In recent decades, the fringes of the Asian summer monsoon, such as Pakistan and Northeast China, have become hotspots of extreme precipitation. Although such increases are often linked to thermodynamic changes in a warming climate, the dynamical drivers behind these trends, particularly the systematic role of extratropical circulations, remain poorly understood. This study identifies the large-scale atmospheric circulation patterns responsible for intense summertime rainfall in Northeast China and Pakistan. Clustering analysis reveals that the recent increase in intense precipitation is driven by a distinct shift in the preferred Rossby wave pathways over Eurasia. Dynamical analysis with an idealized model indicates that this shift is driven by an evolving upper-tropospheric mean flow which modifies atmospheric instability and wave propagation. These results highlight the need to understand the origins of extratropical background flow changes to improve projections of regional precipitation in a changing climate.

Latent heating (LH) influences the extratropical large-scale circulation, but its dynamical role remains difficult to isolate because moist processes simultaneously affect eddies, the zonal-mean state, and their interaction. Here, we investigate how eddy-LH coupling shapes the extratropical circulation in statistical equilibrium by conducting a series of moist idealized general circulation model experiments in which the coupling between LH and eddies is selectively suppressed while preserving the direct effect of LH on the zonal-mean state. As eddy-LH coupling is progressively suppressed, the effective static stability experienced by eddies approaches the dry static stability. The zonal-mean state concurrently undergoes a systematic reorganization characterized by an enhanced meridional temperature gradient in midlatitudes associated with high-latitude cooling and a poleward intensification of the eddy-driven jet. The meridional temperature gradient increases quasi-linearly with the effective static stability, reminiscent of baroclinic adjustment in a moist atmosphere. Despite the loss of diabatic eddy energy input, storm track activity does not weaken owing to the compensating increase in baroclinic energy conversion associated with the enhanced meridional temperature gradient. The dominant length scale of extratropical eddies increases systematically as eddy-LH coupling weakens, consistent with scaling based on the Rossby deformation radius defined using the effective static stability. These results demonstrate that the role of eddy-LH coupling in the equilibrated extratropical circulation extends beyond its established influence on linear moist baroclinic instability to include the regulation of the background mean state.

Abstract This study examines the performance of ensemble simulations in representing precipitation using the Weather Research and Forecasting (WRF) Model in three recent nor’easter events: 14–15 March 2017, 2–3 March 2018, and 29–30 January 2022. Compared to observations, the model tends to overestimate total precipitation along the eastern New England coast, with varying accuracy across the events. The WRF Model reasonably captures heavy precipitation in the March 2018 event, while it largely overestimates moderate and light precipitation during the March 2017 and January 2022 simulations, respectively. A detailed sensitivity analysis of the March 2018 event using four different microphysics schemes reveals distinct strengths: Morrison most accurately simulates total precipitation amount, Thompson captures peak precipitation intensity best, National Severe Storms Laboratory (NSSL) yields overall reliable performance, and WRF double-moment 6-class (WDM6) achieves the strongest spatial correlation with gridded observations. The multiphysics ensemble approach offers modest improvements that are comparable to the gains achieved by a five-member stochastic ensemble. These findings provide insights for refining nor’easter modeling, ultimately advancing preparedness and risk management strategies in vulnerable coastal regions.

The western Maritime Continent exhibits a pronounced regional diurnal cycle of rainfall, to which the propagating Mesoscale Convective Systems (MCSs) make a significant contribution. Using an automated tracking algorithm based on brightness temperature, we quantify the contribution of MCSs to the regional diurnal cycle over Singapore and the South China Sea. MCSs affecting the Singapore sector show two distinct peaks in the diurnal cycle, one in the early morning and the other in the afternoon. These two diurnal peaks reflect the superposition of two opposing MCS propagation pathways: eastward-propagating events originating from Sumatra and westward-propagating events originating from the South China Sea and Borneo, respectively. These propagation regimes occur under systematically different large-scale wind profiles. Eastward and westward propagation are associated with distinct lower- and mid-tropospheric zonal wind conditions, whereas upper-tropospheric winds provide less separation between the two regimes. Seasonal changes in MCS directionality are consistent with changes in the preferred low-level wind direction. The Madden-Julian Oscillation (MJO) modulates the relative frequency of eastward- and westward-propagating events and thereby influences the regional diurnal cycle through its effect on the environmental wind profiles. These results show that the regional diurnal cycle over the western Maritime Continent is shaped not only by local land-sea thermal contrasts but also by distinct populations of propagating MCSs whose preferred pathways are modulated by the large-scale environment.

