Precipitation pattern change

Microclimatev02.30 credits/fixed

Responsible:Niccolò Calandri

To calculate Precipitation pattern change Risk, we use the following variable [Copernicus Climate Data Store (CDS)]:

daily precipitation: total daily precipitation (mm). We analyze two opposite but complementary aspects of rain events, using two key indicators calculated annually:
Maximum 5-day consecutive precipitation (Rx5day): Measures maximum rainfall over a 5-day consecutive period during the year. This indicator is a proxy for intense and prolonged rain events, often associated with soil saturation, river flooding and floods.
Maximum consecutive dry days (CDD): Measures duration of the longest period without significant precipitation (< 1 mm) during the year. This indicator is a proxy for persistence of dry periods and drought risk. Overall risk is not based on absolute thresholds, but compares a future year's annual Rx5day and CDD values with their respective historical statistical distributions (percentiles). Final risk level is determined by the worse value between the two indicators: a year is considered high risk if it presents either an exceptionally intense rain event or an exceptionally long dry period compared to its climate history.

Our methodology for assessing 'Precipitation Pattern Change Risk' captures one of the most critical and paradoxical aspects of climate change: the intensification of the hydrological cycle.

This approach, which simultaneously analyzes wet extremes (Rx5day) and dry ones (CDD), is fully supported by the most recent studies.

The two chosen indicators, Rx5day and CDD, are two of the official climate extreme indices defined and standardized by the Expert Team on Climate Change Detection and Indices (ETCCDI), a working group supported by the World Meteorological Organization (WMO).

ETCCFI Climate Change Indices

The underlying logic of our method — that the same region can become subject to both more intense rains and longer droughts — is one of the central conclusions of the IPCC 2021 report:

Climate Change 2021: The Physical Science Basis Chapter 8 ('Water Cycle Changes') and Chapter 11 ('Weather and Climate Extreme Events...') of this report explain that a warmer atmosphere can hold more water vapor (about 7% more for each degree of warming). This leads to two consequences:
When it rains, events are more intense and concentrated, leading to increased indicators like Rx5day.
In periods between rain events, soil dries faster due to higher temperatures, lengthening duration and intensity of dry periods, leading to increased indicators like CDD. Our methodology is therefore an operational translation of this aspect. Comparing a year's value with its historical distribution through percentiles is standard statistical practice in climatology for assessing rarity and severity of an anomaly.
Indices for monitoring changes in extremes based on daily temperature and precipitation data This methodological document promotes use of percentile-based approaches because they are 'climatically relative'. They allow identifying an 'extreme' event objectively for any location, whether a rainforest or desert. Our choice to use the worse percentile ( $\max(\text{percentile}(\text{Rx5day}), \text{percentile}(\text{CDD}))$ ) is a robust way to capture the year's dominant risk, whether related to excess or deficit of water. In conclusion, our method uses international standard indices (ETCCDI) to measure two sides of the same coin, reflecting IPCC conclusions on hydrological cycle intensification, and evaluates them with a robust and consolidated statistical methodology.

Data Sources & References

Sources

WRD_CDSCM_99

Methodology

Our methodology for assessing 'Precipitation Pattern Change Risk' captures one of the most critical and paradoxical aspects of climate change: the intensification of the hydrological cycle.

This approach, which simultaneously analyzes wet extremes (Rx5day) and dry ones (CDD), is fully supported by the most recent studies.

The underlying logic of our method — that the same region can become subject to both more intense rains and longer droughts — is one of the central conclusions of the IPCC 2021 report:
• [Climate Change 2021: The Physical Science Basis](Climate Change 2021: The Physical Science Basis)
Chapter 8 ('Water Cycle Changes') and Chapter 11 ('Weather and Climate Extreme Events...') of this report explain that a warmer atmosphere can hold more water vapor (about 7% more for each degree of warming). This leads to two consequences:

When it rains, events are more intense and concentrated, leading to increased indicators like Rx5day.
In periods between rain events, soil dries faster due to higher temperatures, lengthening duration and intensity of dry periods, leading to increased indicators like CDD.
Our methodology is therefore an operational translation of this aspect.
Comparing a year's value with its historical distribution through percentiles is standard statistical practice in climatology for assessing rarity and severity of an anomaly.
• [Indices for monitoring changes in extremes based on daily temperature and precipitation data](Indices for monitoring changes in extremes based on daily temperature and precipitation data)
This methodological document promotes use of percentile-based approaches because they are 'climatically relative'. They allow identifying an 'extreme' event objectively for any location, whether a rainforest or desert. Our choice to use the worse percentile ( $\max(\text{percentile}(\text{Rx5day}), \text{percentile}(\text{CDD}))$ ) is a robust way to capture the year's dominant risk, whether related to excess or deficit of water.
In conclusion, our method uses international standard indices (ETCCDI) to measure two sides of the same coin, reflecting IPCC conclusions on hydrological cycle intensification, and evaluates them with a robust and consolidated statistical methodology.