About the NCEP MMAB Global Visibility System
Revised April 2, 2004; last reviewed July 2, 2013.
INTRODUCTIONLowered visibilities affect all forms of transportation to one degree or other. Many incidents of multi-car accidents have occurred in dense fog; while aviation interests may be severely affected by dense fog or heavy snow. A large percentage of the accidents at sea occur with visibilities under one kilometer (Tremant 1987). This branch is principly interested in the marine aspects of lowered visibilities, but since we are using model output from the GFS, we have developed a global product that can be used by all transportation interests.
Although other obstructions may lower visibility below 1 km at sea, the most prevalent obstruction is fog. Dense fog is only reported if the visibility is 1 km or less. Until 1989 there was no objective guidance available to National Weather Service marine forecasters for fog or lowered visibilities. The NCEP MMAB Open Ocean Statistical Fog and Visibility Forecast System was implemented for this purpose. The system provides fog and visibility guidance over the North Pacific and North Atlantic during the prime season for advection fog and lowered visibilities (April - September). The guidance is not applicable to coastal areas. Further, output from the system is available from 00- through 72-h at 12-h intervals and is run at 0000 and 1200 UTC. This is inadequate for the needs of today's NWS Marine Forecasters.
The new system makes use of the cloud model imbedded in the GFS and an algorithm developed by Stoelinga and Warner (1999). The inputs from the GFS include the 2 m air temperature, the 2 m relative humidity, the surface pressure, the synoptic and convective precipitation rates, the precipitable water, the cloud mixing ratio at 1000 hPa, categorical rain, categorical freezing rain, categorical ice pellets, and categorical snow. From these variables the following parameters are computed: local precipitable water, water vapor mixing ratio, and precipitation mixing ratio. These are used with the categorical precipitation variables to determine the extiguishing coefficients for the various precipitation types. The extinguishing coefficients are combined linearly and used to determine the visibility in meters. In addition the sea ice extent and the land/sea mask is used to place constraints on the product and make it more realistic. We have also replaced the Reynolds SST analysis with the RTG_SST analysis which is produced daily and will give a more realistic SST along the east coasts of the continents.
DEVELOPMENTThe Mixing ratio, qv, is computed from the air temperature and relative humidity at a height of 2 m above the surface and the surface pressure. If the air temperature is below freezing, the mixing ratio is computed with respect to ice rather than liquid water. This cuts out most of the spurious lowered visibilities over frozen surfaces especially in the winter. The local precipitable water is computed by integrating the mixing ratio over a one hPa thick layer. The precipitation mixing ratio, qr, is computed by taking the synoptic and convective precipitation rates, combining them, multiplying by one second and dividing the result by the local precipitable water, pw. This gives the precipitation mixing ratio in kg/kg. The cloud water mixing ratio, qc, is taken from the 1000 mb surface layer. Eventually, it will be taken from the lowest sigma layer in the model, but currently that is not available as an output from the model.
Over the ocean, the daily RTG_SST has been substituted for the Reynold's climatological SST. A saturation mixing ratio is computed for the water temperature and compared with the mixing ratio of the air at 2 meters. If the mixing ratio of the air is greater than the saturation mixing ratio at the ocean surface, then the difference (qc2) is compared with qc. If qc2 is greater than qc, qc2 is used in the visibility determination and the relative humidity is raised to 100 percent. To determine the visibility, the categorical precipitation parameters are determined to be on or off. Then the mass concentrations of the precipitation type and the cloud liquid water mixing ratio or the cloud ice mixing ratio are computed. The extinction coefficients for the cloud water and the precipitation types are computed and combined, and finally, the visibility is computed. For marine purposes the maximum visibility is limited to 20,000 m. 13,000 m and over is considered unlimited. 13,000 m (7 n mi) is generally accepted as the maximum distance to the horizon from the bridge of a ship.
Without further constraints, the visual range reflects the precipitation shield associated with convective and non-convective systems and the strength of the rainfall predicted, rather than the visual range. To present a more realistic visual range over land and water, constraints were placed on the visual range according to the humidity over the land and water.
Over land a relative humidity at the two meter level of 95 percent or less over land resulted in an unrestricted visual range, relative humidities greater than 95, but less than 99 percent were adjusted incrementally until no change was made at 100 percent. Over water the humdity range begins at 85 percent and is changed incrementally upto 98 percent and above that no change is made.
Where ice is detected, precip is not allowed, and where the surface temperature is below freezing, mixing ratio is computed over ice, rather than water. Also, where relative humidity is zero, no precipitation is allowed. Over Antarctica the GFS often has precipitation falling into areas where the 2 meter relative humidity is zero. Our ice expert, Robert Grumbine, has said this not too realistic (private communication, 2003).
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AVAILABILITYThese fields are available as global or regional gif files or as global GRIB files. The gif files are available for both animations and stills for various projection hours four times a day at 0000, 0600, 1200, and 1800 GMT. The GRIB files have projections from 03- through 168-h at 3-h intervals.
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