TROPICAL CYCLONES & SHIPPING

Nowadays, a lot of people are familiar with Tropical Cyclones, which often cause severe damage to nature and humanity in certain regions of our globe. However, apart from previous sectors, shipping is equally effective by Cyclones. Nevertheless, what is that separates Tropical Cyclones by normal Storms and do them more powerful and severe? Due to the fact that storms and tropical cyclones in general tent to become more intense as a result of climate change, it is vital that we learn useful knowledge about them. Firstly, the National Hurricane Center (NHC) defines a Tropical Cyclone as an area of low pressure with a warm core at its center that develops over Tropical or Sub-Tropical waters and is depicted on a surface chart by a few closed circular isobars, with associated steep pressure gradients. Tropical Cyclones are often classified according to the intensity of their strongest related winds: ? Tropical Depression (TD) – wind speed at the surface is less than 33 kts. ? Moderate Tropical Storm (TS) – wind speed at the surface ranges between 34 and 47 kts. ? Severe Tropical Storm (STS) – surface wind speeds between 48 and 63 kts. ? Hurricane (Hu) – wind speed at the surface of 64 kts or higher. ? Major Hurricane (MHu) – wind speed at the surface reaches 96 kts. Tropical cyclones are warm season phenomena. The peak frequency of these storms occurs after the maximum in solar radiation is received for the year, which occurs on June 22 in the Northern Hemisphere and December 22 in the Southern Hemisphere. The ocean surface reaches its maximum temperature several weeks after the solar radiation maximum, so most tropical cyclones occur during the late summer to early fall—that is, from July to September in the Northern Hemisphere and from January to March in the Southern Hemisphere.

Tropical cyclone is also called typhoon, hurricane or just cyclone. This depends on the location it forms. More specifically: ? In the Caribbean Sea, the Gulf of Mexico, the North Atlantic Ocean, and the eastern and central North Pacific Ocean, it is called “hurricane”. ? In the western North Pacific, it is called “typhoon” ? In the Bay of Bengal and Arabian Sea, it is called “cyclone” ? In western South Pacific and southeast India Ocean, it is called “severe tropical cyclone” ? In the southwest India Ocean, it is called “tropical cyclone”. Tropical Cyclones develop from preexisting disturbances, which are often convective cloud clusters accompanied by a low-level cyclonic vorticity maximum, such as a tropical wave. Due to the pressure gradient effect, air will flow inward into the low-pressure region where low-level vorticity is present. As a result of the Coriolis force, incoming air is deflected to the right in Northern Hemisphere (left in Southern) and a counterclockwise circulation is produced. Near the area of lowest pressure, the inflow of air causes low-level convergence, which in turn causes rising motion and deep convection. Tropical Cyclone is essentially a heat engine, with the underlying water serving as its heat supply. According to research, the sea surface temperature must be at least 27 degrees Celsius. At this temperature saturated air contains a great deal of water vapor, and when the air ascends, adiabatic cooling induces condensation with the release of latent heat of vaporization, which is the energy source for the development and maintenance of tropical cyclones. In addition, the vertical wind shear, which refers to the variation in wind speed with height, must be sufficiently low, less than 15 to 20 knots from the surface to the high troposphere. The center of the system is dominated by a 55-kilometer-diameter eye, or vortex, in which calm conditions predominate. However, winds of Beaufort Force 12 or higher are present directly outside of this region. The eye is encircled by the eye wall, which is a ring of Cumulonimbus clouds, and has normally cloudless sky. The perimeter is characterized by dense cloud bands from which torrential rains pour, derived by regions with little rain or no precipitation.

The sophistication of guidance products supplied by operational prediction centres across the world and the accuracy with which Tropical Cyclone tracks may be forecasted have vastly improved over the past several decades. For instance, in 1970s, the average 72-hour Atlantic basin tropical storm or hurricane prediction error was greater than 350 nautical miles, but in the present day, the average error is less than 100 nautical miles. Several Regional Specialized Meteorological Centres (RSMCs) are authorized by the World Meteorological Organization (WMO) to provide tropical storm predictions and alerts. When a tropical storm is active, the National Hurricane Center (https://www.nhc.noaa.gov/ ) produces tropical cyclone warning packages that include many official text and graphical products. Every six hours at 0300, 0900, 1500 and 2100 UTC, this suite of advisory products is published. Public Advisory, Forecast/Advisory, Tropical Cyclone Discussion and Wind Speed Probability are the principal text products.

In addition, the Tropical Cyclone Track Forecast Cone and Watch/Warning Graphic depicts the most recent NHC track forecast of a tropical cyclone’s center, as well as an approximation of coastal areas under a hurricane warning (red), hurricane watch (pink), tropical storm warning (blue) and tropical storm watch (green) (yellow). The orange circle indicates the present location the tropical cyclone’s center. The cone indicates the track of a tropical cyclone’s core and is constructed by enclosing the region blown out by a series of rings along the predicted trajectory.

The Tropical Cyclone Danger Graphic is a product of the National Hurricane Center that is based on the “Mariner’s 1-2-3 rule”. The images illustrate the danger zone associated with tropical storms from the equator to 60°N between 0° and 100°W, including the Pacific East of 100°W, and from the equator to 40°N between 80°W and 175°W, including the Gulf of Mexico and Western Caribbean. Add 100, 200 and 300 nautical miles to the tropical storm force 34 knots at the 24, 48, and 72-hour forecast locations, respectively, to establish the danger zone. Given the improvements in forecasting and the increasing availability of receiving these forecasts at sea, the most effective way to avoid an encounter with a tropical cyclone is to monitor the forecast products from the appropriate RSMC or TCWC and take early action, which includes determining the tropical cyclone’s location and direction of travel relative to the vessel and maneuvering the vessel accordingly. To retain situational awareness and safety during severe storms, a mariner must be skilled at recognizing and categorizing environmental changes. If the ship is within the cyclonic circulation, the first step is to calculate the ship’s position relative to the storm center. While the vessel may still make substantial progress through the water, a path should be chosen to take it as far from the center as feasible. Assuming the vessel can travel faster than the storm, it is reasonably easy to outrun the storm if there is sufficient sea room. But when the storm is more intense, the answer is not as straightforward. Prime Navigation Shipping uses in their analyses all available sources derived from global and widely respected independent meteorological organizations which are considered to most reliable nowadays amongst others, to provide timely and valid weather forecasts. Especially in the case of Tropical Cyclones, our team, which apart from well-seasoned ocean going Master Mariners and weather routing operators / meteorologists which are expert in Marine industry, have immediate cooperation with the master’s on board and operators to assist any vessel dealing with difficult conditions and suggest optimum routes not only for safety, but also by taking into account other business priorities such as fuel consumption or arrival time, avoiding bad weather conditions. In recent years, Tropical Cyclones have an increasingly uncertain trajectory due to climate change causing variations in air and sea temperatures. As a result, it’s necessary to immediately monitor the cyclone and to intervene in time by the operator to change the plan voyage. On the chart below, which created by NOAA, it seems that Absolute Track Error is defined as the great-circle distance between a storm’s forecast position and its observed position (i.e., NHC Best Track) at the forecast verification time. Absolute Track Error values are shown along the y-axis in nautical miles (nm), with values closer to 0.0 indicating more accurate absolute track forecasts. Forecast hours are shown along the xaxis. This plot shows the average Absolute Track Error from all storms that formed in the East Pacific during the 2023 Hurricane Season.

Created by: Efstratios Avgerinos Meteorologist – Weather Routing Operator – Prime Navigation Shipping