Atmospheric rivers cause extreme precipitation and sustain water resources in the western United States. Their occurrence has often been attributed to extratropical cyclones. Here, we apply a recently proposed multiscale index to atmospheric rivers identified from reanalysis data and show that their landfalls in this region are not merely synoptic-scale phenomena but are also driven by large-scale circulation independent of extratropical cyclones. Specifically, quasi-stationary waves with centers of action along the Eurasian and North American coasts form a circum-North Pacific pattern. This large-scale teleconnection pattern channels subtropical moisture toward the U.S. West Coast on intraseasonal timescales, enabling its constructive interference with extratropical cyclone-induced moisture transport. The resulting "intermediate" atmospheric rivers account for up to twice as much winter precipitation as purely synoptic atmospheric rivers and exhibit a stronger correspondence with high-category events. Recognizing this multiscale process will be critical for the improved understanding of their predictability, variability and projected changes.

Synoptically forced uplift plays an important role in the initiation of springtime mesoscale convective systems (MCSs) over the U.S. Great Plains by removing convective inhibition. In this study, we quantitatively diagnose these uplift mechanisms by solving the quasi-geostrophic omega equation in Q-vector form. To provide a process-based assessment, Q vectors are decomposed into shearwise (i.e., along-isentrope) and transverse components (i.e., cross-isentrope), representing the Lagrangian changes in the direction and magnitude, respectively, of the potential temperature gradient following the geostrophic motion. The composite analysis reveals the upper- and lower-tropospheric synoptic-scale weather conditions which have been documented as being favorable for springtime MCS initiation. We find that shearwise ascent, induced by a baroclinically organized (i.e., westward tilted with height) trough-ridge couplet, is the leading contributor to total dynamical ascent. The contribution of an upper-level jet streak, which induces upward motion in its right-entrance region through geostrophic frontogenesis, turns out to be secondary, as indicated by weak transverse ascent. The lower-tropospheric transverse ascent, induced by warm frontogenesis at the exit region of the Great Plains low-level jet, is also limited in magnitude. Furthermore, MCSs initiated under stronger shearwise ascent grow into larger, more precipitation-producing storms. This study underscores the critical role of transient baroclinic wave amplification not only in the genesis but also in the subsequent evolution of springtime MCSs in the central United States.

The westward-propagating convectively coupled equatorial wave (CCEW) variability produced by an idealized general circulation model (GCM) is investigated. The model is a zonally symmetric aquaplanet with a slab ocean. Water vapor in the model may condense and produce latent heating, but there is no parameterization of cloud processes, only a quasi-equilibrium convection scheme. The CCEWs produced by the model are found to be sensitive to the heat capacity of the slab and the strength of surface friction. In spectral space, the westward-propagating precipitation variability in the model is dominated by sharp peaks in spectral power at zonal wavenumbers 5 and 6. These precipitation peaks are situated along the dispersion curve of the Rossby-Haurwitz waves, suggesting a connection between the global Rossby modes and precipitation variability. Composites of these disturbances reveal global circulation patterns that extend into the midlatitudes. The moisture variance budget of these disturbances shows that moisture advection by the global Rossby modes maintains the accompanying moisture signal. This is interpreted as downgradient advection of the background moisture gradient of the intertropical convergence zone. The locations of the precipitation peaks are sensitive to Doppler shifting by the zonal winds; when this Doppler shift becomes too weak, the frequencies of the global Rossby modes become too high to effectively couple to convection. A linearized primitive equation model shows that the presence of vertical shear in the background zonal winds is vital for producing a forced response that resembles the modes produced by the GCM. The forced response of the linear model is optimally located to enhance the original circulation of the global mode.

