LIDAR OBSERVATION OF MESOSPHERIC CLOUDS ABOVE BEIJING: A CASE STUDY

We report on the first observation of mesospheric cloud (MC) at the altitude range of 50-65 km with lidars at Yanqing (40.5°N, 116°E) and Pingquan (41°N, 118.7°E) on 30 October 2018. The MC occurred at the 51-56 km altitude range during the hours of dawn. It had an obvious double-layer structure, but the cloud layer was sparse. The MC was re-observed at the 56-62 km altitude range in the twilight, and the double-layer structure was still obvious. However, the cloud layer became thicker with a maximum volume backscatter coefficient (BSC) 3.1×10&'(m&'sr&' . Atmospheric temperature structure was derived according to the lidar observations, and it was found that, several hours before the MC occurrence, a temperature anomaly with coldest temperature ~185 K was propagating downward at the altitudes of 50-65 km. This MC layer was simultaneously observed with lidar at Pingquan in the twilight. It could be a regional MC event and possibly formed locally by a transient cooling due to small-scale disturbances in the mesosphere.


INTRODUCTION
Mesospheric clouds (MCs) observed at midlatitudes were accepted as being very unusual atmospheric phenomena. They are possibly related to exceptional dynamical forcing or perhaps are an early indicator of significant long-term changes in the Earth's climate. For a long time, two kinds of typical MCs, i.e., noctilucent clouds and polar mesospheric clouds, have been extensively studied by imaging or lidar observations [1][2][3]. It was generally agreed that MCs are composed of ice crystals and often occur in summer near polar latitudes in the upper mesosphere between 76 and 86 km [4]. Meanwhile, it was reported that, due to the global climate change, the occurrence frequency of MC has increased and it has been occasionally observed at some mid-latitudes in the last several decades [5][6][7]. However, for the MC event under the mesopause, it has been seldom reported so far. In this paper, the lidar-observed features of an MC layer in the 50-65 altitude range above Beijing will be introduced. On the basis of the simultaneous lidar observations at different locations, the generation mechanism of this rare atmospheric phenomenon will be discussed.

2.INSTRUMENTATION &METHODOLOGY
The Yanqing (40.5°N, 116°E) lidar station was built in early 2010. Its lidar system works with two laser beams (@532 nm & 589 nm), and their emitted energies are approximately 320 mJ and 60 mJ per shot, respectively [8]. For an individual lidar profile, the photon counts accumulate for every 1000 laser shots in 34 s, and the spatial resolution and temporal resolution are 96 m and 34 s, respectively. The Pingquan (41°N, 118.7°E) lidar station was established in the summer of 2016, and its lidar system currently works with one laser beam at 589 nm. All the parameters of sodium lidar are the same as those at Yanqing. At altitudes between approximately 35 km and 110 km the lidar records the photons backscattered from air molecules, the sodium layer and aerosols.
An MC event is detected as an enhanced signal relative to the background noise and the air molecule signal at the altitude range of 50-80 km. Such enhanced signal can last for more than 30 min and not vary when the laser frequency is detuning. The MC is quantified by determining the volume backscatter coefficient, -. , which is defined as (1) where n is the number density of MC particles and 78('9( ∘ ) 7; is the effective cross section for backscatter by an individual MC particle for a specific particle size distribution and the applied wavelength of 589 nm or 532 nm. In practice, -. was calculated with the following two equations: where is the backscatter ratio of MC, FGH ( ) is the BSC of atmospheric molecules, FGH is the backscatter signal from molecules, and JGJKH is the total signal after subtraction of the background. At those altitudes where there was no MC, the backscatter signal was normalized to the atmospheric molecular backscatter signal, and it was calculated from the atmospheric densities taken from NRLSISE-00 model. Then, the constant was used to constrain FGH ( ) at MC altitudes.

