We describe and display the results from the
Global Forecast System (GFS) model of the U.S. National Center for Environmental
Prediction (NCEP). This guide is not meant to be a thorough and complete
description, but to give an overview of the forecasts and their presentation.
The view for the forecasts and analyses in the tropics
consists of two overlaping longitudinal bands between 30ēN-30ēS, one centered
on the Pacific basin, and the other centered over Africa. There is an
analysis, and a series of forecast maps. The forecasts are presented at
12 hour intervals out to three three days, then at 24 hour intervals to
five days. There are five different panels for each period. The
contents and meaning of the five panels is described below in detail.
The analyses represent the initial state for the integration
of the various forecast models. The analyses are produced from observations
at weather stations around the world, as well as ship and buoy reports
at sea, reports from aircraft, radiosonde balloons, and even satellite
data. These data are merged after quality control procedures have been
applied. Even with all of the data sources, there are still tremendous
gaps in coverage over remote areas. An optimal interpolation (OI) procedure
is performed using the previous model forecasts to fill these gaps and
create a complete picture of the state of the atmosphere at the forecast
time T=0. The model is then integrated forward in time to produce the
forecasts which are displayed here.
At the bottom of each map is a bar telling the date
and time for which the analysis or forecast is valid, the number of hours
after the analysis for which the forecast is valid, the fields displayed,
and their units. The five types of maps are described below.
Panels 1 and 2
Winds at 850mb and 200mb
- Winds at 850mb (about 1 mile or 1.5 km above sea
level) and 200mb (over 10 km above sea level) are shown.
- Purple shading indicates the speed of the winds
at that level, in meters per second.
- The streamlines indicate the direction of flow of
the wind, which is generally from west to east in the subtropics, especially
aloft, and from east to west throughout much of the tropics.
- The color of the streamlines indicates a relative
measure of horizontal divergence of the flow. Orange and red indicates
strong divergence, and blue/purple indicate strong convergence. Low-level
convergence with divergence aloft at the same location is usually associated
with strong vertical velocities in the middle troposphere, and severe
weather/heavy rainfall.
Panel 3
Vertical Velocity and Precipitation
- The colored contours in the analysis map indicate
vertical velocity of the wind at the 700 millibar level, in millibars
per hour (since pressure decreases with height, negative values indicate
ascending air, and positive values denote sinking).
- Ascending motion is associated with cloudiness
and rain. Large negative values of vertical velocity correspond
to areas of heavy rainfall if moisture is available. These areas
tend to correspond with the storms in the first two panels.
- The green shading in the forecasts indicate
12 or 24 hour accumulated precipitation, measured in millimeters.
- The total is the amount of rainfall forecast
during the 12 or 24 hours immediately preceding the verification
time in the lower lefthand corner of the map.
- Notice the nearly continuous band of rainfall
around the globe near the equator. That is the Inter-Tropical Convergence
Zone (ITCZ), and is a region where the Trade Winds of both hemispheres
tend to converge. This band will move north and south during the
course of the year, tracking the seasonal cycle of the sun, but
lagging behind by several months (especially over ocean).
- Also prevalent on the annual time scale are
the summertime monsoon rains over India, Southeast Asia, northern
Austalia, western Mexico, and subtropical South America. There are
also strong seasonal rainfall patterns over much of Central Africa.
Panel 4
Temperature of the Sea Surface, and 2 Meter Air
Temperature
- Over the ocean, shading and blue contours
indicate the sea surface temperature (SST), in degrees Celsius.
- The contour interval is 2ēC up to 20ēC, and
1ēC thereafter.
- Sea surface temperatures above 26ēC are shaded
in tones of red.
- Notice the strong precipitation of the ITCZ
and the South Pacific Convergence Zone (SPCZ) tends to align with
the warmest SST, and avoid nearby cooler waters.
- The shading and green contours over land indicates
the air temperature 2m above the land surface.
- The contour interval is 2ēC.
- Air temperatures above 30ēC are shaded in yellow,
orange and brown.
- Air temperatures below 20ēC are shaded in blue
and purple.
- Over monsoon areas a pronounced warming can
be seen in the months preceeding the rainy season. Once the rains
come, the temperatures over land cool markedly. Also notice the
strong daily temperature cycle over desert areas (especially North
Africa and Arabia) by comparing 00Z and 12Z maps.
Panel 5
Cloud Cover and Precipitable Water
- The colored contours indicate total precipitable
water in the atmosphere. Precipitable water is the total depth of liquid
water that would result if all water vapor contained in a vertical column
of air could be "wrung out", leaving the air completely dry. It indicates
the total humidity of the air above a location, and is a good indicator
of the amount of moisture potentially available to supply rainfall.
The contour interval is 10 mm up to 30 mm, and 5 mm thereafter.
- The purple-grey shading indicates the forecast
cloud cover fraction as computed for each model grid box. The fraction
includes low, medium and high clouds. Only coverage above 50% is shaded,
with lighter colors indicating more complete coverage by cloud.
- There are two regimes of cloudiness apparent
in these maps. One is the convective cloudiness that corresponds
with high precipitable water amounts and precipitation rates (see
panel 3) in the ITCZ and monsoon areas. Second is the dense, low
cloudiness found in the otherwise dry areas over the cool oceans
to the west of the continents in the subtropics (e.g. off the coasts
of Chile, Namibia, Western Australia, Baja California and Mauritania).
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