the wind is horizontally homogeneous in order to retrieve the wind speed and direction; this is often not the case in boundary layer meteorology and can introduce errors into the wind estimates. One area where the assumption of horizontal homogeneity is likely invalid is in the study of complex terrain, where different beams of a scan pattern

Objectives

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Results

Conclusion