The main purpose of this work is to describe a major field project on fog and summarize the preliminary results. Three field phases of the Fog Remote Sensing and Modeling (FRAM) project were conducted over the following two regions of Canada: 1) the Center for Atmospheric Research Experiments (CARE), in Toronto, Ontario (FRAM-C), during the winter of 2005/06, and 2) Lunenburg, Nova Scotia (FRAM-L), during June 2006 and June 2007. Fog conditions observed during FRAM-C were continental in nature, while those conditions observed during FRAM-L were of marine origin. The main objectives of the project were to attain 1) a better description of fog environments, 2) the development of microphysical parameterizations for model applications, 3) the development of remote sensing methods for fog nowcasting/forecasting, 4) an understanding of issues related to instrument capabilities and improvement of the analysis, and 5) an integration of model data with observations to predict and detect fog areas and particle phas...

The Fog Remote Sensing and Modeling Field Project

Radar in Meteorology

A review on ice fog measurements and modeling

In order to study the small-scale structure of radition fog, large-eddy simulations (LESs) of a fog case are analysed. The LESs were performed at very high resolution –2 m in the horizontal and 1 m in the vertical. Despite uncertainties in the measurements, particularly for advection, the main characteristics of the fog layer were well captured by the model. Radiation fog forms in statically stable stratification near the ground. During the formation phase, small stripes occur in the middle of the fog layer, associated with a significant burst in the turbulent kinetic energy (TKE). During the development phase, the dynamics of the fog layer change significantly. The maximum of the variance moves to the top of the fog layer where horizontal rolls appear clearly. These eddies have their centre near the mean top of the fog layer and have a depth corresponding to about one third of the fog layer height. This leads to a maximum of TKE at the top of fog, and to very strong scatter on the liquid water content. The energy is clearly produced at a length-scale corresponding to the fog height. The turbulence is 3D homogeneous inside the fog layer, while it is better characterized by 2D turbulence near the top. During the dissipation phase, the radiative heating of the surface increases the convective structure of the fog. The dissipation of fog at ground level takes a long time (about 2 h), even if the soil conditions are homogeneous. The top of the stratus layer is homogeneous, while the spread of the base height reaches a value typical of the cloud thickness. Copyright © 2012 Royal Meteorological Society

Small‐scale structure of radiation fog: a large‐eddy simulation study

Large-eddy simulations (LES) of radiation fog were performed over an airport area to study the effect of urban canopy on fog. These LES were performed with the Meso-NH research model at very high resolution: 1.5 m in the horizontal and 1 m in the vertical and over a domain 4.5 km × 1.5 km. The blocking effect of the airport buildings led to strong wind shear and consequently to the production of turbulent kinetic energy (TKE). The airport buildings also had a strong effect on vertical velocity, with a subsidence region behind the buildings. The increase of both turbulence and vertical velocity strongly modified the fog formation. The fog layer took more time to form in the airport area, but the increase in turbulence facilitated the vertical development of the fog layer. The fog took 1.5 h to form over the whole simulated airport area. The fog height was heterogeneous during the formation phase, with the formation of very low clouds locally. The effect of airport buildings on vertical velocity could explain these heterogeneities of the fog height. During the mature phase of the fog, the buildings had little impact on the fog layer characteristics. The fog dynamics were mainly controlled by processes at its top. These results suggest that the inclusion of high levels of detail in the building representation remains important for the local forecasting of fog formation. Particularly, small-scale heterogeneities can explain the spatial variability of fog formation. It seems necessary to take small-scale variability of the urban canopy into account for local and accurate forecasts of fog formation over airport areas.

Effect of small-scale surface heterogeneities and buildings on radiation fog: Large-eddy simulation study at Paris-Charles de Gaulle airport

Observations of fogs with a millimeter-wave scanning Doppler radar were conducted at Kushiro in Hokkaido, Japan, in the summer seasons of 1999 and 2000. Three typical types of plan position indicator (PPI) displays were observed: cellular echoes with high radar reflectivity factors (∼−10 dBZ), uniformly distributed echoes with high reflectivities (∼−10 dBZ), and uniformly distributed echoes with low reflectivities (∼−30 dBZ). The authors focused on advection fog with cellular echoes observed on 5 August 1999 and 31 July 2000. Echoes showed structures of cells with a reflectivity of −10 dBZ and with intervals of about 1 km. This echo pattern moved northward (i.e., from the sea to the land). There was a vertical shear of the horizontal wind at a height around 200 m in both cases, and structures of each cell were upright above the shear line and were leaning below it. The direction and the speed of the echo pattern in both PPI and range–height indicator (RHI) displays agreed well with that of the ho...

