Satellite observations of humid heat extremes over Africa

背景与理由湿热对人体健康是一个严重的威胁，它会降低人体通过出汗排出热量的能力。在气候变化的情况下，极端高温对人类的影响将会增加，特别是在热带“热点”地区，如赤道非洲，那里人口众多，已经非常炎热潮湿，容易受到气候变化的影响。虽然对干球极端温度的研究已经相当成熟，但对极端湿热的气象驱动因素的理解却非常有限。新出现的研究表明，潮湿热浪的形成是涉及云、降雨、蒸发和水分平流的潮湿过程之间复杂相互作用的结果（Birch等人，2022），同时低层大气缺乏混合，这阻碍了热量的通风（Raymond等人，2021）。湿度，而不是温度，是非洲干旱地区极端湿热的更强驱动因素，而在已经非常潮湿的赤道热带地区，温度和湿度都是驱动因素（Birch et al. 2022）。迫切需要在区域范围内评估大气环流、云、降雨和地表通量对湿热的调节程度，并在区域尺度上评估气候模式中的这些驱动因素。由于缺乏对非洲大陆大部分地区的常规和详细观测，对非洲的气候和大气科学研究具有挑战性。因此，研究必须依靠卫星检索，卫星检索的优点是可以定期覆盖整个非洲大陆。例如，卫星专家和博士共同导师Dominique Bouniol之前的一项研究使用了一系列星上观测数据集来描述西非萨赫勒地区热浪形成的过程，并发现云和水分通过长波温室效应使温度升高。气候预估几乎无处不在地由相对粗分辨率的区域或全球气候模式提供，众所周知，这些模式不能很好地代表可能导致或导致严重湿热胁迫事件的关键潮湿过程。在非洲，这意味着对当前潮湿热浪频率及其未来变化的低估（Birch et al. 2022）。对流尺度（网格间距<5km）区域气候模式以足够高的分辨率运行，可以关闭对流参数化，并明确地表示最大的对流运动。因此，对流尺度模式能够更好地代表极端湿热的重要过程，如强降雨和干旱期、大气水循环和土壤水分-降水反馈。对流尺度模式提供了一个机会，以推进基于过程的理解湿热极端，但尚未充分利用这种方式。目的和目标该项目的目的是描述非洲极端湿热的驱动因素，并评估它们在高分辨率气候模型中的代表性。这一目标将通过以下目标来实现：利用地球能量和辐射收支、云、降雨、温度和水蒸气的卫星检索，量化非洲不同地区观测到的案例研究以及平均而言过去许多事件的极端湿热的驱动因素和关键特征。对比卫星观测评估非洲极端湿热现象在对流尺度和CMIP6气候模式模拟中的表现。使用泛非对流尺度气候模拟，包括任何新的集合模拟，来评估未来极端湿热预测的不确定性

Background and RationaleHumid heat is a serious risk to human health, reducing the body's ability to expel heat through sweating. The human impact of heat extremes will increase under climate change, particularly in tropical 'hot spots', such as equatorial Africa, which is highly populated, already very hot and humid and vulnerable to climate variability. Whilst there is a fairly well established body of research on dry bulb temperature extremes, there is very limited understanding of the meteorological drivers of humid heat extremes.Emerging work suggests that humid heatwave formation is the result of a complex interaction between moist processes involving cloud, rainfall, evaporation and moisture advection (Birch et al. 2022), alongside a lack of mixing in the lower atmosphere, which prevents the ventilation of heat (Raymond et al. 2021). Humidity, rather than temperature, is the stronger driver of humid heat extremes in the arid regions of Africa, whereas both temperature and humidity are drivers in the already very humid equatorial tropics (Birch et al. 2022). There is a critical need to regionally assess the extent to which humid heat is modulated by atmospheric circulation, cloud, rainfall and surface fluxes and evaluate these drivers in climate models at the regional scale.Climate and atmospheric science research is challenging over Africa due to a lack of routine and detailed observations over much of the continent. Studies must therefore rely on satellite retrievals, which have the advantage of providing regular coverage over the entire continent. For example, a previous study by satellite expert and PhD co-supervisor Dominique Bouniol used a range of spaceborne observational datasets to characterise the processes involved in the formation of a heatwave over the Sahel region of West Africa and found that cloud and moisture were increasing temperatures through the longwave greenhouse effect.Climate projections are almost ubiquitously provided by relatively coarse resolution regional or global climate models, which are known to poorly represent the key moist processes that can cause or ventilate severe moist heat stress events. Over Africa, this translates into an underestimation of present day humid heatwave frequency, and their future change (Birch et al. 2022). Convective-scale (grid spacing <5km) regional climate models are run at a sufficiently high resolution to allow the convective parameterisation to be turned off and to represent the largest convective motions explicitly. Convective-scale models are thus better able to represent processes important for humid heat extremes such as intense rainfall and dry spells, the atmospheric water cycle and soil moisture-precipitation feedbacks. Convective-scale models provide an opportunity to advance process-based understanding of humid heat extremes, but have yet to be fully exploited in this way.Aims and ObjectivesThe aim of the project is to characterise the drivers of African humid heat extremes and evaluate their representation in high-resolution climate models. This aim will be addressed through the following objectives:Use satellite retrievals of the Earth's energy and radiative budgets, cloud, rainfall, temperature and water vapour to quantify the drivers and key characteristics of humid heat extremes over different regions of Africa for observed case studies and on average, over many past events.Evaluate the representation of African humid heat extremes and in convective-scale and CMIP6 climate model simulations against satellite observations.Use the pan-African convective-scale climate simulations, including any new ensemble simulations, to assess the uncertainty in future projections of humid heat extremes