Transition from Fog to Stratus over the Northwest Pacific Ocean: Large-eddy Simulation

Liu, Yang; Liu, Jing-Wu; Xie, Shang‐Ping; Shen, Samuel S. P.

doi:10.1175/mwr-d-20-0420.1

Cited by 3 publications

(4 citation statements)

References 49 publications

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“…The positive feedback process makes the fog top another "cold surface" opposite to the colder sea surface and hence produces a peak value of LWC near the top of the fog layer (hereafter termed "fog-top LWC peak"). This is supported by previous numerical modeling with fine vertical resolution [3,19,30]. In particular, a large eddy simulation has revealed it in detail and identifies it as the result of longwave radiative cooling at the fog top [30].…”

Section: Introductionsupporting

confidence: 73%

Section: Introductionsupporting

confidence: 73%

“…This is supported by previous numerical modeling with fine vertical resolution [3,19,30]. In particular, a large eddy simulation has revealed it in detail and identifies it as the result of longwave radiative cooling at the fog top [30]. Besides the feedback process associated with LWC and longwave radiative cooling on the fog top, there is also another important feedback process.…”

Section: Introductionsupporting

confidence: 67%

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Overestimated Fog-Top Entrainment in WRF Simulation Leading to Unrealistic Dissipation of Sea Fog: A Case Study

Zhang,

Shi,

Gao

et al. 2024

Remote Sensing

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Entrainment at the top of the planetary boundary layer (PBL) is of significant importance because it controls the upward growth of the PBL height. An option called ysu_topdown_pblmix, which provides a parameterization of fog-top entrainment, has been proposed for valley fog modeling and introduced into the YSU (Yonsei University) PBL scheme in the Weather Research and Forecasting (WRF) model. However, enabling this option in simulations of sea fog over the Yellow Sea typically results in unrealistic dissipation near the fog bottom and even within the entire fog layer. In this study, we theoretically examine the composition of the option ysu_topdown_pblmix, and then argue that one term in this option might be redundant for sea-fog modeling. The fog-top variables are employed in this term to determine the basic entrainment in the dry PBL, which is already parameterized by the surface variables in the original YSU PBL scheme. This term likely leads to an overestimation of the fog-top entrainment rate, so we refer to it as redundant. To explore the connection between the redundant term and unrealistic dissipation, a widespread sea-fog episode over the Yellow Sea is employed as a case study based on the WRF model. The simulation results clearly attribute the unrealistic dissipation to the extra entrainment rate that the redundant term induces. Fog-top entrainment is unexpectedly overestimated due to this extra entrainment rate, resulting in a significantly drier and warmer bias within the interior of sea fog. When sea fog develops and reaches a temperature lower than the sea surface, the sea surface functions as a warming source to heat the fog bottom jointly with the downward heat flux brought by the fog-top entrainment, leading the dissipation to initially occur near the fog bottom and then gradually expand upwards. We suggest a straightforward method to modify the option ysu_topdown_pblmix for sea-fog modeling that eliminates the redundant term. The improvement effect of this method was supported by the results of sensitivity tests. However, more sea-fog cases are required to validate the modification method.

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Section: Introductionsupporting

confidence: 73%

Section: Introductionsupporting

confidence: 73%

Section: Introductionsupporting

confidence: 67%

See 1 more Smart Citation

Overestimated Fog-Top Entrainment in WRF Simulation Leading to Unrealistic Dissipation of Sea Fog: A Case Study

Zhang,

Shi,

Gao

et al. 2024

Remote Sensing

View full text Add to dashboard Cite

show abstract

“…According to the Clausius-Clapeyron equation, the change in RH (zRH/zt) is mainly controlled by specific humidity (q) and temperature (T). Therefore, to understand what induces zRH/zt variation, we quantitatively decompose zRH/zt to the contributions of q and T as (Yang et al, 2021) zRH zt z zt…”