Moving Cellular Structure of Fog Echoes Obtained with a Millimeter-Wave Scanning Doppler Radar at Kushiro, Japan

[1] Three-dimensional fog roll structure induced by Kelvin-Helmholtz instability (KHI) is studied, based on observations made with a millimeter-wave scanning Doppler radar and a rawinsonde. Roll structure in a fog layer was observed below 150 m altitude when extremely large vertical shear (exceeding 100 m s−1 km−1) existed at 50–60 m altitude. The direction of the shear was perpendicular to the roll direction. The Richardson number was less than 0.25 at the shear altitude, which indicates that the fog roll structure was induced by KHI.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005GL022423

Shear-induced roll structure of fog observed by a millimeter-wave scanning Doppler radar

A special fog observation campaign was conducted in the Miyoshi basin, Hiroshima prefecture, Japan during the period November 7–15, 2000. We observed the spatial distributions of fogs and their movements using a millimeterwave scanning radar. This is the first time that the distribution of basin fogs associated with fog development and decay processes has been examined. Echo intensity observed with the radar, which is mainly associated with fog particle size, was almost under −23 dBZ at levels below 200 m in height. The horizontal distribution of echo intensity changed with time. Namely, weak echoes were observed over nearly all observation areas at first, and then the echoes gradually became stronger as the fogs developed, although the echoes were weaker at higher levels. After sunrise, the echoes decayed. During the developing periods, the occurrence ratio of the echo intensity between −38 and −23 dBZ increased from the lower height, while the ratio decreased from the higher levels during the decay periods. This feature in the developing period is consistent with the results of optical measurements but the feature in the decaying period is inconsistent. It is suggested that this inconsistency is due to the difference in sensitivity between the two measurement approaches.

https://link.springer.com/content/pdf/10.1186%2FBF03353408.pdf

Fog observations with a millimeter-wave scanning radar at Miyoshi basin, Japan

[1] Multiscale band structure in fog influenced by gravity waves and Kelvin-Helmholtz instability (KHI) is studied based on observations with a millimeter-wave scanning Doppler radar on 1 August 2000 in Kushiro District, Hokkaido Prefecture, Japan. Band structure in fog having radar reflectivity >−20 dBZ and a scale of ∼1.5 km was observed around 50 m altitude. The band structure was also observed in Doppler velocity. The band structure in both radar reflectivity and Doppler velocity propagated northwestward with a speed of 4.17 m s−1. From linear gravity wave theory it is estimated that the observed gravity waves had a horizontal wavelength of 1.5 km and period of 6 min. The updraft induced by gravity waves is estimated to be 7.8 cm s−1. The increase in liquid water content (LWC) caused by the updraft is 0.035 g m−1, and the increase of radar reflectivity is 2 dBZ. Within the band structure induced by the gravity waves (horizontal scale ∼1.5 km), roll structure with a smaller horizontal scale (∼300–600 m) was also observed in radar reflectivity. A previous study has shown that this smaller-scale roll structure is caused by shear-induced KHI. This study shows that both gravity waves and KHI modulated the structure of fog and resulted in the existence of multiscale (1.5 km and 300–600 m scale) fog structure.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005JD006648

Akihisa Uematsu

Papers

Moving Cellular Structure of Fog Echoes Obtained with a Millimeter-Wave Scanning Doppler Radar at Kushiro, Japan

Shear-induced roll structure of fog observed by a millimeter-wave scanning Doppler radar

Fog observations with a millimeter-wave scanning radar at Miyoshi basin, Japan

Influence of gravity waves on fog structure revealed by a millimeter‐wave scanning Doppler radar