Thesis defense J.M. Loriaux: precipitation

18 January 2017 | 15:00
location: Aula, TU Delft
by Webredactie

Convective extreme precipitation at midlatitudes. Promotor: Prof.dr. A.P. Siebesma (CiTG).

Reports of extreme precipitation events are becoming more frequent in the news; they are causing traffic jams, floods, and even loss of life. As a result of global warming, the atmosphere will be able to retain more moisture, which means that globally, precipitation will increase. On average, this increase amounts to approximately 1-3% per degree warming, but locally the precipitation increase can deviate from this quite a bit. And although we have a fairly good understanding of the global mean response to climate change, the behavior of the extremes is more difficult to determine. Therefore, an important question is how events of extreme precipitation will manifest in a future climate.
To determine this, it is necessary to understand the circumstances leading to extreme events. This is complicated by the large range of spatial and temporal scales at which relevant processes take place. In this thesis we try to assess the processes and conditions controlling precipitation extremes, and determine the behavior of these events in a future climate. This is done by using observations and various models with different resolutions. The focus of this thesis is on convective precipitation extremes. These are heavy rainfall events that are caused by instability, where the air rises quickly.
The amount of water vapor that air can hold until saturation occurs is described by the Clausius-Clapeyron equation. If the moisture supply remains unchanged, in the limit where all the moisture that the air can take in is precipitated out, the precipitation intensity would also be described by this equation. However, observations over the Netherlands indicate a 14% per degree rise in extreme precipitation intensity with temperature, which corresponds to approximately twice the Clausius-Clapeyron relation.
It has been suggested in the literature that this enhanced increase could be a statistical artifact caused by the presence of different precipitation types affecting the statistics. If only convective precipitation extremes were analyzed, this enhanced increase would vanish. However, using observations, in this thesis we show that this relation does hold for convective extremes. By increasing the temporal resolution from hourly to 10 minutes, extremes in the dataset are made up of a larger amount of convective events. Rather than a Clausius-Clapeyron induced increase of 7% per degree, we see that the observed 14% per degree increase is valid over a larger  temperature interval than for the hourly data. From this we conclude that the observed increase of 14% per degree is robust for convective extremes over the Netherlands.
Using a conceptual model we can better assess the local processes that determine the relationship between rainfall intensity and temperature. Two processes seem to determine the increase in the precipitation intensity; the moisture flux at the cloud base and the lateral moisture convergence in the cloud. The first process is mainly determined by the Clausius-Clapeyron relation. Lateral moisture convergence (the amount of moisture that is drawn into the cloud) is also enhanced by the strength of the updraft in the cloud, and as a result leads to a stronger increase in rainfall intensity. After all, when upward motions increase, continuity dictates that this air must also be replenished. As a result, more of that moist air is drawn into the cloud per degree warming, allowing the precipitation intensity to increase by more than expected based on the Clausius-Clapeyron equation alone.
This enhanced increase appears to be related to the extent to which the temperature profile of the atmosphere, and hence also the stability, changes as a result of the global warming. In the tropics the atmospheric stability is considered to be fairly constant with warming, but in the midlatitudes, a decrease in the atmospheric stability is expected. Therefore we can expect a stronger climate response of extreme precipitation there.
Using model results and observations we have mapped the large-scale conditions typical for showers of different strengths. Showers with high precipitation intensities are shown to take place under warmer, more humid conditions than weaker events. Moreover, strong precipitation events take place in an unstable atmosphere with stronger large-scale moisture convergence than weaker events. These properties show a strengthening temporal signal with increasing rainfall intensity.
The aforementioned insights have been used to perform realistic simulations of extreme precipitation using a high resolution cloud resolving model. What makes this setup stand out amongst other things is that the model is driven by realistic, time dependent conditions. Within this experimental setup, sensitivity experiments have been performed by systematically perturbing the relative humidity, stability and large-scale moisture convergence. This has lead to several important results. The total precipitation is shown to increase with humidity, instability and large-scale moisture convergence. Decomposition of the total precipitation into the precipitation intensity and the area fraction (the fraction of the domain where it has been raining) shows that the instability and moisture convergence realize growth in different ways. While the instability increases the precipitation intensity, but has little impact on the area fraction, the large-scale moisture convergence mainly increases the area fraction, without having a large effect on the precipitation intensity.
These sensitivity experiments have been repeated for a warmer climate. They show that the relations found for the present-day climate, continue to hold in a future climate. With an increase of approximately 7% per degree warming, the climate response of extreme precipitation intensities is lower than expected, but higher than the response of the total rainfall, because the area fraction decreases in the climate simulations. This means that in a warmer climate, it will rain less, but more intensely.

More information?
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