Section: Contributions Of Specific Humidity (Q) and Temperature (T)mentioning

confidence: 99%

Springtime sea fog penetration in Qingdao: Anomalous moistening and diurnal cooling

et al. 2022

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Sea fog often penetrates adjacent coastal areas, a process called sea fog penetration (SFP). SFP can cause traffic accidents and other economic losses. Qingdao, an international port city with a dense population, suffers from SFP originating over the Yellow Sea in the boreal spring (March–May); the process, however, is not well-studied. Based on hourly observations from buoys and automatic weather stations distributed in Qingdao and its adjacent islands, we composite SFP events to reveal their spatiotemporal features and to investigate the mechanisms involved. Results show that these SFP events often penetrate inland areas from southeast to northwest and last 5–8 h at night. We further use reanalysis data to reveal that during the daytime before SFP, strong moisture advection at 925–975 hPa brings sufficient water vapor from the Yellow Sea to Qingdao; the water vapor then transfers downward to the surface via background descending motion and turbulent mixing. The daytime anomalous moistening, together with the following diurnal cooling at night, saturates the surface atmosphere and, hence, facilitates SFP. The strength of SFP depends on the strength of daytime anomalous moistening. Considering that moistening leads SFP by about a day, we use this relationship to predict the intensity of SFP. The accuracy of predicting SFP events could reach 50–80%, which highlights the predictability of intensity of SFP in Qingdao.

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Effects of radiative cooling on advection fog over the northwest Pacific Ocean: observations and large-eddy simulations

Yang,

Ding,

Liu

et al. 2024

Atmos. Chem. Phys.

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Abstract. During boreal summer, prevailing southerlies traverse the sharp sea surface temperature (SST) front in the northwest Pacific (NWP) Ocean, creating a stable air–sea interface characterized by surface air temperature (SAT) higher than SST, which promotes the frequent occurrence of advection fog. However, long-term shipborne observations reveal that during episodes of advection fog, SAT usually decreases below SST, with a peak relative frequency (∼ 34.5 %) in all fog observations before sunrise and a minimum relative frequency (∼ 18.8 %) before sunset. From a Lagrangian perspective, this study employs a turbulence-closure large-eddy simulation (LES) model to trace a fog column across the SST front and investigates how SAT drops below SST during an advection fog event. The LES model, incorporating constant solar radiation, successfully simulates the evolution of advection fog and the negative difference between SAT and SST. Simulation results show that once the near-surface air condenses, thermal turbulence is generated by strong longwave radiation cooling (LWC) at the fog top. The influence of LWC on the fog layer surpasses the cooling effect of the near-surface mechanical turbulence ∼ 2 h after the fog formation while the fog column is still positioned over the SST front. When the fog column arrives at the cold flank of the SST front, the top-down-developing mixed layer induced by the LWC reaches the surface, causing SAT to drop below SST. The LES model with diurnal solar radiation successfully simulates the observational diurnal variation in SAT and SST (SAT-SST) during the fog event, suggesting that the model captures the essential processes responsible for negative SAT-SST. This study highlights the significance of fog-top cooling and its associated thermal turbulence in the evolution of advection fog. Given the challenges faced by numerical weather prediction models in forecasting sea fog, our findings suggest that observations of negative SAT-SST during advection fog episodes present an opportunity to enhance the performance of these models in simulating the thermal turbulence induced by the LWC at the fog top.

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Transition from Fog to Stratus over the Northwest Pacific Ocean: Large-eddy Simulation

Cited by 3 publications

References 49 publications

Overestimated Fog-Top Entrainment in WRF Simulation Leading to Unrealistic Dissipation of Sea Fog: A Case Study

Overestimated Fog-Top Entrainment in WRF Simulation Leading to Unrealistic Dissipation of Sea Fog: A Case Study

Springtime sea fog penetration in Qingdao: Anomalous moistening and diurnal cooling

Effects of radiative cooling on advection fog over the northwest Pacific Ocean: observations and large-eddy simulations

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