Guide to the Tropical Forecasts and Analyses
and display the results from the Medium-Range Forecast (MRF) 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)
- 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
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).
last update: 23 August 1995
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