Summary

The World Climate Research Program has identified six Grand Challenges that “ represent major areas of scientific research, modeling, analysis and observations” for the coming decade. These Grand Challenges formed the nuclei for sessions at the Climate Symposium, convened in Darmstadt in October 2014, that gathered the world’s leading space agencies to survey needs for future space-based observation systems. This workshop proposal grows out of discussions by participants in the session on Clouds, Circulation, and Climate Sensitivity as to what new satellite observations would be most useful in responding to our challenge. ! ! Themes for the workshop arise in part from recent modeling results exploiting increasingly powerful computing capabilities. This allows for a new class of simulations that highlight the coupling between small-scale cloud processes and large-scale climate processes. The observational background includes almost ten years of vertically-resolved cloud observations from active sensors (radars and lidars) in orbit, the value of which continues to grow as the new observations are synthesized with existing understanding. A single follow-on mission is planned for roughly 2016 but there are no further plans to obtain such observations.

Specific Background

The Grand Challenge on Clouds, Circulation, and Climate Sensitivity has identified four questions with the potential to accelerate progress in understanding the Earth system and anticipating global and regional climate changes. Two questions are related to how clouds mediate the strength and intensity of large-scale rainfall maxima in the tropics and mid-latitudes. The other two are focused on the the role of cloud-scale processes in climate, particularly at low latitudes where cloud-environment interactions have a large impact on climate and its sensitivity to forcing. The question ’What role does convective aggregation play in climate?’ links observed and modeled relationships between the degree of convective organization and the distribution of clear-sky humidity, on the one hand, to the temperature dependence of convective selfaggregation in models, on the other, suggesting an unexplored feedback of convective organization on climate sensitivity. The question ‘What role does convection play in cloud feedbacks?’ focuses on the role of tropical deep convection in setting the atmospheric conditions to which climatically-important shallow clouds are exquisitely sensitive, on how the response of these clouds to changes to this environment may be linked to climate sensitivity, and how this response is mediated by somewhat deeper precipitating shallow convection. ! ! One common thread in these questions is the need to better understand the environmental conditions that control shallow clouds. The two most important conditions are the large-scale vertical velocity and, especially in the lowest portion of the atmosphere, the distribution of water vapor. Dynamics in the Tropics cannot support large temperature gradients so water vapor is the dynamically-relevant variable controlling, for example, the timing and intensity of deep convection and heavy rainfall. Because water vapor scales nonlinearly with temperature the vast majority of water vapor (and water vapor variability) is concentrated in the lowest kilometer of two of the atmosphere. This is precisely where current satellite observations from infrared and microwave sounders are most lacking (as was demonstrated in Darmstadt by the large uncertainty in atmospheric analyses that include such observations). Thus the known sensitivities of shallow and deep convection, along with radiatively-important shallow clouds, are dwarfed by uncertainty in the spatial and temporal distribution of water vapor. ! ! Many of the links between convection and future climate, mediated by circulation and water vapor in the lower troposphere, ought also to be observable in the day-to-day weather, so the opportunity to closely observe the interplay between lower tropospheric water vapor, circulation, and cloudiness has the potential to greatly advance our understanding of the climate system. Observations over land are fairly dense and can be integrated using techniques from weather forecasting, but much of the tropics is oceanic. This is where climatically-relevant low clouds are most prevalent, our existing observations most lacking, and satellite observations hold the most promise.

Participation & Objectives

We will gather representatives from three distinct communities in roughly equal proportions. Participants with expertise in theory and modeling will provide perspective on what information would be most useful in constraining particular hypothesis or expanding overall understanding. We will ask members of the data analysis community to help identify how available observations may be best used or synthesized and which prospective observations would bring the most benefit. We will solicit members of the (sometimes distinct) instrumental and retrieval communities for information about the technical aspects and feasibility of current and prospective measurements made from in situ, ground-based, airborne, or satellite platforms.

Desired Outcomes

We seek outcomes relevant to four time scales of increasing length. First, we will assess the current state-of-the-art in our ability to understand the distribution of lower tropospheric water vapor with emphasis on opportunities for more thoroughly exploiting existing satellite observations from current hyper-spectral microwave and infrared instruments. We will identify ripe near-term opportunities for novel analysis of existing observations to help elucidate the interactions between shallow clouds and their environment. We will consider opportunities for deployment of surface observing networks and/or intensive field campaigns that might be realized in the five-to-eight year time frame, with the intent of focusing the community around planned deployments or developing plans for new field observations. These three activities will be used to highlight the measurement gaps which could be met in the long term by future satellite observing systems. !

 

Last update: September 15, 2016