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  • Author or Editor: Feng Xiao x
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Zhaolin Gu
,
Yongzhi Zhao
,
Yun Li
,
Yongzhang Yu
, and
Xiao Feng

Abstract

Based on an advanced dust devil–scale large-eddy simulation (LES) model, the atmosphere flow of a modeled dust devil in a quasi–steady state was first simulated to illustrate the characteristics of the gas phase field in the mature stage, including the prediction of the lower pressure and higher temperature in the vortex core. The dust-lifting physics is examined in two aspects. Through the experimental data analysis, it is verified again that the horizontal swirling wind can only make solid particles saltate along the ground surface. Based on a Lagrangian reference frame, the tracks of dust grains with different density (material) and diameter are calculated to show the effect of dust particles entrained by the vertical swirling wind field. The movement of solid particles depends on the interactions between the aloft dust particles and the airflow field of dust devils, in which the drag and the centrifugal force component on the horizontal plane are the key force components. There is the trend of the fine dust grains rising along the inner helical tracks while the large dust grains are lifting along the outer helical tracks and then descending beyond the corner region, resulting in the impact between different-sized dust grains in the swirling atmospheric flow. This trend will make the dust stratification, developing a top small-sized grain domain and a bottom large-sized grain domain in dust devils.

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Jingyi Chen
,
Samson Hagos
,
Zhe Feng
,
Jerome D. Fast
, and
Heng Xiao

Abstract

Some of the climate research puzzles relate to a limited understanding of the critical factors governing the life cycle of cumulus clouds. These factors force the initiation and the various mixing processes during cloud life cycles. To shed some light into these processes, we tracked the life cycle of thousands of individual shallow cumulus clouds in a large-eddy simulation during the Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems field campaign in the U.S. southern Great Plains. Concurrent evolution of clouds is tracked and their respective neighboring clouds are examined. Results show that the clouds initially smaller than neighboring clouds can grow larger than the neighboring clouds by a factor of 2 within 20% of their lifetime. Two groups of the tracked clouds with growing and decaying neighboring clouds, respectively, show distinct characteristics in their life cycles. Clouds with growing neighboring clouds form above regions with larger surface heterogeneity, whereas clouds with decaying neighboring clouds are associated with less heterogeneous surfaces. Also, those with decaying neighboring clouds experience larger instability and a more humid boundary layer, indicating evaporation below the cloud base is likely occurring before those clouds are formed. Larger instability leads to higher vertical velocity and convergence within the cloud, which causes stronger surrounding downdrafts and water vapor removal in the surrounding area. The latter appears to be the reason for the decaying neighboring clouds. Understanding those processes provide insights into how cloud–cloud interactions modulate the evolution of cloud population and into how this evolution can be represented in future cumulus parameterizations.

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Jingyi Chen
,
Samson Hagos
,
Heng Xiao
,
Jerome Fast
, and
Zhe Feng

Abstract

This study uses semi-idealized simulations to investigate multiscale processes induced by the heterogeneity of soil moisture observed during the 2016 Holistic Interactions of Shallow Clouds, Aerosols, and Land-Ecosystems (HI-SCALE) field campaign. The semi-idealized simulations have realistic land heterogeneity, but large-scale winds are removed. Analysis on isentropic coordinates enables the tracking of circulation that transports energy vertically and facilitates the identification of the primary convective processes induced by realistic land heterogeneity. The isentropes associated with upward motion are found to connect the ground characterized by high latent heat flux to cloud bases directly over the ground with high sensible heat flux, while isentropes associated with downward motion connect precipitation to the ground characterized by high sensible heat fluxes. The mixing of dry, warm parcels ascending from the ground with high sensible heat fluxes and moist parcels from high latent heat regions leads to cloud formation. This new mechanism explains how soil moisture heterogeneity provides the key ingredients such as buoyancy and moisture for shallow cloud formation. We also found that the submesoscale dominates upward energy transport in the boundary layer, while mesoscale circulations contribute to vertical energy transport above the boundary layer. Our novel method better illustrates and elucidates the nature of land atmospheric interactions under irregular and realistic soil moisture patterns.

Significance Statement

Models that resolve boundary layer turbulence and clouds have been used extensively to understand processes controlling land–atmosphere interactions, but many of their configurations and computational expense limit the use of variable land properties. This study aims to understand how heterogeneous land properties over multiple spatial scales affect energy redistribution by moist convection. Using a more realistic land representation and isentropic analyses, we found that high sensible heat flux regions are associated with relatively higher vertical velocity near the surface, and the high latent heat flux regions are associated with relatively higher moist energy. The mixing of parcels rising from these two regions results in the formation of shallow clouds.

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Xiangfeng Hu
,
Hao Huang
,
Haixia Xiao
,
Yi Cui
,
Feng Lv
,
Liwei Zhao
, and
Xueshuai Ji

Abstract

Microphysical structures and processes in a case of precipitating stratiform clouds in North China on 21 May 2018 are investigated using joint observations from an aircraft and an X-band polarimetric radar. The results show that there are enhancements in differential reflectivity (Z DR) and specific differential phase (K DP) above the 7-km altitude, consistent with the existence of dendrites and platelike ice crystals. The horizontal reflectivity factor (ZH ) increases and Z DR decreases downward above the melting layer (ML), due to the prevalent aggregation process, which is confirmed by the downward increasing volume-weighted mean diameter (Dm ) and decreasing total number concentration (Nt ) observed by the aircraft. Within the ML, the concentration of median-sized particles (2–5 mm) decreases rapidly downward due to the melting process. Within approximately the top 2/3 of the ML, the melting particles’ mean and maximum sizes increase, demonstrating the dominance of the aggregation process. This causes the enhancements of ZH and Z DR within the radar bright band together with the increase in the dielectric constant. Within the bottom 1/3 of the ML, the breakup process is responsible for the decreasing Dm and increasing Nt observed by the aircraft. Below the ML, the measurements by the polarimetric radar and the aircraft only show slight variance with altitude, indicating the near balance between microphysical processes favored by the nearly saturated air. The results of the microphysics in the stratiform case would help improve the microphysical parameterization of numerical modeling in the future.

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