In an article published in Remote Sensing, researchers have analyzed the properties of various profiles of atmospheric refractive index structure constant over the Lhasa Region. The valuable in situ sounding data necessary for the analysis were collected at different strength conditions of an Asian summer monsoon anticyclone (ASMA) in August 2018.
The optical turbulence features of the Tibetan Plateau (TP) are different from those in low-elevation areas due to its complex topography, peculiar atmospheric circulations, and high elevation.
Within a few days of the Asian summer monsoon anticyclone, the atmospheric refractive index structure constant of the upper troposphere-lower stratosphere changes considerably, particularly in the upper troposphere region. The impact of the Asian summer monsoon anticyclone on atmospheric refractive index structure constant varies between the upper troposphere-lower stratosphere and the tropopause regions.
An "upper highs-lower lows" pressure field pattern, favorable for reducing the possible temperature lapse rate, becomes more evident when the Asian summer monsoon anticyclone is stronger and closer to Lhasa. The decrease in static stability collectively influences the development of optical turbulence, increases tropopause height, and reduces tropopause temperature.
However, a pressure field structure characterized as the "upper highs-lower highs" develops over the Lhasa if potent high-pressure activity occurs at the low-pressure layer. The formation of such a field increases the potential temperature lapse rate and limits the convective intensity.
The Asian summer monsoon anticyclone is not affected by low-level atmospheric high-pressure activity. However, it suppresses atmospheric refractive index structure constant in the lower stratosphere and tropopause. The upper troposphere-lower stratosphere optical turbulence intensity fluctuations produced by Asian summer monsoon anticyclone activities also significantly impact astronomical parameters.
The Tibetan Plateau and the Effects of ASMA
The Tibetan Plateau differs significantly from low-elevation plain regions due to its unique characteristics, including:
- Its complex terrain of high mountains and valleys
- Variation in atmospheric composition
- Dramatic changes in the atmospheric environment
- Unique circulation characteristics and geographical climate
The Tibetan plateau exhibits atmospheric optical turbulence with different characteristics than the low-elevation plain regions.
Based on the optical turbulence process that occurs during mass and energy exchanges between the atmosphere and the land, the Tibetan plateau is the primary energy source supplying latent heat fluxes to the atmosphere. Optical turbulence can develop in the upper troposphere-lower stratosphere region due to the combined effect of the heat source's interaction and the complex geography of the Tibetan plateau.
Intense Asian summer monsoon circulations, such as deep convective activity and planetary-scale anticyclones such as the Asian summer monsoon anticyclone, exist over the Tibetan plateau. The upper troposphere-lower stratosphere, closely associated with the Tibetan plateau land-atmosphere heat transfer, is almost entirely occupied by the ASMA.
Strong convective events and the Asian summer monsoon anticyclone on the Tibetan plateau impact the distribution of atmospheric components in the upper troposphere-lower stratosphere of the Asian monsoon zone by elevating the lower atmosphere. A turbulent atmosphere is an essential channel for transport in stratosphere-troposphere exchange (STE).
As the Earth’s third pole, the Tibetan plateau has drawn much interest. Numerous atmospheric scientific investigations on the Tibetan plateau have been carried out over the past three decades with numerical simulations. However, the relation between meteorological parameters and the optical turbulence structure in the upper troposphere-lower stratosphere has rarely been researched.
The causes for the significant short-term variations of atmospheric refractive index structure constant in the Lhasa area from the features of atmospheric optical turbulence parameters, high-pressure activities at 500 hPa, atmospheric circulation, and the Asian summer monsoon anticyclone were analyzed in this paper.
The thermal turbulence sounding experiment was performed at the Lhasa Meteorological Bureau from August 3 to August 18, 2018. The valuable data of the high vertical resolution thermal turbulence sounding obtained during the experiment served as a solid foundation for investigating the fine atmospheric structure in the upper troposphere-lower stratosphere and validating model simulation over the Tibetan plateau.
As the geopotential height increased, the intensity of the high-pressure system above the Tibetan plateau gradually increased. The Asian summer monsoon anticyclone center shifted to the Tibetan plateau after August 12 (onset), where it generally intensified (13–15 August, early-stage formation) along with forming a high-pressure system.
The ASMA fully developed over the Tibetan plateau after August 16 and reached its peak intensity.
A more potent "upper highs and lower lows" pressure field structure was visible over Lhasa when the Asian summer monsoon anticyclone intensity was higher or the Asian summer monsoon anticyclone center was nearer to Lhasa.
The value of the atmospheric refractive index structure constant increased as convective activity increased. On the other hand, alterations to the low-level pressure field, such as those at 500 hPa, significantly affected the atmospheric refractive index structure constant vertical profile.
The potential temperature profiles in the upper troposphere-lower stratosphere changed considerably during the experimental period, particularly in the high troposphere. The four days' potential temperature lapse rates were generally at the same amplitudes between 12.5 and 16 km. On the other hand, the trend of the potential temperature within 12.5–16 km on August 16, 2018, was notably different from the previous three days.
Study Demonstrates Optical Turbulence in the Upper Troposphere-Lower Stratosphere Region
This paper analyzed the impacts of the ASMA and high-pressure events in the 500 hPa zone on atmospheric refractive index structure constant under various ASMA strength states in the Lhasa region based on valuable in situ sounding data.
The ASMA generally influenced the atmospheric refractive index structure constant in the upper troposphere-lower stratosphere and tropopause through various mechanisms. It was discovered that during high-intensity ASMA, optical turbulence activity was inhibited in the tropopause and lower stratosphere, which was not conducive to the STE process and astronomical observations.
ASMA's degree of convection promotion was related to its center's location and strength. It was also affected by the high-pressure events of the lower atmosphere.
However, due to the limited radiosonde data considered in this paper, determining whether the proposed discussions were regional and the causes for the short-term atmospheric refractive index structure constant variations necessitated further analyses and exploration with other extensive data.
K. Zhang, F. Wang, N. Weng, X. Wu, X. Li, T. Luo, (2022) Optical Turbulence Characteristics in the Upper Troposphere–Lower Stratosphere over the Lhasa within the Asian Summer Monsoon Anticyclone. Remote Sensing. https://www.mdpi.com/2072-4292/14/16/4104