Weather associated with TRS
Atmospheric pressure
In the outer storm area of a TRS, the fall of pressure ahead of the trough, and the rise of pressure behind it, is slow. The semi-diurnal variation of pressure may still be visible on the trace of a barograph. In the eye-wall, the fall of pressure ahead of the trough, and the rise of pressure behind it, is very sharp. The trace of the barograph is very steep, nearly vertical. Semi-diurnal variation is not visible on it. In the eye, the lowest pressure is reached.
Wind
Wind direction: If a stationary observer is in the Right Hand Semi Circle (RHSC), the wind will veer steadily and if he is in the Left Hand Semi Circle (LHSC), it will back steadily. This holds good for both NH and SH. If he is in the direct path of the storm, wind direction will remain fairly steady.
Angle of indraft: The angle of indraft, in the outer fringes of the storm, is about 45° and this gradually decreases until it is 0° in the eye-wall. The application of Buys Ballot’s law, therefore, is appropriate.
Also read: 9 Essential IG System Alarms Onboard: Ensuring Safety at Sea
Wind force: The wind force will increase as the atmospheric pressure falls and after the trough, or eye as the case may be, has passed, the wind force will gradually decrease as the atmospheric pressure increases. The wind force in the outer storm area may be force 6 to 7, whereas in the eye-wall of a violent TRS it may be force 12 or over. In the eye-wall, the strongest winds usually lie in the rear
quadrant on the polar side of the storm.
Wind direction & force in the eye: As soon as a vessel passes from the eye-wall into the eye, the wind dies down into light airs but the swell is mountainous and confused. It must not be presumed that a vessel in the eye of a TRS is in a comfortable and safe position. On the contrary, she is in a most dangerous situation. After a short while, as the vessel passes into the eye-wall behind the trough, the sudden hurricane force wind from the opposite direction as before, strikes
the vessel and may cause it to heel over by as much as 80° or more and would hold it like that, leaving practically no margin for rolling further.
Over and above possible extensive wind-damage, cargo may break loose and
many openings may go below water causing the vessel to capsize.
Atmospheric temperature
Since a TRS exists in one air-mass only, no drastic changes of atmospheric temperature are experienced on its passage. However, atmospheric temperature would decrease during rain. In the eye, a slight increase may be registered due to adiabatic heating of the subsiding air.
Clouds & precipitation
In the outer fringes of the storm, cirrus in the form of strands or filaments generally so aligned, that they may be said to point towards the storm centre. Then cirrostratus followed by altostratus.
Around the eye-wall, thick nimbostratus (giving continuous rain) and small patches of cumulus, may be seen. At the eye-wall, towering anvil- may be seen. Towering anvil- shaped cumulonimbus gives torrential rain. Directly above the eye, a small circular patch of blue sky may be seen, indicating an absence of cloud therein and consequent cessation of precipitation.
Visibility
In the outer fringes of a TRS, visibility
is usually excellent. In the middle of the outer storm area, it becomes good except in occasional showers. Around the eye-wall, under the nimbostratus clouds, it becomes poor due to rain. In the eye-wall, it is poor due to driving rain and spray. In the eye, it is poor due to mist or fog.
Storm surge or tidal wave
The very violent winds of the eye-wall stir up mountainous waves, as high as 20 metres (from crest to trough). Since the TRS moves comparatively slowly, the winds act for a long time on the same area, setting up strong currents of water as deep as 25 metres below the surface. As these strong currents approach the shallow water near the coast, the water level suddenly rises well above the usual level (as much as 5 metres) and floods coastal areas. Such sudden rises of water level, caused by TRSs, are called “storm surges” or “tidal waves“. The storm surge may be experienced from about 300 to 400 miles ahead of the storm and may last until the storm passes.
Ref: Marine Meteorology by Capt. H Subramaniam
