scholarly journals Convective Boundary Layer Clouds as Observed with Ground-Based Lidar at a Mid-Latitude Plain Site

2021 ◽  
Vol 13 (7) ◽  
pp. 1281
Author(s):  
Yifan Zhan ◽  
Fan Yi ◽  
Fuchao Liu ◽  
Yunpeng Zhang ◽  
Changming Yu ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Surface Air Temperature ◽  
Cloud Base ◽  
Convective Boundary ◽  
Height Range ◽  
Near Surface ◽  
Short Period ◽  
Ground Based Lidar ◽  
Water Vapor Mixing Ratio

A total of 3047 individual shallow cumuli were identified from 9 years of polarization lidar measurements (2011–2019) at Wuhan, China (30.5°N, 114.4°E). These fair-weather shallow cumuli occurred at the top edge of the convective boundary layer between April and October with the maximum occurrence in July over the 30°N plain site. They persisted mostly (>92%) for a short period of ~1–10 min and had a geometrical thickness of ~50–600 m (a mean of 209 ± 138 m). The majority (>94%) of the cloud bases of these cumuli were found to appear ~50–560 m (a mean of 308 ± 254 m) above the lifting condensation level (LCL). In this height range from the LCL to the cloud base, the lidar volume depolarization ratio (δδV) slightly decreased with increasing height, showing gradually increasing condensation in this sub-cloud region due to penetrative thermals. Most of the observed shallow cumuli (79%) formed under the conditions of high near-surface air temperature (>30 °C) and water vapor mixing ratio (>15 g kg−1).

Subcloud and Cloud-Base Latent Heat Fluxes during Shallow Cumulus Convection

2019 ◽  
Vol 77 (3) ◽  
pp. 1081-1100 ◽  
Author(s):  
Neil P. Lareau
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Latent Heat ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
Cloud Base ◽  
Cumulus Convection ◽  
Mixing Ratio ◽  
Convective Boundary ◽  
Water Vapor Mixing Ratio

Abstract Doppler and Raman lidar observations of vertical velocity and water vapor mixing ratio are used to probe the physics and statistics of subcloud and cloud-base latent heat fluxes during cumulus convection at the ARM Southern Great Plains (SGP) site in Oklahoma, United States. The statistical results show that latent heat fluxes increase with height from the surface up to ~0.8Zi (where Zi is the convective boundary layer depth) and then decrease to ~0 at Zi. Peak fluxes aloft exceeding 500 W m−2 are associated with periods of increased cumulus cloud cover and stronger jumps in the mean humidity profile. These entrainment fluxes are much larger than the surface fluxes, indicating substantial drying over the 0–0.8Zi layer accompanied by moistening aloft as the CBL deepens over the diurnal cycle. We also show that the boundary layer humidity budget is approximately closed by computing the flux divergence across the 0–0.8Zi layer. Composite subcloud velocity and water vapor anomalies show that clouds are linked to coherent updraft and moisture plumes. The moisture anomaly is Gaussian, most pronounced above 0.8Zi and systematically wider than the velocity anomaly, which has a narrow central updraft flanked by downdrafts. This size and shape disparity results in downdrafts characterized by a high water vapor mixing ratio and thus a broad joint probability density function (JPDF) of velocity and mixing ratio in the upper CBL. We also show that cloud-base latent heat fluxes can be both positive and negative and that the instantaneous positive fluxes can be very large (~10 000 W m−2). However, since cloud fraction tends to be small, the net impact of these fluxes remains modest.

A Thermal Plume Model for the Convective Boundary Layer: Representation of Cumulus Clouds

2008 ◽  
Vol 65 (2) ◽  
pp. 407-425 ◽  
Author(s):  
Catherine Rio ◽  
Frédéric Hourdin
Keyword(s):  
Boundary Layer ◽  
Mass Flux ◽  
Convective Boundary Layer ◽  
Turbulent Transport ◽  
Thermal Plume ◽  
Cumulus Clouds ◽  
Convective Boundary ◽  
Near Surface ◽  
Plume Model ◽  
Surface Drying