Spatial footprint, that is, the length scale of precipitation extremes, directly impacts the affected area and flooding [1, 2]. However, a physical framework for analyzing it has yet to be developed. Here, we investigate the seasonal and spatial distribution of the spatial footprint of precipitation extremes, their observed changes, and the underlying physical processes using the observational records of the last four decades. We show that subtropical arid regions are global hotspots for large-scale precipitation extremes, which are triggered mainly by the breaking of planetary-scale waves. The eddy length scales measure the spatial scales at which the weather systems are most prevalent. In the extratropics (poleward of 30◦ N/S), the eddy length scales exhibit a significant positive correlation with the length scale of precipitation extremes. Our analysis indicates that the eddy length scales offer a useful framework to assess the spatial footprint of precipitation extremes in the extratropics. Although precipitation is scarce in arid regions, sudden deluges with large spatial footprints over just a few hours to days can make them highly vulnerable to flash floods. We emphasize that the frameworks for design rainfall estimation [3–5] should account for the spatial footprint of events in addition to the conventional characteristics of intensity, duration, and frequency.

Atmospheric rivers (ARs) play a crucial role in extreme precipitation and water resources management in the western United States. Their occurrence has been widely attributed to extratropical cyclones. Here, we find that landfalling ARs in this region are not merely synoptic-scale phenomena but are also driven by large-scale circulation. Specifically, quasi-stationary Rossby waves with centers of action along the Eurasian and North American coasts form a newly identified circum-North Pacific pattern. This teleconnection pattern channels subtropical moisture toward the U.S. West Coast on intraseasonal timescales, enabling its constructive interference with extratropical cyclone-induced moisture transport. The resulting “intermediate” ARs account for up to twice as much winter precipitation as purely synoptic ARs. Moreover, they exhibit a stronger correspondence with high-category AR events. This previously unrecognized multiscale process deserves greater attention for the skillful prediction of ARs and the improved understanding of their variability and projected changes.

A high-resolution (;50 km) atmospheric model from GFDL showed promise for simulating observed climatological features of mesoscale convective systems (MCSs) despite nonnegligible biases. This study provides a comprehensive evaluation of precipitation and life cycle characteristics of simulated MCSs in the continental United States. The MCSs are tracked using cloud-top brightness temperature as the sole proxy, enabling independent process-based attribution of MCS biases to their cold cloud systems and precipitation. The MCS precipitation is underestimated in the central United States and overestimated in the eastern United States, with biases being more pronounced in summer than in spring. The pattern of MCS precipitation bias differs from that of total precipitation bias, suggesting that its improvement may not necessarily eliminate the long-standing precipitation deficit in the central United States. Most MCS precipitation is explicitly resolved through large-scale cloud processes. The model well reproduces observed distributions of various MCS diagnostics and their interrelations. However, it produces spurious systems producing very weak and spatially unorganized precipitation (referred to as "dry MCS problem"), which is particularly severe in summer. The linear relation between MCS size and precipitating area is also underestimated, even when the dry MCSs are excluded. While the model accurately captures observed parabolic-shaped MCS size evolution, it misrepresents the fractional precipitating area as a monotonic decrease throughout the life cycle. The MCS precipitation is largely overestimated at initiation, followed by a sharp decrease. This reflects overly efficient precipitation in the convective towers during the upscale growth phase of MCSs, which is hypothesized to be responsible for suppressed stratiform precipitation at maturity.

Abstract We examine tropical rainfall from the Geophysical Fluid Dynamics Laboratory's Atmosphere Model version 4 (GFDL AM4) at three horizontal resolutions of 100 km, 50 km, and 25 km. The model produces more intense rainfall at finer resolutions, but a large discrepancy still exists between the simulated and the observed frequency distribution. We use a theoretical precipitation scaling diagnostic to examine the frequency distribution of the simulated rainfall. The scaling accurately produces the frequency distribution at moderate‐to‐high intensity (≥10 mm day−1). Intense tropical rainfall at finer resolutions is produced primarily from the increased contribution of resolved precipitation and enhanced updrafts. The model becomes more sensitive to the grid‐scale updrafts than local thermodynamics at high rain rates as the contribution from the resolved precipitation increases.

