Applying Anomaly-based Weather Analysis to the Prediction of Low Visibility Associated with the Coastal Fog at Ningbo-Zhoushan Port in East China

Weihong Qian,Jeremy Cheuk-Hin Leung,Siyuan Huang,Youli Chen

doi:10.1007/s00376-019-8252-5

Weihong Qian, Jeremy Cheuk-Hin Leung + Show 2 more

Open Access

https://doi.org/10.1007/s00376-019-8252-5

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Abstract

Low visibility episodes (visibility < 1000 m) were studied by applying the anomaly-based weather analysis method. A regional episode of low visibility associated with a coastal fog that occurred from 27 to 28 January 2016 over Ningbo-Zhoushan Port, Zhejiang Province, East China, was first examined. Some basic features from the anomalous weather analysis for this case were identified: (1) the process of low visibility mainly caused by coastal fog was a direct response to anomalous temperature inversion in the lower troposphere, with a warm center around the 925 hPa level, which was formed by a positive geopotential height (GPH) anomaly in the upper troposphere and a negative GPH anomaly near the surface; (2) the positive humidity anomaly was conducive to the formation of coastal fog and rain; (3) regional coastal fog formed at the moment when the southwesterly wind anomalies transferred to northeasterly wind anomalies. Other cases confirmed that the low visibility associated with coastal fog depends upon low-level inversion, a positive humidity anomaly, and a change of wind anomalies from southwesterly to northeasterly, rain and stratus cloud amount. The correlation coefficients of six-hourly inversion, 850–925-hPa-averaged temperature, GPH and humidity anomalies against visibility are −0.31, 0.40 and −0.48, respectively, reaching the 99% confidence level in the first half-years of 2015 and 2016. By applying the anomaly-based weather analysis method to medium-range model output products, such as ensemble prediction systems, the anomalous temperature-pressure pattern and humidity—wind pattern can be used to predict the process of low visibility associated with coastal fog at several days in advance.

Highlights

Article Highlights: Anomaly-based weather analysis is a useful method for extracting and visualizing the atmospheric conditions that induce coastal fog. Coastal fog mainly results from lower-tropospheric anomalous temperature inversion and specific humidity. Coastal fog can be predicted several days in advance by applying anomaly-based weather analysis to model products
We examine whether the anomaly-based weather analysis method can be applied to fog episodes with visibility less than 1 km for more than 24 hours
Comparing the four basic anomalous variables—geopotential height (GPH), temperature, humidity and wind anomalies—we find that the vertical patterns and evolutions of GPH−temperature and humidity anomalies are better than wind anomalies at indicating the fog formation and rain that resulted in low visibility

Summary

2.ĂĂĂĂDatasets and methods

Four datasets were used in this study. The first was the global atmospheric reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF), named ERA-Interim (Dee et al, 2011; http://apps.ecmwf.int/datasets/data/interimfull-daily/levtype=sfc/). The green and red lines indicate that the visibility of regions 3 and 4 sometimes reached 1 km from 27 to 28 January in 2016 This happened with the surface relative humidity higher than 95%−100%, the surface temperature at about 7°C−8°C, rain, and a change from southwesterly to northeasterly winds, based on the automatic station observations. A shear line of wind anomalies crossed the port, so there was a convergence center of southerly wind anomalies at the port These features observed from the anomalous weather analysis can be seen as some basic signals to predict the formation of fog, rain and low visibility from both a temporal and spatial perspective

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6.ĂĂĂĂConclusion and discussion