@article{21755, abstract = {Tropical shallow clouds are a major source of uncertainty in Earth's climate sensitivity, especially through their spatial arrangement, which global climate models do not represent. Efforts to understand their organization have partly relied on classifying observed scenes, identifying four patterns as archetypal regimes. Here we analyze geostationary satellite imagery of the western tropical Atlantic using the L‐function, a tool based on point pattern theory that quantifies cloud organization across spatial scales. Classical examples of the four patterns show distinct L‐function fingerprints, revealing their characteristic clustering and regularity scales and aiding physical interpretation. Yet, when evaluating many scenes at fixed spatial scales, the L‐function distribution lacks the distinct modes expected from discrete regimes. This is corroborated by analyses of other organization indices employing diverse approaches, from inter‐cloud nearest‐neighbor distances to fractal analysis. Implications for the parameterization of mesoscale cloud organization in climate models are discussed.}, author = {Biagioli, Giovanni and Mandorli, Giulio and Freischem, Lilli Johanna and Casallas Garcia, Alejandro and Tompkins, Adrian Mark}, issn = {1944-8007}, journal = {Geophysical Research Letters}, number = {8}, publisher = {Wiley}, title = {{Spatial patterns of shallow clouds: Challenging the concept of defined regimes}}, doi = {10.1029/2025gl119921}, volume = {53}, year = {2026}, } @article{15165, abstract = {Current knowledge suggests a drought Indian monsoon (perhaps a severe one) when the El Nino Southern Oscillation and Pacific Decadal Oscillation each exhibit positive phases (a joint positive phase). For the monsoons, which are exceptions in this regard, we found northeast India often gets excess pre-monsoon rainfall. Further investigation reveals that this excess pre-monsoon rainfall is produced by the interaction of the large-scale circulation associated with the joint phase with the mountains in northeast India. We posit that a warmer troposphere, a consequence of excess rainfall over northeast India, drives a stronger monsoon circulation and enhances monsoon rainfall over central India. Hence, we argue that pre-monsoon rainfall over northeast India can be used for seasonal monsoon rainfall prediction over central India. Most importantly, its predictive value is at its peak when the Pacific Ocean exhibits a joint positive phase and the threat of extreme drought monsoon looms over India.}, author = {Goswami, Bidyut B}, issn = {1944-8007}, journal = {Geophysical Research Letters}, number = {5}, publisher = {Wiley}, title = {{A pre-monsoon signal of false alarms of Indian monsoon droughts}}, doi = {10.1029/2023GL106569}, volume = {51}, year = {2024}, } @article{15186, abstract = {The elimination of rain evaporation in the planetary boundary layer (PBL) has been found to lead to convective self‐aggregation (CSA) even without radiative feedback, but the precise mechanisms underlying this phenomenon remain unclear. We conducted cloud‐resolving simulations with two domain sizes and progressively reduced rain evaporation in the PBL. Surprisingly, CSA only occurred when rain evaporation was almost completely removed. The additional convective heating resulting from the reduction of evaporative cooling in the moist patch was found to be the trigger, thereafter a dry subsidence intrusion into the PBL in the dry patch takes over and sets CSA in motion. Temperature and moisture anomalies oppose each other in their buoyancy effects, hence explaining the need for almost total rain evaporation removal. We also found radiative cooling and not cold pools to be the leading cause for the comparative ease of CSA to take place in the larger domain.