Periodic Reporting for period 3 - INTERACTION (Cloud-cloud interaction in convective precipitation)
Okres sprawozdawczy: 2021-07-01 do 2022-12-31
Thunderstorm clouds are observed to cluster in space, which can lead to tropical cyclones and extreme precipitation events both in the tropics and in mid-latitudes. This clustering is observed from satellite and ground based instruments, and can be simulated, but the fundamental mechanisms for why it occurs, are not well-understood.
Society is most directly affected by extreme precipitation events from clustered thunderstorms, as these can lead to flash floods - floods occurring as a result of heavy precipitation (downpours) over a relatively short period (few hours). Flash floods can be devastating to human lives and infrastructure, and appear to occur more frequently over recent years. Understanding the mechanisms that lead to flash floods will allow for better predictions of flood risk, today and in the future climate. Furthermore, cloud clusters can influence, how Earth's atmosphere interacts with radiation, such as sunlight and thermal radiation. A change in clustering under increasing surface temperatures could feed back on the heating through changes in the radiation that reaches and leaves the surface - thus again modifying the climate change signal.
The project has the overall objective to develop a clear understanding of how thunderstorms cluster. The working hypothesis is, that convective clouds interact through so-called "cold pools," air masses cooled by rainfall beneath thunderstorm clouds. When cold pools collide as they spread along the surface, they can excite new thunderstorms. This mechanism encodes a spatial interaction effect, which we describe theoretically and by conceptual modeling. This will lead to novel convective parameterizations in climate models, which in turn can yield more reliable future climate projections.
Society is most directly affected by extreme precipitation events from clustered thunderstorms, as these can lead to flash floods - floods occurring as a result of heavy precipitation (downpours) over a relatively short period (few hours). Flash floods can be devastating to human lives and infrastructure, and appear to occur more frequently over recent years. Understanding the mechanisms that lead to flash floods will allow for better predictions of flood risk, today and in the future climate. Furthermore, cloud clusters can influence, how Earth's atmosphere interacts with radiation, such as sunlight and thermal radiation. A change in clustering under increasing surface temperatures could feed back on the heating through changes in the radiation that reaches and leaves the surface - thus again modifying the climate change signal.
The project has the overall objective to develop a clear understanding of how thunderstorms cluster. The working hypothesis is, that convective clouds interact through so-called "cold pools," air masses cooled by rainfall beneath thunderstorm clouds. When cold pools collide as they spread along the surface, they can excite new thunderstorms. This mechanism encodes a spatial interaction effect, which we describe theoretically and by conceptual modeling. This will lead to novel convective parameterizations in climate models, which in turn can yield more reliable future climate projections.
A key hypothesis within INTERACTION is, that convective clouds "communicate" through so-called cold pools (CPs). A theoretical, conceptual basis was laid out within the first part of the project. The key hypothesis was investigated within high-resolution simulations and has indeed been confirmed. CP interaction is fundamental to the thunderstorm organization under a range of boundary conditions. We have shown that there are different types of CP interactions, e.g. those where two versus three CPs collide. The different modes of interaction allowed us to formulate conceptual models, describing spatial organization of thunderstorms.
Key achievements:
(1) conceptually, when CPs are approximated to spread as circles, increasing their radii with time, and interactions take place when three circles coincide in a single point, intricate clustering emerges. The spatial scales at which convective cells are organized in mid-latitude thunderstorm cloud fields, should systematically increase over time. We show analogous behavior using much more expensive large-eddy fluid dynamics simulations – demonstrating that simple conceptual explanations exist for the much more costly fluid dynamics simulations. Thus, we have also reached better understanding of how thunderstorms organize in reality;
(2) when exploiting, that CPs do not reach infinite size (already energy constraints would prevent this), we apply our theory to tropical “convective self-aggregation (CSA),” a heavily-studied phenomenon linked to the Madden-Julian Oscillation and hurricane formation. Our theory predicts that CSA, by which the domain spontaneously segregates into a cloudy and a cloud-free sub-region, can be achieved purely by the interaction between CPs. This brings a new perspective to the research field of CSA, where previous research has implicated a combination of processes, e.g. radiation physics and surface heat fluxes;
(3) we show that the diurnal cycle amplitude is crucial in determining, whether the cloud field clusters or not. In sub-regions, where clustering is strong, so are precipitation extremes and the risk of flash floods. These results bring forward testable predictions, which we aim to confirm using observations.
To summarize, we have addressed the following tasks outlined in the proposal: (1) scale increase of convection; (2) cold pool tracking; (3) mesoscale convective systems; (4) observational data analysis; (5) tailored numerical experiments; (6) prototype cloud-cloud interaction model (partially accomplished).
In the second project period, our theoretical modeling will be built into a parameterization for convective organization. Stronger emphasis will be on observational analysis, making use of high-resolution satellite and radar observations.
We are organizing an international focus meeting (virtual format due to Covid-19, ~ 50 leading experts expected from Europe, USA, Canada, Brazil) hosted at the Niels Bohr Institute, Univ. Copenhagen (May 5-7, 2021). We expect a strong dissemination and networking perspective, and we will co-fund this conference from the INTERACTION budget. Another, hopefully in-person, workshop is planned in 2022.