Abstract The “thermal plume model,” a mass-flux scheme combined with a classical diffusive approach, originally developed to represent turbulent transport in the dry convective boundary layer, is extended here to the representation of cloud processes. The modified parameterization is validated in a 1D configuration against results of large eddy simulations (LES), as well as in a 3D configuration against in situ measurements, for a series of cases of dry and cloudy convective boundary layers. Accounting for coherent structures of the mixed layer with the mass-flux scheme improves the representation of the diurnal cycle of the boundary layer, particularly its progressive deepening during the day and the associated near-surface drying. Results also underline the role of the prescription of the mixing of air between the plume and its environment, and of submean-plume fluctuations.

scholarly journals On the Summertime Planetary Boundary Layer with Different Thermodynamic Stability in China: A Radiosonde Perspective

2018 ◽  
Vol 31 (4) ◽  
pp. 1451-1465 ◽  
Author(s):  
Wanchun Zhang ◽  
Jianping Guo ◽  
Yucong Miao ◽  
Huan Liu ◽  
Yu Song ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Thermodynamic Stability ◽  
Ground Surface ◽  
Surface Air Temperature ◽  
Northwest China ◽  
Occurrence Frequency ◽  
Convective Boundary ◽  
Near Surface ◽  
Neutral Boundary Layer

Strongly influenced by thermodynamic stability, the planetary boundary layer (PBL) is key to the exchange of heat, momentum, and moisture between the ground surface and free troposphere. The PBL with different thermodynamic stability across the whole of China, however, is not yet well understood. In this study, the occurrence frequency and spatial distribution of the convective boundary layer (CBL), neutral boundary layer (NBL), and stable boundary layer (SBL) were systematically investigated, based on intensive summertime soundings launched at 1400 Beijing time (BJT) throughout China’s radiosonde network (CRN) for the period 2012 to 2016. Overall, the occurrences of CBL, NBL, and SBL account for 70%, 26%, and 4%, respectively, suggesting that CBL dominates in summer throughout China. In terms of the spatial pattern of PBL height, a prominent north–south gradient can be found with higher PBL height in northwest China. In addition, the PBL heights of CBL and NBL were found to be positively (negatively) associated with near-surface air temperature (humidity), whereas no apparent relationship was found for SBL. Furthermore, clouds tend to reduce the occurrence frequency, irrespective of PBL type. Roughly 70% of SBL cases occur under overcast conditions, much higher than those for NBL and CBL, indicating that clouds govern to some extent the occurrence of SBL. In contrast, except for the discernible changes in PBL height under overcast conditions relative to those under clear-sky conditions, the changes in PBL height under partly cloudy conditions are no more than 170 m for both NBL and CBL types.

scholarly journals Stochastic Backscatter for Cloud-Resolving Models. Part I: Implementation and Testing in a Dry Convective Boundary Layer

2008 ◽  
Vol 65 (1) ◽  
pp. 123-139 ◽  
Author(s):  
Sonja Weinbrecht ◽  
Paul J. Mason
Keyword(s):  
Boundary Layer ◽  
Free Convection ◽  
Convective Boundary Layer ◽  
Deep Convection ◽  
Time Development ◽  
Convective Boundary ◽  
Near Surface ◽  
Complex Issue ◽  
Marked Influence ◽  
Mesh Refinements

Abstract In simulations of deep convection with cloud-resolving models the turbulence is often rather poorly resolved, and the influence of the subfilter-scale parameterization used in such circumstances is probably greater than in better-resolved simulations. Therefore a study to investigate the influence of stochastic backscatter was performed and presented in two papers. This first paper focuses on a description of the stochastic backscatter model and its effect on a neutral and a dry convective boundary layer. The second paper then deals with two cases of deep convection. The dry convective boundary layer is typical of the subcloud layer in deep convection and this study allows for influences on this layer to be investigated separately. As a simple case of convection it also allows for general effects to be identified. The implementation of stochastic backscatter was improved to ensure an appropriate scale of backscatter that is independent of any mesh refinements and always spatially isotropic. It can also be applied in a deep atmosphere with use of the anelastic approximation. Generally the backscatter is found to affect the velocity and scalar variances as well as the spatial structure and time development of the simulations. An additional issue is the marked influence of both varying resolution and backscatter on the near-surface temperature differences. This is an element of the complex issue of surface transfers under free convection conditions. Overall, the investigations show that the backscatter gives some beneficial changes to the simulations, which tend to keep results in less well- resolved cases closer to those in better-resolved simulations.

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