}, author = {Hwong, Yi-Ling and Muller, Caroline J}, issn = {1944-8007}, journal = {Geophysical Research Letters}, keywords = {General Earth and Planetary Sciences, Geophysics}, number = {6}, publisher = {Wiley}, title = {{The unreasonable efficiency of total rain evaporation removal in triggering convective self‐aggregation}}, doi = {10.1029/2023gl106523}, volume = {51}, year = {2024}, } @article{14779, abstract = {The presence of a developed boundary layer decouples a glacier's response from ambient conditions, suggesting that sensitivity to climate change is increased by glacier retreat. To test this hypothesis, we explore six years of distributed meteorological data on a small Swiss glacier in the period 2001–2022. Large glacier fragmentation has occurred since 2001 (−35% area change up to 2022) coinciding with notable frontal retreat, an observed switch from down‐glacier katabatic to up‐glacier valley winds and an increased sensitivity (ratio) of on‐glacier to off‐glacier temperature. As the glacier ceases to develop density‐driven katabatic winds, sensible heat fluxes on the glacier are increasingly determined by the conditions occurring outside the boundary layer of the glacier, sealing the glacier's demise as the climate continues to warm and experience an increased frequency of extreme summers.}, author = {Shaw, Thomas E. and Buri, Pascal and McCarthy, Michael and Miles, Evan S. and Ayala, Álvaro and Pellicciotti, Francesca}, issn = {1944-8007}, journal = {Geophysical Research Letters}, keywords = {General Earth and Planetary Sciences, Geophysics}, number = {11}, publisher = {American Geophysical Union}, title = {{The decaying near‐surface boundary layer of a retreating alpine glacier}}, doi = {10.1029/2023gl103043}, volume = {50}, year = {2023}, } @article{12107, abstract = {The sensitivity of coarse-grained daily extreme precipitation to sea surface temperature is analyzed using satellite precipitation estimates over the 300–302.5 K range. A theoretical scaling is proposed, linking changes in coarse-grained precipitation to changes in fine-scale hourly precipitation area fraction and changes in conditional fine-scale precipitation rates. The analysis reveals that the extreme coarse-grained precipitation scaling with temperature (∼7%/K) is dominated by the fine-scale precipitating fraction scaling (∼6.5%/K) when using a 3 mm/h fine-scale threshold to delineate the precipitating fraction. These results are shown to be robust to the selection of the precipitation product and to the percentile used to characterize the extreme. This new coarse-grained scaling is further related to the well-known scaling for fine-scale precipitation extremes, and suggests a compensation between thermodynamic and dynamic contributions or that both contributions are small with respect to that of fractional coverage. These results suggest that processes responsible for the changes in fractional coverage are to be accounted for to assess the sensitivity of coarse-grained extreme daily precipitation to surface temperature.}, author = {Roca, Rémy and De Meyer, Victorien and Muller, Caroline J}, issn = {1944-8007}, journal = {Geophysical Research Letters}, number = {24}, publisher = {Wiley}, title = {{Precipitating fraction, not intensity, explains extreme coarse-grained precipitation Clausius-Clapeyron scaling with sea surface temperature over tropical oceans}}, doi = {10.1029/2022GL100624}, volume = {49}, year = {2022}, } @article{10653, abstract = {Squall lines are known to be the consequence of the interaction of low-level shear with cold pools associated with convective downdrafts. Also, as the magnitude of the shear increases beyond a critical shear, squall lines tend to orient themselves. The existing literature suggests that this orientation reduces incoming wind shear to the squall line, and maintains equilibrium between wind shear and cold pool spreading. Although this theory is widely accepted, very few quantitative studies have been conducted on supercritical regime especially. Here, we test this hypothesis with tropical squall lines obtained by imposing a vertical wind shear in cloud resolving simulations in radiative convective equilibrium. In the sub-critical regime, squall lines are perpendicular to the shear. In the super-critical regime, their orientation maintain the equilibrium, supporting existing theories. We also find that as shear increases, cold pools become more intense. However, this intensification has little impact on squall line orientation.