Geoscience Reference
In-Depth Information
MOS may be applied to any model and aims to
objectively interpolate gridded model output to a
single station based on its climate and weather
history. Multiple regression equations are devel-
oped which relate the actual weather observed at
a station over the course of time with the
conditions predicted by the model. With a long
enough history, MOS can make a correction for
local effects not simulated in the model and
for certain model biases. MOS variables include
daily maximum/minimum temperature, 12-hour
probability of precipitation occurrences and
precipitation amount, probability of frozen
precipitation, thunderstorm occurrence, cloud
cover and surface winds.
Various types of specialty forecasts are also
regularly made. In the United States, the National
Hurricane Center in Miami is responsible for
issuing forecast as to hurricane intensity changes
and the track the storm will follow in the Atlantic
and eastern Pacific areas. Forecasts are issued for
72 hours in advance four times daily. The central
Pacific Hurricane Center performs similar fore-
casts for storms west of 140
°
W and east of the
dateline. The US Weather Service also uses
numerical models to predict the evolution of El
Niño-Southern Oscillation which is important
for long-range forecasts discussed below. Special
events, such as the Olympic Games, are beginning
to regularly employ numerical weather forecasting
in their preparations and to use regional models
designed to be most accurate at the single point of
interest. MOS techniques are also used to improve
these very specialized forecasts.
precipitation. Mesoscale models with grid cells
which can be less than 10km on a side are regularly
used to study such phenomena in detail. The
development of radar networks (
Plate 7.2
), new
instruments and high-speed communication links
has provided a means of issuing warnings of severe
weather within the next hour. Several countries
have recently developed integrated satellite and
radar systems to provide information on the
horizontal and vertical extent of thunderstorms,
for example. Networks of automatic weather
stations (including buoys) that measure wind,
temperature and humidity supplement such data.
In addition, for detailed boundary layer and lower
troposphere data, there is now an array of vertical
sounders. These include: acoustic sounders
(measuring wind speed and direction from
echoes created by thermal eddies), and specialized
(Doppler) radar measuring winds in clear air by
returns either from insects (3.5cm wavelength
radar) or from variations in the air's refractive
index (10cm wavelength radar).
Nowcasting
techniques use highly automated computers and
image-analysis systems to integrate data from a
variety of sources rapidly. Interpretation of
the data displays requires skilled personnel
and/or extensive software to provide appropriate
information. The prompt warning of wind shear
and downburst hazards at airports is one example
of the importance of nowcasting procedures.
Overall, the greatest benefits from improved
forecasting can be expected in aviation and the
electric power industry for forecasts less than six
hours ahead, in transportation, construction and
manufacturing for 12- to 24-hour forecasts and in
agriculture for two- to five-day forecasts. In terms
of economic losses, the last category could benefit
the most from more reliable and more precise
forecasts.
2 Nowcasting
Severe weather is typically short-lived (<2 hours)
and, due to its mesoscale character (<100km), it
affects local/regional areas, necessitating site-
specific forecasts. Included in this category
are thunderstorms, flash floods, gust fronts,
tornadoes, high winds especially along coasts, over
lakes and mountains, heavy snow and freezing
3
Long-range outlooks
The atmosphere-ocean system is a non-linear
(chaotic) system making exact long-term predic-
