Preprints
https://doi.org/10.5194/egusphere-2023-1494
https://doi.org/10.5194/egusphere-2023-1494
11 Sep 2023
 | 11 Sep 2023

Effects of longwave radiative cooling on advection fog over the Northwest Pacific Ocean: Observations and large eddy simulations

Liu Yang, Saisai Ding, Jing-Wu Liu, and Su-Ping Zhang

Abstract. During the boreal summer, the 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 %) to 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 the SST during an advection fog event. The LES model, incorporating constant solar radiation, successfully simulates the evolution of advection fog and the negative SAT-SST. Simulation results show that once the near-surface air condenses, the 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 hours after the fog formation, while the fog column is still positioned over the SST front. When the fog column arrives the cold flank of the SST front, the top-down developing mixed layer induced by the LWC reaches the surface, causing the SAT to drop below SST. The LES model with diurnal solar radiation well simulates the diurnal variation in 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.

Liu Yang, Saisai Ding, Jing-Wu Liu, and Su-Ping Zhang

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2023-1494', Hermann Gerber, 13 Sep 2023
  • RC1: 'Comment on egusphere-2023-1494', Anonymous Referee #1, 10 Oct 2023
  • RC2: 'Comment on egusphere-2023-1494', Mónica Zamora Zapata, 17 Oct 2023

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2023-1494', Hermann Gerber, 13 Sep 2023
  • RC1: 'Comment on egusphere-2023-1494', Anonymous Referee #1, 10 Oct 2023
  • RC2: 'Comment on egusphere-2023-1494', Mónica Zamora Zapata, 17 Oct 2023
Liu Yang, Saisai Ding, Jing-Wu Liu, and Su-Ping Zhang
Liu Yang, Saisai Ding, Jing-Wu Liu, and Su-Ping Zhang

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Short summary
Advection fog occurs when warm and moist air moves over a cold sea surface. In this situation, the temperature of the foggy air usually drops below the sea surface temperature (SST), particularly at night. High-resolution simulations show that the cooling effect of longwave radiation from the top of the fog layer permeates through the fog, resulting in a cooling of the surface air below SST. This study emphasizes the significance of monitoring air temperature to enhance sea fog forecasting.