The extended 3DVAR assimilation method is then applied to 10 more sea fog cases to further evaluate its effect on the model simulations. For the narrowly spread coastal case, the model completely fails to reproduce the sea fog event without the assimilation of MTSAT-derived humidity. The improvement is attributed to a more realistic representation of the marine boundary layer (MBL) and better descriptions of moisture and temperature profiles. For the widespread-fog case, the assimilation of MTSAT-derived information significantly improves the forecast of the sea fog area, increasing the probability of detection and equitable threat scores by about 20% and 15%, respectively. Two sea fog cases, one spread widely over the Yellow Sea and the other spread narrowly along the coast, are first studied in detail with a suite of experiments. Assuming a relative humidity of 100% in fog, the MTSAT-derived humidity is assimilated by the extended 3DVAR assimilation method. The sea fog properties, including its horizontal distribution and thickness, are retrieved empirically from the infrared and visible cloud imageries of the Multifunctional Transport Satellite (MTSAT). Strong winds- these can cause turbulence within the fog and disperse the water droplets.An extended three-dimensional variational data assimilation (3DVAR) method based on the Weather Research and Forecasting Model (WRF) is developed to assimilate satellite-derived humidity from sea fog at its initial stage over the Yellow Sea. This is often called ‘burning off the fog’.Ģ. This effect spreads up through the fog and the fog dissipates. Morning sunrise- the sun’s rays heat the ground beneath the fog and the water droplets evaporate to become water vapour. The photo shown was taken in the Canadian province of Quebec.ġ. Because of the convection, the water appears to ‘steam’. This convection causes the mixed air to rise a metre or more, thus enhancing the process of fog formation. However, convection also occurs, because the water is so much warmer than the overlying air. What is the science behind this type of fog?Ĭondensation results mainly from the cold air mixing with the air that is in contact with the water surface. Over sea water, steam fog is called sea smoke. This kind of fog forms when cold air flows over water that is more than 9° or 10☌ warmer than the air. The fog then dissipates (disappears), often very quickly. Most of the rays are actually reflected from the top of the fog but some reach the surface, otherwise it would not be daylight in the fog! The ground is gradually heated until the dew-point temperature is exceeded. During the day, the sun’s rays heat the ground beneath the fog. This type of fog does not form over the sea because the temperature of the sea’s surface stays much the same day and night. If there is a very gentle breeze, the tiny water droplets are stirred upwards to form a shallow layer of radiation fog, as in the picture of fog at Cardiff shown. If there is no wind, droplets of dew form on, for example, grass. If the air in contact with a surface is cooled to its dew-point temperature, small water droplets form (condensation). When the sky is clear at night, land surfaces radiate heat to space and therefore cool.Sea and lake surfaces do not, however, cool by more than a small amount overnight (much less than 1☌). What is the science behind this type of fog? Radiation fog is particularly common in autumn and winter in the UK. It commonly forms in the dips with sources of moisture such as streams and rivers. In mid-winter, however, particularly in latitudes where the sun is low in the sky (e.g. This type of fog usually forms at night and dissipates (disappears) during the day. This kind of fog forms when the sky is clear and the wind speeds are low (1-5 kilometres per hour). In fog the distance you can see (known as visibility) is less than 1km, but in mist the visibility distance can be 1km-2km. The density of fog and mist are different. Fogs seen over the sea are different to those found in valley lowland areas and over mountains for example.ĭownload fog fact file for printing, or have a look at our Experiments and Demonstrations page for experiments which demonstrate how clouds can look dark from below but white from above, or how to make a hygrometer to measure air humidity.For a deeper understanding of how and where clouds form, have a look at our exercise using height/ temperature graphs to investigate atmospheric stability, lapse rates and cloud formation with a worksheet for students with an introductory PowerPoint or this paper. It can cover vast areas and comes in a variety of types, just like clouds do.
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