Tropical Cyclone Rainfall Rates and Climate Change

3 11 2017

Dale C. S. Destin |

This is a continuation of our series – Tropical Cyclones and Climate Change – TCs (tropical depressions, tropical storms, hurricanes). In our previous blog in this series, we looked at Tropical Cyclone Frequency and Climate Change. In this blog, we will look at whether climate change is having an impact on TC rainfall rates.

To date, the 2017 Atlantic hurricane season has produced 16 named storms – four more than the average or 33% more than usual. This is the most since 2012, when 19 formed. Meanwhile, there have been 10 hurricanes – four more than the average or 67% more than usual. This tied with 2010 for the most hurricanes since 2012. Further, there have been six major hurricanes – three more than average or 100% more than usual. This is the most major hurricanes since 2005.

Have TC rainfall rates increased?

According to the Intergovernmental Panel on Climate Change (IPCC), the answer to the above question is no. Recall that the IPCC is the United Nations international body assigned with the task of assessing climate change.

Rainfall totals from Hurricane Maria based on (IMERG) or satellite data

Rainfall totals from Hurricane Maria based on (IMERG) or satellite data

Here is what the IPCC Assessment Report Five (AR5) actually says: “…no broad-scale, detectable long-term changes in tropical cyclone rainfall rates have been reported…”; hence, there can be no credible claim of  TCs producing increased rainfall.

This conclusion by the IPCC is supported by many TC researchers, including Kevin J.S. Walsh et al. and Thomas R. Knutson et al. They all indicate that although the moisture content of the Atmosphere has increased and will continue to do so, as the earth warms, there has been no detectable change in tropical cyclone rainfall rates.

Will TC rainfall rates increase?

To this question, the IPCC says yes! Here are the exact words from the IPCC AR5, “…the increase in rainfall rates associated with tropical cyclones is a consistent feature of the numerical models under greenhouse warming as atmospheric moisture content in the tropics and tropical cyclone moisture convergence is projected to increase.”

According to the IPCC, rainfall rates within 200 km of the centre of TCs are likely to increase by 5 to 20% by the year 2100, due to climate change. This conclusion was also arrived at by many TC researchers including Kevin J.S. Walsh et al. and Thomas R. Knutson et al.

HurricaneIrma_6Sep17

Hurricane Irma rainfall rates – Sep 5-6, 2017. The highest rates are near the centre

The science behind the IPCC conclusion is quite robust. The Clausius-Clapeyron equation indicates that the water holding capacity of the atmosphere increases by around 7% for every degree °C rise in temperature; hence, one can expect increasing water vapour levels in the atmosphere as the earth continues to warm. There is evidence to show that this has happened in terms of rising specific humidity levels; however, studies are yet to detect increased rainfall rates from TCs.

Going forward, TCs will likely take advantage of the increasing moisture levels, in the Atmosphere, to produce higher rainfall rates. However, this is not to say that overall rainfall will increase from these systems as decreasing frequency of TCs could counteract increases. Also, rainfall rates only speak to the amount of rainfall per unit time and not total overall rainfall, which could remain unchanged.

Based on the IPCC and virtually all TC researchers, climate change has not changed TC rainfall rates; the rates remain unchanged. However, they are projected to increase by the year 2100, as TCs will likely extract higher rainfall rates from a moistening Atmosphere, being caused by climate change.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone size.


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