Abstract
Abstract. Mid-level stratiform precipitations during the passage of warm fronts were detailedly observed on two occasions (light and moderate rain) by a 355 nm polarization lidar and water vapor Raman lidar, both equipped with waterproof transparent roof windows. The hours-long precipitation streaks shown in the lidar signal (X) and volume depolarization ratio (δv) reveal some ubiquitous features of the microphysical process of precipitating hydrometeors. We find that for the light-rain case precipitation that reaches the surface begins as ice-phase-dominant hydrometeors that fall out of a shallow liquid cloud layer at altitudes above the 0 ∘C isotherm level, and the depolarization ratio magnitude of falling hydrometeors increases from the liquid-water values (δv<0.09) to the ice/snow values (δv>0.20) during the first 100–200 m of their descent. Subsequently, the falling hydrometeors yield a dense layer with an ice/snow bright band occurring above and a liquid-water bright band occurring below (separated by a lidar dark band) as a result of crossing the 0 ∘C level. The ice/snow bright band might be a manifestation of local hydrometeor accumulation. Most falling raindrops shrink or vanish in the liquid-water bright band due to evaporation, whereas a few large raindrops fall out of the layer. We also find that a prominent δv peak (0.10–0.40) always occurs at an altitude of approximately 0.6 km when precipitation reaches the surface, reflecting the collision–coalescence growth of falling large raindrops and their subsequent spontaneous breakup. The microphysical process (at ice-bright-band altitudes and below) of moderate rain resembles that of the light-rain case, but more large-sized hydrometeors are involved.
Highlights
An observation-based understanding of the microphysical processes of precipitation is essential for weather/climate modeling and predictions
To study the microphysical processes that occur at altitudes ranging from the apparent source cloud base down to the near-surface during surface precipitation, a 355 nm polarization lidar and a water vapor Raman lidar at the Wuhan University atmospheric observation site were equipped with waterproof transparent roof windows
Observations of precipitation and associated precursor clouds were made with two co-located lidars (a 355 nm polarization lidar and water vapor Raman lidar) equipped with waterproof transparent roof windows at the Wuhan University atmospheric observatory (30.5◦ N, 114.4◦ E; 73 m above sea level)
Summary
An observation-based understanding of the microphysical processes of precipitation is essential for weather/climate modeling and predictions. The ice formation process has been studied extensively by observing liquid-layer-topped ice virgae, because ground-based lidar and radar can reliably sample the entire height ranges of ice virgae and their apparent source cloud bases (Ansmann et al, 2009; de Boer et al, 2011; Bühl et al, 2016, 2019). To study the microphysical processes that occur at altitudes ranging from the apparent source cloud base down to the near-surface during surface precipitation, a 355 nm polarization lidar and a water vapor Raman lidar at the Wuhan University atmospheric observation site were equipped with waterproof transparent roof windows. Water accumulation on the roof windows hardly impacted the lidar-observed subcloud water vapor mixing ratio (qv) profiles This allows us to systematically observe precipitation processes (light and moderate rains).
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