Impacts of surface heterogeneity on dry planetary boundary layers in an urban‐rural setting

Zhu, Xiaoliang; Ni, Guangheng; Cong, Zhen; Sun, Ting; Li, Dan

doi:10.1002/2016jd024982

Cited by 16 publications

(21 citation statements)

References 51 publications

(71 reference statements)

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“…The higher sensible heat flux over urban areas can result in urban-rural circulations and stronger vertical motions over urban areas (not shown here but see e.g. Kang and Lenschow (2014), Zhang et al (2014a) and Zhu et al (2016)), which facilitate cloud formation and moist convection. Figure 6 shows the vertical profiles of potential temperature, relative humidity, and cloud water mixing ratio of case CTL and case URB.…”

Section: Impacts Of the Existence Of An Urban Islandmentioning

confidence: 86%

“…It has proven to be a useful simulation tool to investigate convective PBL characteristics under heterogeneous heating conditions (Liu et al, 2011;Kang and Lenschow, 2014). In a previous study, we have made some necessary modifications in the WRF Version 3.5.1 to take into account the urban features (Zhu et al, 2016).…”

Section: Model Description and Configurationmentioning

confidence: 99%

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An idealized LES study of urban modification of moist convection

Zhu

Zhou

et al. 2017

Quart J Royal Meteoro Soc

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To understand the impacts of urbanization on moist convection, we explore how an idealized circular urban island affects the diurnal cycle and spatial distribution of rainfall over urban and surrounding rural areas at the diurnal equilibrium state using large‐eddy simulations (LES) performed with the Weather Research and Forecasting (WRF) model. Compared to the control case where the whole domain is covered by grassland, the existence of an urban island significantly enhances the rainfall rate over the urban area as the stronger surface heating creates convergence zones and stronger vertical motions over the urban area. A suite of experiments is then conducted to investigate the effects of soil moisture of the surrounding rural land and the urban size on precipitation. Results show that as the rural soil moisture increases, both urban and rural precipitation rates increase almost linearly. This increase is not attributed to the urban heat island (UHI) effect but rather a stronger moisture deficit effect in the urban area creating a stronger moisture inflow. When the urban area becomes larger but the initial available water remains the same in the domain, the UHI effect and moisture deficit effect increase but the total water supply decreases. As a result, the urban rainfall rate increases first and then decreases as the urban size increases. This suggests that there is an ‘optimal’ scale at which the urban rainfall rate is maximized, at least in our modelling framework. Our simulations further suggest that this optimal scale occurs when the urban fraction lies between 1 and 10%.

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Section: Impacts Of the Existence Of An Urban Islandmentioning

confidence: 86%

Section: Model Description and Configurationmentioning

confidence: 99%

An idealized LES study of urban modification of moist convection

Zhu

Zhou

et al. 2017

Quart J Royal Meteoro Soc

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“…A similar approach is often adopted in LES studies (e.g. Vihma and others, 2011; Schalkwijk and others, 2013), particularly those in which large-scale effects are disregarded (Darbieu and others, 2015) and when the impact of small-scale heterogeneities is analyzed in the context of larger-scale averages over short periods of time (Zhu and others, 2016). Nevertheless, the simulations initialized with strong wind have to be treated with caution, since in the real atmosphere, high winds are always associated with high larger-scale pressure gradients not prescribed in the present study.…”

Section: Model Configurationmentioning

confidence: 99%

The influence of the spatial distribution of leads and ice floes on the atmospheric boundary layer over fragmented sea ice

Wenta¹,

Herman²

2018

Ann. Glaciol.

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The response of the atmospheric boundary layer (ABL) to subgrid-scale variations of sea ice properties and fracturing is poorly understood and not taken into account in mesoscale Numerical Weather Prediction (NWP) model parametrizations. In this paper we analyze three-dimensional air circulation within the ABL over fragmented sea ice. A series of idealized high-resolution simulations with the Weather Research and Forecasting (WRF) model is performed for several spatial distributions of ice floes and leads for two values of sea ice concentration (0.5 and 0.9) and several ambient wind speed profiles. The results show that the convective circulation within the ABL is sensitive to the subgrid-scale spatial distribution of sea ice. Considerable variability of several domain-averaged quantities – cloud liquid water content, surface turbulent heat flux (THF) – is found for different arrangements of floes. Moreover, the organized structure of air circulation leads to spatial covariance of variables characterizing the ABL. Based on the example of THF, it is demonstrated that this covariance may lead to substantial errors when THF values are estimated from area-averaged quantities, as it is done in mesoscale NWP models. This suggests the need for developing suitable parametrizations of ABL effects related to subgrid-scale sea ice features for these models.

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“…We used the Weather Research and Forecasting (WRF) model, version 3.9, which has the LES capability and has been widely used to investigate convective boundary layers (CBLs) over homogeneous and heterogeneous surfaces (Moeng et al, ; Talbot et al, ; Zhu et al, ). Following Zhu et al (), the original WRF‐LES model was modified so that the surface temperature, instead of the surface heat flux, can be prescribed as the surface boundary condition. We choose to do this because some recent studies (Basu et al, ; Holtslag et al, ) suggest that prescribing surface temperature is a better option than prescribing sensible heat flux under stable conditions.…”

Section: Experimental Designsmentioning

confidence: 99%

The Effects of Surface Heterogeneity Scale on the Flux Imbalance under Free Convection

Zhou

2019

JGR Atmospheres

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It is well known that the available energy (i.e., the net radiation minus the ground heat flux) is often 10-30% larger than the sum of turbulent fluxes measured by the eddy-covariance method. Although field observations and previous large-eddy simulation studies have shown that surface heterogeneity can induce flux imbalance, the relationship between the flux imbalance magnitude and the surface heterogeneity scale remains to be investigated in more detail. Here we examine the flux imbalance over landscapes characterized by different surface heterogeneity scales in a dry freely convective boundary layer. We reveal that the flux imbalance initially increases with increasing surface heterogeneity scale. However, when the surface heterogeneity scale becomes larger than the boundary layer height, the surface starts to behave locally homogeneous, which leads to a lower flux imbalance. Based on large-eddy simulation results, we propose a conceptual model to explain how the domain average flux imbalance is influenced by surface heterogeneity. The flux imbalance is found to be controlled by the ratio of the boundary layer height to the Obukhov length (−z i /L), the integral length scale of vertical velocity (l w ), the mean horizontal speed (U), and the time averaging interval (T). Among these four variables, l w determines the size of turbulent coherent structures (i.e., large eddies), whereas −z i /L affects the form of these large eddies. Meanwhile, the U and T determine how many these large eddies can be sampled by the eddy covariance. This finding indicates that it may be possible to diagnose the flux imbalance using these four variables under convective conditions. Key Points: • The relation between flux imbalance and surface heterogeneity scale is investigated using large-eddy simulations and a cospectral model • A diagnostic equation for flux imbalance is proposed • The qualitative relations between flux imbalance and various factors reported in the literature can be explained by this equation

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Impacts of surface heterogeneity on dry planetary boundary layers in an urban‐rural setting

Cited by 16 publications

References 51 publications

An idealized LES study of urban modification of moist convection

An idealized LES study of urban modification of moist convection

The influence of the spatial distribution of leads and ice floes on the atmospheric boundary layer over fragmented sea ice

The Effects of Surface Heterogeneity Scale on the Flux Imbalance under Free Convection

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