Abstract
A squall line is a type of strongly organized mesoscale convective system that can cause severe weather disasters. Thus, it is crucial to explore the dynamic structure and hydrometeor distributions in squall lines. This study analyzed a squall line over Guangdong Province on 6 May 2016 that was observed using a Ka-band millimeter-wave cloud radar (CR) and an S-band dual-polarization radar (PR). Doppler spectral density data obtained by the CR were used to retrieve the vertical air motions and raindrop size distribution (DSD). The results showed the following: First, the CR detected detailed vertical profiles and their evolution before and during the squall line passage. In the convection time segment (segment B), heavy rain existed with a reflectivity factor exceeding 35 dBZ and a velocity spectrum width exceeding 1.3 m s−1. In the PR detection, the differential reflectivity factor (Zdr) was 1–2 dB, and the large specific differential phase (Kdp) also represented large liquid water content. In the transition and stratiform cloud time segments (segments B and C), the rain stabilized gradually, with decreasing cloud tops, stable precipitation, and a 0 °C layer bright band. Smaller Kdp values (less than 0.9) were distributed around the 0 °C layer, which may have been caused by the melting of ice crystal particles. Second, from the CR-retrieved vertical air velocity, before squall line passage, downdrafts dominated in local convection and weak updrafts existed in higher-altitude altostratus clouds. In segment B, the updraft air velocity reached more than 8 m s−1 below the 0 °C layer. From segments C to D, the updrafts changed gradually into weak and wide-ranging downdrafts. Third, in the comparison of DSD values retrieved at 1.5 km and DSD values on the ground, the retrieved DSD line was lower than the disdrometer, the overall magnitude of the DSD retrieved was smaller, and the difference decreased from segments C to D. The standardized intercept parameter (Nw) and shape parameter (μ) of the DSD retrieved at 1.8 km showed good agreement with the disdrometer results, and the mass-weighted mean diameter (Dm) was smaller than that on the ground, but very close to the PR-retrieved Dm result at 2 km. Therefore, comparing with the DSD retrieved at around 2 km, the overall number concentration remained unchanged and Dm got larger on the ground, possibly reflecting the process of raindrop coalescence. Lastly, the average vertical profiles of several quantities in all segments showed that, first of all, the decrease of Nw and Dm with height in segments C and D was similar, reflecting the collision effect of falling raindrops. The trends were opposite in segment B, indicating that raindrops underwent intense mixing and rapid collision and growth in this segment. Then, PR-retrieved Dm profiles can verify the rationality of the CR-retrieved Dm. Finally, a vertical velocity profile peak generated a larger Dm especially in segments C and D.
Highlights
A squall line is a type of strongly organized mesoscale convective system
There was a weak updraft in near-surface clouds and precipitation, which may have been caused by the divergence of rain falling rapidly on the ground
Cloud radar retrievals on 66May, May,2016: 2016: Vertical air velocity (b) free-falling
Summary
A squall line is a type of strongly organized mesoscale convective system. When a squall line passes, barometric pressure rises, temperature drops, and the wind speed increases suddenly, causing severe weather disasters, such as hailstorms, heavy rain, and severe wind. Inversion methods, such as the velocity azimuth processing method and four-dimensional variational assimilation inversion technology, can be used to acquire the wind field structure inside squall lines These studies mostly provide a general dynamic structure of the squall lines from a mesoscale perspective and seldom provide hydrometeor distributions from a microphysical perspective. Liu et al [12] and Peng et al [13] tried to retrieve the dynamic structure and DSD in clouds and precipitation using CR, and compared and verified the results retrieved with ground disdrometer data and inversion results from other radar bands These studies focused mainly on weak stratiform cloud precipitation or clouds with no rain and did not retrieve and analyze typical convective cloud precipitation with single-band CR.
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