Convective updraught evaluation in high‐resolution NWP simulations using single‐Doppler radar measurements

Abstract

This study presents an evaluation of the size and strength of convective updraughts in high‐resolution simulations by the UK Met Office Unified Model (UM). Updraught velocities have been estimated from range–height indicator (RHI) Doppler velocity measurements using the Chilbolton advanced meteorological radar, as part of the Dynamical and Microphysical Evolution of Convective Storms (DYMECS) project. Based on mass continuity and the vertical integration of the observed radial convergence, vertical velocities tend to be underestimated for convective clouds due to the undetected cross‐radial convergence. Velocity fields from the UM at a resolution corresponding to the radar observations are used to scale such estimates to mitigate the inherent biases. The analysis of more than 100 observed and simulated storms indicates that the horizontal scale of updraughts in simulations tend to decrease with grid length; the 200 m grid length agreed most closely with the observations. Typical updraught mass fluxes in the 500 m grid length simulations were up to an order of magnitude greater than observed, and greater still in the 1.5 km grid length simulations. The effect of increasing the mixing length in the sub‐grid turbulence scheme depends on the grid length. For the 1.5 km simulations, updraughts were weakened though their horizontal scale remained largely unchanged. Progressively more so for the sub‐kilometre grid lengths, updraughts were broadened and intensified; horizontal scale was now determined by the mixing length rather than the grid length. In general, simulated updraughts were found to weaken too quickly with height. The findings were supported by the analysis of the widths of reflectivity patterns in both the simulations and observations.

The DYMECS Project: A Statistical Approach for the Evaluation of Convective Storms in High-Resolution NWP Models

“…In this section, radar estimates of vertical velocities are derived and rescaled following this method and are used to statistically evaluate the true model vertical velocities. For a full description of the method, see Nicol et al (2015).…”

A highly localized high‐precipitation event over Corsica

“…At the cloud base however, BL89 is restricted not only by TKE and stability but also by remote stable zones above and below due to its non‐local nature. The underestimation of the cell size could indicate insufficient mixing along the lateral cloud edges (Nicol et al ; Verrelle et al ). For the presented case, this is seen for both mixing‐length formulations, DEAR and BL89.…”

Characteristics of vertical air motion in isolated convective clouds

“…Remote sensing has the advantage of being able to measure the vertical velocity at different heights simultaneously (Tonttila et al, 2011), and some of the techniques can detect the strongest updraft cores in convective clouds (Heymsfield et al, 2010;Collis et al, 2013). Volumetric radars can also provide three-dimensional (3-D) structure of air motion in convective clouds Nicol et al, 2015;Jorgensen et al, 2000). However, remote sensing measurements are not as accurate as aircraft measurements because of the assumptions needed to account for the contribution of hydrometeor fall speed in the observed Doppler velocity in order to ultimately estimate air velocity.…”