DISCUSSION: As we head deeper into the 2018 Tropical Atlantic Basin hurricane season, there is no question that there are many different questions which loom on the minds of many people around the world. Among them, is how to understand and interpret the critical differences between what is referred to as a tropical storm and a subtropical storm. During any given tropical cyclone season in many different ocean basins around the world, there are often occurrences by which a low-pressure system of a tropical and/or subtropical nature develop. Moreover, people and the general media often confuse people regarding the key differences between what constitutes a tropical vs. subtropical storm.
First off, and above all else, tropical storms are warm-core low-pressure systems which often develop deep convection around the circulation center of the tropical storm. This ring of deep convection is often observed from above and referred to as a tropical storm’s central dense overcast (CDO). A CDO is defined via infrared satellite imagery as the continuous or near-continuous ring of coldest cloud tops which surround the center of the tropical storm. On the flip side, subtropical storms are predominantly cold-core low-pressure systems which extract the bulk of their energy from (Sun-based) diurnal heating cycles which is why the convection around the center of subtropical storms is often most vigorous near and around the peak of the day-time heating hours over a given ocean basin.
In addition, since tropical storms being warm-core low-pressure systems, they are often readily identified by the fact that isobars (i.e., lines of constant atmospheric pressure) which chronologically extend out from the center of the storm (i.e., from the point of lowest minimum central pressure) increase in value which makes sense since a storm’s intensity is weaker as you go outward from the center. This is found to be the same with both tropical and subtropical low-pressure systems. However, the marque difference between tropical and sub-tropical storms is that from a vertical perspective, these isobars tend to dip down near the center of the storm with tropical low-pressure systems which helps to explain why they tend to be weaker with height. On the other hand, isobars tend to bump up a bit near the center of subtropical storms which also explains why subtropical storms (and non-tropical low-pressure systems as well for that matter) tend to generally strengthen with increasing height.
To elaborate a bit more on this issue, we will revert to the official definition of a subtropical cyclone. Per the definition from the National Oceanic and Atmospheric Administration (NOAA), “A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. Like tropical cyclones, they are non-frontal, synoptic-scale (i.e., often systems which have life-cycles of between 5 and 7 days or so) cyclones that originate over tropical or subtropical waters and have a closed surface wind circulation about a well-defined center. In addition, they have organized moderate to deep convection, but lack a central dense overcast. Unlike, tropical cyclones, subtropical cyclones derive a significant portion of their energy from baroclinic sources and are generally cold-core in the upper troposphere, often being associated with an upper-level low or trough. In comparison to tropical cyclones, these systems generally have a radius of maximum winds occurring relatively far from the center (i.e., usually greater than 60 nautical miles), and generally have a less symmetric wind field and distribution of convection.” Hence, there are some similarities as well as some key differences between tropical and subtropical storms.
Attached above you will find a recent still image of the ongoing Subtropical Storm Alberto from 3:31 AM EDT on 27 May 2018 and then just below it, a brief corresponding infrared satellite imagery loop.
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© 2018 Meteorologist Jordan Rabinowitz