}, author = {Abramian, Sophie and Muller, Caroline J and Risi, Camille}, issn = {1944-8007}, journal = {Geophysical Research Letters}, number = {1}, publisher = {Wiley}, title = {{Shear-convection interactions and orientation of tropical squall lines}}, doi = {10.1029/2021GL095184}, volume = {49}, year = {2022}, } @article{12588, abstract = {The thinning patterns of debris-covered glaciers in High Mountain Asia are not well understood. Here we calculate the effect of supraglacial ice cliffs on the mass balance of all glaciers in a Himalayan catchment, using a process-based ice cliff melt model. We show that ice cliffs are responsible for higher than expected thinning rates of debris-covered glacier tongues, leading to an underestimation of their ice mass loss of 17% ± 4% in the catchment if not considered. We also show that cliffs do enhance melt where other processes would suppress it, that is, at high elevations, or where debris is thick, and that they contribute relatively more to glacier mass loss if oriented north. Our approach provides a key contribution to our understanding of the mass losses of debris-covered glaciers, and a new quantification of their catchment wide melt and mass balance.}, author = {Buri, Pascal and Miles, Evan S. and Steiner, Jakob F. and Ragettli, Silvan and Pellicciotti, Francesca}, issn = {1944-8007}, journal = {Geophysical Research Letters}, keywords = {General Earth and Planetary Sciences, Geophysics}, number = {6}, publisher = {American Geophysical Union}, title = {{Supraglacial ice cliffs can substantially increase the mass loss of debris‐covered glaciers}}, doi = {10.1029/2020gl092150}, volume = {48}, year = {2021}, } @article{12604, abstract = {Glaciers in the high mountains of Asia provide an important water resource for millions of people. Many of these glaciers are partially covered by rocky debris, which protects the ice from solar radiation and warm air. However, studies have found that the surface of these debris-covered glaciers is actually lowering as fast as glaciers without debris. Water ponded on the surface of the glaciers may be partially responsible, as water can absorb atmospheric energy very efficiently. However, the overall effect of these ponds has not been thoroughly assessed yet. We study a valley in Nepal for which we have extensive weather measurements, and we use a numerical model to calculate the energy absorbed by ponds on the surface of the glaciers over 6 months. As we have not observed each individual pond thoroughly, we run the model 5,000 times with different setups. We find that ponds are extremely important for glacier melt and absorb energy 14 times as quickly as the debris-covered ice. Although the ponds account for 1% of the glacier area covered by rocks, and only 0.3% of the total glacier area, they absorb enough energy to account for one eighth of the whole valley's ice loss.}, author = {Miles, Evan S. and Willis, Ian and Buri, Pascal and Steiner, Jakob F. and Arnold, Neil S. and Pellicciotti, Francesca}, issn = {1944-8007}, journal = {Geophysical Research Letters}, keywords = {General Earth and Planetary Sciences, Geophysics}, number = {19}, pages = {10464--10473}, publisher = {American Geophysical Union}, title = {{Surface pond energy absorption across four Himalayan Glaciers accounts for 1/8 of total catchment ice loss}}, doi = {10.1029/2018gl079678}, volume = {45}, year = {2018}, } @article{9148, abstract = {Several observational studies have shown a tight relationship between tropical precipitation and column‐integrated water vapor. We show that the observed relationship in the tropics between column‐integrated water vapor, precipitation, and its variance can be qualitatively reproduced by a simple and physically motivated two‐layer model. It has previously been argued that features of this relationship could be explained by analogy with the theory of continuous phase transitions. Instead, our model explicitly assumes that the onset of precipitation is governed by a stability threshold involving boundary‐layer water vapor. This allows us to explain the precipitation‐humidity relationship over a broader range of water vapor values, and may explain the observed temperature dependence of the relationship.}, author = {Muller, Caroline J and Back, Larissa E. and O'Gorman, Paul A. and Emanuel, Kerry A.}, issn = {0094-8276}, journal = {Geophysical Research Letters}, keywords = {General Earth and Planetary Sciences, Geophysics}, number = {16}, publisher = {American Geophysical Union}, title = {{A model for the relationship between tropical precipitation and column water vapor}}, doi = {10.1029/2009gl039667}, volume = {36}, year = {2009}, }