The Summertime Low-Level Jet and Marine Boundary Layer Structure along the California Coast

Abstract The sensitivity of a regional climate model to physical parameterizations and model resolution is investigated in terms of its simulation of boundary layer stratocumulus (SCu) clouds over the southeast Pacific. Specifically, the physical schemes being tested include shallow cumulus convection, subgrid vertical mixing, cloud droplet number concentration (CDNC), and drizzle. As described in Part I, the model with standard settings captures the major features of the boundary layer in the region, including a well-mixed marine boundary layer, a capping temperature inversion, SCu clouds, and the boundary layer regime transition from the well-mixed layer near the coast of South America to a decoupled cloud layer over warmer water to the west. Turning off the shallow cumulus parameterization results in a dramatic increase in the simulated SCu clouds while the boundary layer structure becomes unrealistic, losing the decoupled regime over warm water. With reduced penetrative mixing at the top of shallow cumuli, the simulated SCu clouds are somewhat increased while the boundary layer structure remained largely unchanged. Reducing the CDNC increases the size of cloud droplets and reduces the cloud albedo but has little effect on the vertical structure of the boundary layer and clouds. Allowing more drizzle decreases boundary layer clouds considerably. It is also shown that the simulated depth of the boundary layer and its decoupling is highly sensitive to the model horizontal and vertical resolutions. Insufficient horizontal or vertical resolutions produce a temperature inversion and cloud layer too close to the sea surface, a typical problem for global general circulation models. Implications of these results for global and regional modeling of boundary layer clouds and the areas that need more attention in future model development are discussed.

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Marine boundary layer structure and fractional cloudiness

Journal of Geophysical Research Atmospheres ◽

10.1029/95jd00827 ◽

1995 ◽

Vol 100 (D7) ◽

pp. 14209 ◽

Cited By ~ 69

Author(s):

Bruce A. Albrecht ◽

Michael P. Jensen ◽

William J. Syrett

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Layer Structure ◽

Boundary Layer Structure

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Influence of Boundary Layer Structure and Low-Level Jet on PM2.5 Pollution in Beijing: A Case Study

International Journal of Environmental Research and Public Health ◽

10.3390/ijerph16040616 ◽

2019 ◽

Vol 16 (4) ◽

pp. 616 ◽

Cited By ~ 4

Author(s):

Yucong Miao ◽

Shuhua Liu ◽

Li Sheng ◽

Shunxiang Huang ◽

Jian Li

Keyword(s):

Boundary Layer ◽

Large Scale ◽

Layer Structure ◽

Three Dimensional ◽

Near Surface ◽

Thermally Induced ◽

Low Level ◽

Relative Contribution ◽

Low Level Jet ◽

Meteorological Observations

Beijing experiences frequent PM2.5 pollution, which is influenced by the planetary boundary layer (PBL) structure/process. Partly due to a lack of appropriate observations, the impacts of PBL on PM2.5 pollution are not yet fully understood. Combining wind-profiler data, radiosonde measurements, near-surface meteorological observations, aerosol measurements, and three-dimensional simulations, this study investigated the influence of PBL structure and the low-level jet (LLJ) on the pollution in Beijing from 19 to 20 September 2015. The evolution of the LLJ was generally well simulated by the model, although the wind speed within the PBL was overestimated. Being influenced by the large-scale southerly prevailing winds, the aerosols emitted from the southern polluted regions could be easily transported to Beijing, contributing to ~68% of the PM2.5 measured in Beijing on 20 September. The relative contribution of external transport of PM2.5 to Beijing was high in the afternoon (≥80%), which was related to the strong southerly PBL winds and the presence of thermally-induced upslope winds. On 20 September, the LLJ in Beijing demonstrated a prominent diurnal variation, which was predominant in the morning and after sunset. The occurrence of the LLJ could enhance the dilution capacity in Beijing to some extent, which favors the dilution of pollutants at a local scale. This study has important implications for better understanding the complexity of PBL structure/process associated with PM2.5 pollution in Beijing.

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Orographic gravity wave and low-level jet interaction above a tall and dense Amazonian forest

10.5194/egusphere-egu2020-7341 ◽

2020 ◽

Author(s):

Luca Mortarini ◽

Polari Batista Corrêa ◽

Daniela Cava ◽

Cléo Quaresma Dias-Júnior ◽

Antônio Ocimar Manzi Manzi ◽

...

Keyword(s):

Boundary Layer ◽

Gravity Wave ◽

Coherent Structures ◽

Forest Canopy ◽

Layer Structure ◽

Low Frequency ◽

Stable Stratification ◽

Low Level ◽

Canopy Waves ◽

Low Level Jet

The Wavelet and the Multiresolution analysis are applied to ten nocturnal hours of observations of 3-D wind velocity taken within and above a forest canopy in Central Amazonia. Data from the ATTO Project, consisting in 7 levels of turbulence observations along both 81 and 325-meter towers, are used. The presented night is dynamically rich presenting three distinct periods. In the first one the boundary layer is characterized by canopy waves and coherent structures generated at the canopy top. In the second period an intense orographic gravity wave generated at around 150 m strongly influences the boundary layer structure, both above and below the canopy. In the third period, a very stable stratification at the canopy top enables the development of a low-level jet that interferes and disrupts the vertical orographic wave. During the night the wavelet cospectra identified turbulent and non-turbulent structures with different length and time-scales that are generated at different levels above the canopy and propagated inside it. The contributions of the different temporal scales of the flow above and within the canopy were identified using Wavelet and Multiresolution two-point cospectra. The analysis showed how turbulent and wave-like structures propagates in different ways and, further, the ability of low-frequency processes to penetrate within the canopy and to influence the transport of energy and scalar in the roughness sublayer and within canopy.Keywords: Coherent structures, Canopy Waves, Gravity Waves, Stable Boundary Layer, Low-Level Jet, wave-turbulence interaction.&#160;

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