This study presents a multiscale assessment of the springtime U.S. Mesoscale Convective Systems (MCSs) in the NOAA Geophysical Fluid Dynamics Laboratory (GFDL)’s Atmosphere Model version 4 (AM4). In AM4, MCSs exhibit lower intensity but longer duration, producing more precipitation compared to observation. The overall MCS activity demonstrates a “location bias” with its peak shifting from the Southern Great Plains to the Midwest in AM4, causing an eastward shift in associated precipitation. However, the dry bias of MCS precipitation over the Great Plains due to this shift is compensated by additional precipitation from amplified extratropical cyclone activities. Further analysis reveals that AM4 effectively reproduces the spatiotemporal distribution and relative frequency contribution of large-scale forcing patterns driving MCS genesis. The MCS location bias emerges under all forms of large-scale forcing patterns and is further attributed to local dynamic and thermodynamic factors including weaker surface lows, eastward-shifted fronts, and suppressed low-level jets (LLJs). Here we argue that the MCS location bias results from AM4 biases in both synoptic-mesoscale anomalies (i.e., fronts and LLJs) and seasonal mean circulations. The lack of two-way air-sea interaction in AM4 creates a hemispheric-scale sea level pressure bias, which is ultimately responsible for a seasonal mean northerly bias in lower-tropospheric winds and the subsequent weakening of LLJs. The existence of such biases in prescribed sea surface temperature (SST) experiments implies the need for extra caution when utilizing extended-range forecasts for MCSs over the continental U.S.

A wide range of aerosol effects on precipitation have been proposed, from the scale of individual clouds to that of the globe.This presentation, based on the findings of an expert workshop under the umbrella of the GEWEX Aerosol Precipitation initiative, reviews the evidence and scientific consensus behind these effects and the underlying set of physical mechanisms, categorised into i) radiative effects via modification of radiative fluxes and the energy balance and ii) microphysical effects via modification of cloud droplets and ice crystals.There exists broad consensus and strong theoretical evidence that, because global mean precipitation is constrained by energetics and surface evaporation, aerosol radiative effects (aerosol-radiation interactions and aerosol-cloud interactions) act as drivers of precipitation changes. Likewise, aerosol radiative effects cause well-documented shifts of large-scale precipitation patterns, such as the Inter-Tropical Convergence Zone (ITCZ). The extent to which aerosol effects on precipitation are applicable at smaller scales and driven or buffered by compensating microphysical and dynamical mechanisms and budgetary constraints is less clear. Although there exists broad consensus and strong evidence that suitable aerosol perturbations increase cloud droplet numbers, reducing the efficiency of warm rain formation across cloud regimes, the overall aerosol effect on cloud microphysics and dynamics as well as the subsequent impact on local, regional and global precipitation is less constrained.This presentation provides a review of the physical mechanisms of aerosol effects on precipitation backed up by evidence from recent cloud-resolving and global modelling simulations as well as from satellite observations.

The Taklamakan and Gobi Desert (TGD) region has experienced a pronounced increase in summer precipitation, including high-impact extreme events, over recent decades. Despite identifying large-scale circulation changes as a key driver of the wetting trend, understanding the relative contributions of internal variability and external forcings remains limited. Here, we approach this problem by using a hierarchy of numerical simulations, complemented by diverse statistical analysis tools. Our results offer strong evidence that the atmospheric internal variations primarily drive this observed trend. Specifically, recent changes in the North Atlantic Oscillation have redirected the storm track, leading to increased extratropical storms entering TGD and subsequently more precipitation. A clustering analysis further demonstrates that these linkages predominantly operate at the synoptic scale, with larger contributions from large precipitation events. Our analysis highlights the crucial role of internal variability, in addition to anthropogenic forcing, when seeking a comprehensive understanding of future precipitation trends in TGD. The Taklamakan and Gobi Desert region has witnessed a significant rise in summer precipitation in recent decades. This study shows that atmospheric internal variability, rather than external forcings, is the main driver shaping this wetting trend.

Here, we show that the Last Glacial Maximum (LGM) provides a stronger constraint on equilibrium climate sensitivity (ECS), the global warming from increasing greenhouse gases, after accounting for temperature patterns. Feedbacks governing ECS depend on spatial patterns of surface temperature (“pattern effects”); hence, using the LGM to constrain future warming requires quantifying how temperature patterns produce different feedbacks during LGM cooling versus modern-day warming. Combining data assimilation reconstructions with atmospheric models, we show that the climate is more sensitive to LGM forcing because ice sheets amplify extratropical cooling where feedbacks are destabilizing. Accounting for LGM pattern effects yields a median modern-day ECS of 2.4°C, 66% range 1.7° to 3.5°C (1.4° to 5.0°C, 5 to 95%), from LGM evidence alone. Combining the LGM with other lines of evidence, the best estimate becomes 2.9°C, 66% range 2.4° to 3.5°C (2.1° to 4.1°C, 5 to 95%), substantially narrowing uncertainty compared to recent assessments.

