Atmospheric Boundary Layer and Free Atmosphere: Dynamics, Physical Processes, and Measuring Methods

Abstract

The article presents the main conclusions obtained in the special issue “Atmospheric Boundary Layer and Free Atmosphere: Dynamics, Physical Processes, and Measuring Methods”. The average meteorological quantities as well as the turbulent characteristics in different atmospheric conditions are considered.

The study of the structure of air flows in a wide range of spatial and temporal scales is relevant for the development of physical concepts of various processes in the atmosphere. Appropriate knowledge of atmospheric flow structure and further improvements in understanding atmospheric characteristics are essential for different fields of science including atmospheric physics, meteorology, atmospheric and adaptive optics, and remote sensing of the atmosphere. The Special Issue entitled “Atmospheric Boundary Layer and Free Atmosphere: Dynamics, Physical Processes, and Measuring Methods” contains five articles. Particular attention is paid to the energy characteristics of atmospheric flows estimated from measurement data.

In the article “Thermodynamic and Kinematic Structures in the Rainband Region of Typhoon Lekima (2019) at Landfall”, the authors discuss Super Typhoon Lekima and its effects on spatial fields of meteorological characteristics. The authors study the characteristics of the total and disturbed airflow during the passage of typhoons [1,2]. In particular, the structure of Typhoon Lekima and the surrounding air is studied using the radiosonde balloons measurements and the weather radar at Taizhou station [3]. The authors showed that the typhoon contained a concentric eyewall that did not disappear until the storm made landfall. Vertical profiles of wind speed, air temperature, and other characteristics are given, which demonstrate significant disturbances during the passage of a typhoon. The authors also indicated that the air flows at the outflow layer are considerably turbulent compared to the flows in the tropopause region. These studies are relevant for the diagnostics and forecasting of dangerous weather phenomena, including intense phenomena within the mesoscale and micrometeorological ranges of the energy spectra of atmospheric flows.

The second article, “Diurnal Dynamics of the Umov Kinetic Energy Density Vector in the Atmospheric Boundary Layer from Minisodar Measurements”, is also devoted to the study of atmospheric flows using measurement data [4]. By applying a special program of sodar data preprocessing [5,6] the authors study the behavior of the mean kinetic energy as well as turbulent kinetic energy using minisodar measurements in the lower atmosphere. A novel method is proposed for analyzing the structure of the wind field based on the spatiotemporal dynamics of the Umov vector from minisodar measurements. Using the method three atmospheric layers have been revealed:

(i)

The near-surface layer at heights of 5–15 m;

(ii)

The intermediate atmospheric layer at heights of 15–150 m;

(iii)

The upper atmospheric layer of enhanced turbulence (above the second layer).

We should note that obtained ratios of the turbulent kinetic energy to the mean kinetic energy for different heights in the atmosphere vary within a fairly narrow range. Knowledge of these relationships is the basis for correct parametrization of turbulent characteristics through the mean energy characteristics of the airflow at the site of interest.

The third paper, “Astroclimatic Conditions at the Hoa Lac and Nha Trang Astronomical Observatories”, is also aimed at analyzing the energy characteristics of airflow [7]. The authors analyze the deformations of the turbulence energy spectra at different isobaric levels in the atmosphere. The following conclusions were obtained in the paper:

(i)

The mean annual heights of the jet streams at the sites of the Hoa Lac and Nha Trang astronomical observatories correspond to 175 hPa and 125 hPa, respectively. At the Hoa Lac Observatory, the mean wind speed at the jet axis is 4.3 m/s higher compared to Nha Trang Observatory;

(ii)

The effective turbulent velocity is 5.9 and 7.4 m/s for Nha Trang and Hoa Lac observatories, respectively;

(iii)

At the site of the Nha Trang Astronomical Observatory, the amplitude of daily air temperature variations in the surface layer is approximately 1.5–2.5 times smaller compared to the Hoa Lac Observatory. At the sites of the Nha Trang and Hoa Lac observatories the spectra have “−5/3” slope in the lower part of the optically active atmosphere. With height the slopes of the spectra change. The slopes become steeper;

(iv)

The low-frequency maximum in the spectra is pronounced only in the lower layers of the atmosphere. We associate the variations in the low-frequency maximum in the spectrum with processes in the atmospheric boundary layer (including wave perturbations).

The results obtained in the paper can be used for astronomical applications [8,9].

In the paper “Characteristics of Turbulence and Aerosol Optical and Radiative Properties during Haze–Fog Episodes in Shenyang, Northeast China” the authors analyze haze and haze-fog episodes [10]. Variations of PM2.5 as well as the profiles of average meteorological and turbulent characteristics are estimated. Physically representative variations of the structure constant of air refractive index fluctuations $C_n^2$ in the lower layer of the atmosphere (from the underlying surface to 2 km) are obtained. The authors found that the turbulent kinetic energy and friction velocity remained at low levels during pollution events. Furthermore, Li X. et al. indicated that turbulent fluctuations in the atmospheric boundary layer directly affect the horizontal transport and vertical mixing of aerosols/fog droplets and alter the near-surface visibility and air quality [11,12].

The paper “Statistical Characteristics of Cloud Heights over Lanzhou, China from Multiple Years of Micro-Pulse Lidar Observation” presents the macroscopic characteristics of clouds [13]. These characteristics are the cloud base height, cloud peak height, cloud top height and cloud thickness. Using micro-pulse lidar as well as modified cloud discrimination algorithm [14], the following results were obtained:

(i)

The mean height of the cloud base, cloud peak, cloud top and mean cloud thickness was about 4.0 km, 4.8 km, 5.5 km and 1.5 km, respectively;

(ii)

The repeatability of cloudiness in the middle tier is the highest (41.4%). The low and high clouds are observed with probabilities of 33.7% and 24.9%, respectively;

(iii)

The distributions of the cloud base heights during the year were rather similar;

(iv)

The frequency distributions of cloud peak height in spring and summer were similar; the maximum frequency in spring and summer was 15.8% in the range of 3–4 km and 18% in the range of 4–5 km, respectively. The frequency distributions of the cloud peak height in autumn and winter were also basically similar, and the maximum frequency was 20% in the range of 2–3 km in autumn and 18.6% in the range of 5–6 km in winter, respectively;

(v)

The frequency distributions of the cloud top height in spring and summer were similar, and the frequency distribution of the cloud top height in autumn was similar to that in winter; the maximum frequency was 14% in the range of 3–4 km in spring, 16% in the range of 4–5 km in summer, 20.1% in the range of 2–3 km in autumn and 17.8% in the range of 7–8 km in winter;

(vi)

The cloud thickness was mostly less than 3 km at 94.2%, and generally the thicker the cloud the less the frequency. The frequency distributions of the cloud thickness in spring, summer and winter were similar with maximum frequencies of 44.9%, 35.6% and 52%, respectively, in the range of 1–2 km. The frequency of the cloud thickness in autumn decreased with increasing cloud thickness, and the maximum frequency was 44.9% in the range of 0–1 km.