The freezing-level model interpolates the freez-ing level from the temperatures reported at the mandatory constant-pressure levels. Starting at 1,000 millibar, the computer checks the tempera-ture at each mandatory level until it encounters the first level with a temperature below 0C. The model uses this level and the one preceding it to interpolate the freezing level.

There are some problems associated with this linear interpolation process. It does not account for the following: (1) poor constant-pressure height and/or temperature analyses, (2) inver-sions, or (3) multiple freezing levels. Even with the above limitations, interpolated heights are normally within 100 feet of observed values. The freezing-level chart is widely used in aviation forecasting. It is used to outline areas of potential aircraft icing, and in thunderstorm forecasting. The most severe icing occurs at temperatures between 0C and 10C. With regard to thunderstorms, lightning strikes are most prevalent at the freezing level. Pilots must be advised of this when their aircraft is cleared through a thunderstorm area. Freezing-level charts are also used to forecast changes from rain to snow and vice versa.

Learning Objective: Identify FLENUMOCEANCEN oceanographic analysis models and their uses.

The daily real-time global data base used by EOTS consists of 150 to 200 XBT observations, 1,200 to 2,000 SST observations (ship injection or bucket), and 50,000 to 80,000 satellite SST readouts.

EOTS can also accept synthetic data inputs such as horizontal surface and subsurface thermal gradients. The regional centers supply the synthetic data in message form.

EOTS analyzes 26 thermal parameters in the upper 400 meters of the sea on a 63 by 63 or 125 by 125 vertical grid. Below 400 meters, the thermal field is derived from climatology and is modified to blend smoothly with the temperature profile analyzed above 400 meters.

The primary layer depth (PLD) is the first parameter analyzed by the model. This is generally the depth of the seasonal thermocline. The remaining 25 parameters are temperatures and vertical temperature derivatives. They are ana-lyzed at fixed and floating (fluctuating) levels. The floating levels are relative to the PLD: PLD-25 meters, PLD + 12.5 meters, PLD + 25 meters and PLD + 50 meters. Consequently, when the PLD changes, so do the floating levels. FIB methodology is the heart of the EOTS analysis. A three-cycle FIB technique is used to analyze the fixed- and floating-level temperatures on a horizontal plane and to analyze the vertical temperature gradients between the fixed levels. These analyses are based purely on information blending techniques. EOTS does not consider the effect of oceanic physics and air-sea interaction processes.

Learning Objective: Identify the model that couples EOTS with atmospheric processes.

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