Research

 

Below is a summary of some of our research findings related to our past research interests on the organization of convection as well as to our long-lasting research interests on land-atmosphere interactions.

 

Organization of convection

The tendency of convection to organize  is a well-known ability of convection, clearly visible in satellite images. But even in purely homogeneous conditions, as in studies of radiative convective equilibrium (RCE), convection likes to spontaneously organize (Fig. 1), a process referred to as convective self-aggregation. Our research on the organization of convection aimed to better understand the mechanisms underlying the self-aggregation of convection and investigate the importance of convective organization for climate.  The RCE conceptualization helps us isolate mechanisms that may control the spatial distribution of convection in the real world, and through this, provides us with hypotheses testable against observations or more complex simulations. In contrast to most studies on the self-aggregation of convection, we applied the RCE concept to less idealized boundary bottom boundary conditions such as including sea surface temperature gradients, a dynamical land surface or islands. Our research shows that convection fundamentally strives to organize, and its resulting spatial distribution is set dynamically by shallow circulations, confined in the planetary boundary layer and triggered by boundary layer heterogeneities. Those heterogeneities are linked to the convection itself (cold pools and radiative heating anomalies that develop between convective and non-convective regions) and to the underlying surface (gradients in sea surface temperature or in soil moisture). Those circulations compete with each other for determining the spatial distribution of convection.  In a nutshell, the circulation associated with the largest density anomaly wins, a result that can be understood using a unified framework of gravity current theory (see [1,2,3]). The consequence is that convection is more organized over  ocean than over land, and strongly dynamical surfaces act to homogenize the precipitation distribution. Organization matters for climate as, in the RCE world, it is requested to achieve a stable climate [4]. Its change with warming, together with changes in the shallow cloud fraction, also explains more than 70% of the intermodel spread in climate sensitivity [5].  Organization matters for climate in the real world as the large-scale degree of organization of the ITCZ correlates with the degree of dryness in the subtropics [6], as expected from the results of the RCE studies. But, perhaps surprisingly, mesoscale organization does not matter for tropical precipitation amounts [7], and even coarse-resolution simulations with explicit convection, as coarse as 80 km, can still capture many basic statistics of the climate systems [8].

 

Land-atmosphere interactions

That the land surface may affect the precipitation has long been speculated. S. Aughey expressed in 1880 the idea that soil moisture may affect the precipitation amounts. And already in 1686, Halley recognized that the stronger heating of the continental land masses causes the monsoon. Still, the importance of the land surface for setting the lifecycle, distribution and climatological characteristics of convective precipitation remains debated, particularly due to the use of inappropriate modelling tools that may overemphasize the feedback between the land surface and convection. Our research on land-atmosphere interactions aimed to investigate whether the land surface, or the convection itself, ends up setting aspects of the precipitation distribution over land by taking advantage of models that do not have to rely on convective parameterizations. A key property of the land surface that emerges from our research is its ability to dry out. This leads to a homogenization of the precipitation distribution over land on longer time scales, in contrast to the ocean [1]. Also, it leads to a weak cooling of tropospheric temperature in the tropics by the presence of islands [9]. Beside the ability of the land to dry out, the presence of  surface heterogeneity, e.g. due to different land cover types, can affect the daily triggering of convection through the generation of surface-induced mesoscale circulations. This is well known, but few studies have investigated the feedback of convection on the surface-induced circulation. Our research shows that once convection develops at the front of the surface-induced circulation, it masks the effect of the initial surface heterogeneity and fully sets the final characteristics of the circulation, such as its propagation velocity. This is partly due to the generation of cold pools (10). It follows that biases in the representation of convection projects on the characteristics of a surface-induced circulation, even in the presence of perfect surface heterogeneities (11). As another consequence, the atmospheric conditions scaled by the available water capacity, and not soil moisture, actually control the strength of the variations of precipitation with soil moisture over a heterogeneous surface made of prescribed patches of distinct soil moisture (12).

 

[1] Hohenegger C. and B. Stevens, 2018: The role of the permanent wilting point in controlling the spatial distribution of precipitation. Proc. Natl. Acad. Sci., 115, 5692-5697, doi:10.1073/pnas.1718842115

[2] Windmiller, J. and C. Hohenegger, 2019: Convection on the edge. J. Adv. Mod. Earth Systems, 11, 3959-3972, doi:10.1029/2019MS001820.

[3] Müller, S. K. M. and C. Hohenegger, 2020:  Self-aggregation of convection in spatially-varying sea surface temperatures. J. Adv. Mod. Earth Systems, 12, e2019MS001698, doi:10.1029/2019MS001698.

[4] Hohenegger C. and B. Stevens, 2016: Coupled radiative convective equilibrium simulations with explicit and parameterized convection. J. Adv. Mod. Earth Systems, 8, 1468-1482, doi: 10.1002/2016MS000666.

[5] Becker, T. and A. Wing, 2020: Understanding the extreme spread in climate sensitivity within the radiative-convective equilibrium model intercomparison project. J. Adv. Mod. Earth Systems, 12, e2020MS002165, doi:10.1029/2020MS002165.

[6] Hohenegger, C. and C. Jakob, 2020: A relationship between ITCZ organization and subtropical humidity. Geophys. Res. Let., 47, e2020GL088515, doi:10.1029/2020GL088515

[7] Brueck, M., C. Hohenegger and B. Stevens, 2020: Mesoscale marine precipitation varies independently from the spatial arrangement from its convective cells. Quart. J. Roy. Meteor. Soc.,146, 1391-1402, doi:10.1002/qj.3742

[8] Hohenegger, C., L. Kornblueh, D. Klocke, T. Becker, G. Cioni, J. F. Engels, U. Schulzweida and B. Stevens, 2020: Climate statistics in global simulations of the atmosphere, from 80 to 2.5 km grid spacing. J. Meteorol. Society Japan, 98, 73-91, doi:10.2151/jmsj.2020-005

[9] Leutwyler, D. and C. Hohenegger, revised: Weak cooling of the troposphere by tropical islands in simulations of the radiative-convective equilibrium. Quart. J. Roy. Meteor. Soc., revised.

[10] Rieck M., C. Hohenegger and P. Gentine, 2015: The effect of moist convection on thermally induced mesoscale circulations. Quart. J. Roy. Meteor. Soc., 141, 2418-2428, doi:10.1002/qj.2532

[11]Hohenegger C., L. Schlemmer and L. Silvers, 2015: Coupling of convection and circulation at various resolutions. Tellus A, 67, 26678, doi:10.3402/tellusa.v67.26678

[12] Cioni G. and C. Hohenegger, 2018: A simplified model of precipitation enhancement over a heterogeneous surface. Hydrol. Earth Syst. Sci., 22, 3197-3212,doi:10.5194/hess-22-3197-2018

 

                            

Figure 1: Frequency of convective occurrence in a radiative convective equilibrium simulation. Each plot corresponds to one simulation day; the frequency at each grid point is computed as the number of times cloud water is present at 1 km (based on hourly instantaneous output). On the last plot, the daily mean of the horizontal wind from the lowest atmospheric layer is added. From [4].