Key achievements:
(1) conceptually, when CPs are approximated to spread as circles, increasing their radii with time, and interactions take place when three circles coincide in a single point, intricate clustering emerges. The spatial scales at which convective cells are organized in mid-latitude thunderstorm cloud fields, should systematically increase over time. We show analogous behavior using much more expensive large-eddy fluid dynamics simulations – demonstrating that simple conceptual explanations exist for the much more costly fluid dynamics simulations. Thus, we have also reached better understanding of how thunderstorms organize in reality;
(2) when exploiting, that CPs do not reach infinite size (already energy constraints would prevent this), we apply our theory to tropical “convective self-aggregation (CSA),” a heavily-studied phenomenon linked to the Madden-Julian Oscillation and hurricane formation. Our theory predicts that CSA, by which the domain spontaneously segregates into a cloudy and a cloud-free sub-region, can be achieved purely by the interaction between CPs. This brings a new perspective to the research field of CSA, where previous research has implicated a combination of processes, e.g. radiation physics and surface heat fluxes;
(3) we show that the diurnal cycle amplitude is crucial in determining, whether the cloud field clusters or not. In sub-regions, where clustering is strong, so are precipitation extremes and the risk of flash floods. These results bring forward testable predictions, which we aim to confirm using observations.
To summarize, we have addressed the following tasks outlined in the proposal: (1) scale increase of convection; (2) cold pool tracking; (3) mesoscale convective systems; (4) observational data analysis; (5) tailored numerical experiments; (6) prototype cloud-cloud interaction model (partially accomplished).
In the second project period, our theoretical modeling will be built into a parameterization for convective organization. Stronger emphasis will be on observational analysis, making use of high-resolution satellite and radar observations.
We are organizing an international focus meeting (virtual format due to Covid-19, ~ 50 leading experts expected from Europe, USA, Canada, Brazil) hosted at the Niels Bohr Institute, Univ. Copenhagen (May 5-7, 2021). We expect a strong dissemination and networking perspective, and we will co-fund this conference from the INTERACTION budget. Another, hopefully in-person, workshop is planned in 2022.
Progress beyond the state of the art: We show that cold pool interaction leads to complex cloud clustering effects. This relates directly to the emergent risk of flash floods and was previously not known. We explain, why scales increase during the convective diurnal cycle, a feature we previously mimicked using large-eddy simulations. We have now gained theoretical understanding of this. Further, we have shown theoretically, that, when cloud interactions are incorporated into a so-called radiative convective equilibrium framework, "convective self-aggregation" (CSA) can emerge through cold pool interaction - under fairly general assumptions. This is a new perspective on the heavily-studied CSA problem, with practical relevance to tropical cyclones an the Madden-Julian Oscillation.
Building on this using a substantial suite of high-resolution fluid dynamics simulations, we addressed the emergence of mesoscale convective systems (MCS), i.e. "clumped-together" sets of convective clouds. We show that the surface temperature amplitude determines, whether MCS emerge or not - an important effect previously not known. We can now show that MCS can bring about CSA. In our findings, the emergence of CSA is facilitated by finer model resolution (the realistic limit), whereas traditional studies on CSA require model resolution to be coarse. The latter dilemma stumped the community for decades and we helped resolve this dilemma.
Expected results until the end of the project: The second part of INTERACTION will place a strong focus on satellite imagery, to explore, how populations of MCSs organize in the tropical atmosphere and how they can grow into tropical cyclones, when advected over the ocean. We assess how populations of MCSs (so-called "super clusters") interact using pattern recognition methods (e.g. machine learning) on the satellite-observed tropical cloud field. Our theoretical methods will be assessed against these observational results. We will implement a sub-grid parameterization for cold pools (CPs), which we now understand to be critical in triggering new convective events. The CP gust fronts are too narrow to be properly simulated by current climate models. We aim to implement their effective action into coarse models, so that thunderstorm organization can properly be captured even by global climate models. The model will form the basis for novel convective parameterizations, which properly capture clustered convection.
Building on this using a substantial suite of high-resolution fluid dynamics simulations, we addressed the emergence of mesoscale convective systems (MCS), i.e. "clumped-together" sets of convective clouds. We show that the surface temperature amplitude determines, whether MCS emerge or not - an important effect previously not known. We can now show that MCS can bring about CSA. In our findings, the emergence of CSA is facilitated by finer model resolution (the realistic limit), whereas traditional studies on CSA require model resolution to be coarse. The latter dilemma stumped the community for decades and we helped resolve this dilemma.
Expected results until the end of the project: The second part of INTERACTION will place a strong focus on satellite imagery, to explore, how populations of MCSs organize in the tropical atmosphere and how they can grow into tropical cyclones, when advected over the ocean. We assess how populations of MCSs (so-called "super clusters") interact using pattern recognition methods (e.g. machine learning) on the satellite-observed tropical cloud field. Our theoretical methods will be assessed against these observational results. We will implement a sub-grid parameterization for cold pools (CPs), which we now understand to be critical in triggering new convective events. The CP gust fronts are too narrow to be properly simulated by current climate models. We aim to implement their effective action into coarse models, so that thunderstorm organization can properly be captured even by global climate models. The model will form the basis for novel convective parameterizations, which properly capture clustered convection.