Marine cloud brightening (MCB) is the deliberate injection of aerosol particles into shallow marine clouds to increase their reflection of solar radiation and reduce the amount of energy absorbed by the climate system. From the physical science perspective, the consensus of a broad international group of scientists is that the viability of MCB will ultimately depend on whether observations and models can robustly assess the scale-up of local-to-global brightening in today’s climate and identify strategies that will ensure an equitable geographical distribution of the benefits and risks associated with projected regional changes in temperature and precipitation. To address the physical science knowledge gaps required to assess the societal implications of MCB, we propose a substantial and targeted program of research—field and laboratory experiments, monitoring, and numerical modeling across a range of scales.

General circulation models' (GCMs) estimates of the liquid water path adjustment to anthropogenic aerosol emissions differ in sign from other lines of evidence. This reduces confidence in estimates of the effective radiative forcing of the climate by aerosol–cloud interactions (ERFaci). The discrepancy is thought to stem in part from GCMs' inability to represent the turbulence–microphysics interactions in cloud-top entrainment, a mechanism that leads to a reduction in liquid water in response to an anthropogenic increase in aerosols. In the real atmosphere, enhanced cloud-top entrainment is thought to be the dominant adjustment mechanism for liquid water path, weakening the overall ERFaci. We show that the latest generation of GCMs includes models that produce a negative correlation between the present-day cloud droplet number and liquid water path, a key piece of observational evidence supporting liquid water path reduction by anthropogenic aerosols and one that earlier-generation GCMs could not reproduce. However, even in GCMs with this negative correlation, the increase in anthropogenic aerosols from preindustrial to present-day values still leads to an increase in the simulated liquid water path due to the parameterized precipitation suppression mechanism. This adds to the evidence that correlations in the present-day climate are not necessarily causal. We investigate sources of confounding to explain the noncausal correlation between liquid water path and droplet number. These results are a reminder that assessments of climate parameters based on multiple lines of evidence must carefully consider the complementary strengths of different lines when the lines disagree.

Coupled ocean and prescribed sea surface temperature (SST) experiments are performed to investigate the drivers of Northern Hemisphere (NH) midlatitude winter circulation and blocking changes in warmer climates. In coupled experiments, a historical simulation is compared to a simulation following an end of the twenty-first-century shared socioeconomic pathway (SSP5-8.5) emission scenario. The SSP5-8.5 simulation yields poleward-shifted jets and an enhanced stationary wave pattern compared to the historical simulation. In terms of blocking, a reduction is found across North America and over the Pacific Ocean with the suggestion of more blocking over parts of Eurasia. Separately, prescribed SST experiments are performed decomposing the SSP5-8.5 SST response into a uniform warming component plus a spatially dependent change in SST pattern. SSP5-8.5 changes in circulation are primarily driven by a uniform warming of SST. Uniform warming is also found to account for most of the SSP5-8.5 blocking reduction over North America and the Pacific Ocean, but not over Eurasia. El Ni & ntilde;o-like changes to the SST pattern also yield less blocking over the Pacific and North America. However, adding the responses of uniform and pattern experiments yields a nonlinear overreduction of blocking compared to the SSP5-8.5 experiment. Regional analyses of block energetics suggest that much of the reductions in blocking in warming simulations are driven by decreased baroclinic conversion in some regions and enhanced dissipation from diabatic sources in others. SIGNIFICANCE STATEMENT: Atmospheric blocks are persistent anticyclones that can cause severe weather such as heat waves and cold spells. Climate models generally project that on a warmer Earth, blocking frequency is poised to decrease in the Northern Hemisphere by the end of the twenty-first century. The cause, however, remains unclear. In this study, we investigate the response of mean atmospheric circulation and atmospheric blocking when separately considering the warming of sea surface temperatures (SST) and changing the SST pattern. We fi nd that most of the reduction in blocking can be explained by a uniform warming of SST. Energetics analyses suggest that this reduction is driven by blocks' inhibited extraction of mean fl ow potential energy in some regions and by enhanced diabatic dissipation in others.