Satellite observations of humid heat extremes over Africa

湿热对人体健康是一个严重的风险，降低了身体通过出汗排出热量的能力。在气候变化下，极端高温对人类的影响将增加，特别是在热带“热点”，如赤道非洲，那里人口稠密，已经非常炎热和潮湿，容易受到气候变化的影响。虽然对干球温度极端事件的研究已经相当成熟，但对湿热极端事件的气象驱动因素的了解非常有限。新的研究表明，潮湿热浪的形成是潮湿过程之间复杂相互作用的结果，包括云，降雨，蒸发和水分平流（Birch等人，2022年），同时低层大气缺乏混合，这阻止了热量的通风（Raymond等人，2021年）。在非洲干旱地区，湿度而不是温度是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等人，2022年）。迫切需要在区域一级评估湿热受大气环流、云、降雨和地表通量调节的程度，并在区域一级评估气候模型中的这些驱动因素，由于缺乏对非洲大陆大部分地区的常规和详细观测，非洲的气候和大气科学研究面临挑战。因此，研究必须依靠卫星检索，卫星检索的优点是定期覆盖整个大陆。例如，卫星专家和博士生联合导师Dominique Bouncent之前的一项研究使用了一系列星载观测数据集来模拟西非萨赫勒地区热浪形成的过程，发现云和水分通过长波温室效应增加了温度。气候预测几乎普遍由相对粗糙的分辨率区域或全球气候模型提供，已知其不能很好地代表可引起或阻止严重湿热应力事件的关键潮湿过程。在非洲，这意味着低估了当今潮湿热浪的频率及其未来的变化（Birch et al. 2022）。对流尺度（网格间距<5公里）区域气候模式以足够高的分辨率运行，以便关闭对流参数化，并明确表示最大的对流运动。因此，对流尺度模型能够更好地代表对极端湿热天气重要的过程，如强降雨和干旱、大气水循环和土壤水分-降水反馈。对流尺度模型提供了一个机会，以推进基于过程的理解湿热极端，但还没有得到充分利用，以这种方式。Aims and ObjectivesThe项目的目的是，非洲湿热极端的驱动程序，并评估其代表性在高分辨率的气候模型。将通过以下目标实现这一目标：利用卫星对地球能量和辐射收支、云、降雨、温度和水蒸气的检索，量化非洲不同区域的极端湿热现象的驱动因素和关键特征，用于观测案例研究，并对过去许多事件的平均值进行量化;评价非洲极端湿热现象的代表性，并根据卫星观测结果在对流尺度和CMIP 6气候模型模拟中进行评估;利用泛非对流尺度气候模拟，包括任何新的集合模拟，评估未来极端湿热现象预测的不确定性

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