Mesospheric cloud observed in the dawn on 30 October 2018.
During the regular lidar observation at Yanqing (40.5°N, 116°E) in the dawn on 30 October 2018, it was found that some obvious enhancement began to occurred in the photon-count profiles at the altitude range of 51-56 km due to the occurrence of MCs. The time variation of the volume backscatter coefficient of this MC was analyzed and plotted in Fig.1. It was shown that, around 04:30 LT, the cloud began to occur near the altitude of 51 km, and it was very sparse. About 15 min later, another cloud layer occurred at the altitude range of 53.5-55.5 km, and a double layer structure was observed during the rest of lidar observations. Unfortunately, this MC observation has been carried out for only ~90 min, and it was terminated around the sunrise (~06:00 LT) because the lidar observation can be performed just in nighttime.
According to the definition in Eq.(1), the volume BSC of cloud layer is directly related to the number density of MC particles. It is presented in Fig.1 that the maximum -. was only 1.
although the density of cloud particles increased gradually with time. That means the MC layer was very sparse during the hours of dawn, and the number density of MC particles was extremely tiny.

Mesospheric cloud observed in the twilight on 30 October 2018.
The lidar observations restarted around 18:30 LT in the twilight, and the MC layers were observed again at the very beginning of the observation. The time-altitude variation of the volume BSC of cloud layer was plotted in Fig.2

Comparison and discussion
To compare the two simultaneous observation results from lidars at Yanqing (40.5°N, 116°E) and Pingquan (41°N, 118.7°E), although an MC event has been simultaneously observed at both the two locations, the double-layer structure of MC over Pingquan was not such obvious as that observed at Yanqing. In Fig.3, the upper layer was very tiny, and it was visible to lidar for just several minutes around 19:15 LT. Meanwhile, the lower layer mainly located in the altitude range of 53-56 km, and it was ~3 km lower in altitude than that shown in Fig.2. In terms of time, at Pingquan, the maximum concentration of cloud occurred around 19:30 LT, and it was ~40 min earlier than that occurred over Yanqing. Moreover, the MC event disappeared around 20:45 LT at Pingquan, and it was around 20:00 LT at Yanqing. That is, different features of the cloud layer have been observed by lidars at the two locations which are ~230 km far from to each other. This suggests that the MC layer was probably inhomogeneous in horizontal direction.
In additional, at Yanqing, the same MC event was simultaneously observed by the lidar working at 532 nm, and identical features of the MC layer were found. However, at Pingquan, the MC event was observed by lidar in the twilight only, and no clue of MC has been found when the lidar observations in the dawn were checked. This may indicate that a small-scale MC event was observed above Beijing on 30 October 2018.
According to lidar-observed features above, this MC observed above Beijing is likely not a noctilucent cloud or a polar mesospheric cloud which are often occur in summer near polar latitudes in the upper mesosphere between 76 and 86 km. It occurred in the late autumn of Beijing, and the main altitude located near stratopause. However, this lidar-observed MC possibly had similar generation mechanism as them.
By employing the method introduced by Chanin and Hauchecorne [10], the atmospheric temperature structure before the MC occurrence has been retrieved according to the lidar observations at Yanqing. It was found in Fig.4  It is well known that, in the past few decades, greenhouse-gas concentrations in the atmosphere have unequivocally risen, and weather extremes (e.g., extreme temperatures and precipitations) frequently occurred in the Norther Hemisphere mid-latitudes due to the global warming [11]. The mesosphere probably became cooler because of the increasing levels of CO 2 in the atmosphere. Meanwhile, more water vapor occurred in the mesosphere due to the oxidation of elevated levels of methane in the atmosphere.
Therefore, for this lidar-observed MC with a small scale at middle latitudes, it was possibly related to an exceptional dynamical forcing (e.g., gravity wave activities) which was leading to greater adiabatic cooling in a small region and forming a temperature minimum near the altitude range of 50-65 km. When the atmospheric temperature was sufficiently cold, water vapor formed those mesospheric clouds at these altitudes.

CONCLUSIONS
At the dawn and twilight on 30 October 2018, an MC event in the 50-65 km altitude range was observed with lidars at Yanqing (40.5°N, 116°E) and Pingquan (41°N, 118.7°E). It is an extremely rare atmospheric phenomenon over Beijing and has not been observed and reported in China so far. Analysis results presented that, this MC layer had a double structure with a small scale, and it was probably inhomogeneous in horizontal direction.
Although the composition and the formation mechanism of this MC are still not such clear for us, but this mid-latitude detection is significant because of what it may signify about the global changes of the Earth's climate.