Large-scale Moisture Convergence Research Articles

Extreme Rainfall Risk in Hurricane Ida’s Extratropical Stage: An Analysis with Convection-Permitting Ensemble Hindcasts

Abstract The extratropical stage of Hurricane Ida (2021) brought extreme subdaily rainfall and devastating flooding to parts of eastern Pennsylvania, New Jersey, and New York. We investigate the predictability and character of this event using 31-member ensembles of perturbed initial condition hindcasts with the Tropical Atlantic version of GFDL’s System for High-resolution prediction on Earth-to-Local Domains (T-SHiELD), a ∼13-km global weather forecast model with a ∼3-km nested grid. At lead times of up to 4 days, the ensembles are able to capture the most extreme observed hourly and daily rainfall accumulations but are negatively biased in the spatial extent of heavy precipitation. Large intraensemble differences in the magnitudes and locations of simulated extremes suggest that although impacts were highly localized, risks were widespread. In Ida’s tropical stage, interensemble spread in extreme hourly rainfall is well predicted by large-scale moisture convergence; by contrast, in Ida’s extratropical stage, the most extreme rainfall is governed by mesoscale processes that exhibit chaotic and diverse forms across the ensembles. Our results are relevant to forecasting and communication in advance of extratropical transition and imply that flood preparedness efforts should account for the widespread possibility of severe localized impacts. Significance Statement After making landfall in Louisiana, Hurricane Ida (2021) transitioned to an extratropical storm which brought extreme rainfall and unprecedented flooding to parts of the northeastern United States. To what extent were these impacts knowable in advance? We use a numerical weather model with very high resolution to produce ensemble hindcasts—simulations of a past weather event initialized with tiny perturbations to the initial conditions, representing dozens of equally plausible versions of Ida’s extratropical stage. We find that the observed hourly and daily rainfall maxima fall within the simulated outcomes of ensembles initialized with lead times of about 4 days or less. The location and intensity of the heaviest rainfall vary widely across these ensembles, suggesting that many locations across the Northeast were exposed to some likelihood of extreme rainfall.

Unraveling the Mechanism of Extreme (More than 30 Sigma) Precipitation during August 2018 and 2019 over Kerala, India

AbstractDuring August 2018 and 2019 the southern state of India, Kerala, received unprecedented heavy rainfall, which led to widespread flooding. We aim to characterize the convective nature of these events and the large-scale atmospheric forcing, while exploring their predictability by three state-of-the-art global prediction systems: the National Centers for Environmental Prediction (NCEP)-based India Meteorological Department (IMD) operational Global Forecast System (GFS), the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS), and the Unified Model–based NCUM being run at the National Centre for Medium Range Weather Forecasting (NCMRWF). Satellite, radar, and lightning observations suggest that these rain events were dominated by cumulus congestus and shallow convection with strong zonal flow leading to orographically enhanced rainfall over the Ghats mountain range; sporadic deep convection was also present during the 2019 event. A moisture budget analyses using the fifth major global reanalysis produced by ECMWF (ERA5) and forecast output revealed significantly increased moisture convergence below 800 hPa during the main rain events compared to August climatology. The total column-integrated precipitable water tendency, however, is found to be small throughout the month of August, indicating a balance between moisture convergence and drying by precipitation. By applying a Rossby wave filter to the rainfall anomalies it is shown that the large-scale moisture convergence is associated with westward-propagating barotropic Rossby waves over Kerala, leading to increased predictability of these events, especially for 2019. Evaluation of the deterministic and ensemble rainfall predictions revealed systematic rainfall differences over the Ghats mountains and the coastline. The ensemble predictions were more skillful than the deterministic forecasts, as they were able to predict rainfall anomalies (greater than three standard deviations from climatology) beyond day 5 for August 2019 and up to day 3 for 2018.

Impact of Indian Ocean warming on increasing trend in pre-monsoon rainfall and Hadley circulation over Bay of Bengal

In the present manuscript, we analyzed the pre-monsoon (March–April–May) rainfall over the Indian summer monsoon region using the Global Precipitation Climatology Project (GPCP) rainfall datasets during 1979–2015. It is found that the rainfall over the Bay of Bengal (BOB) has increased significantly and it is associated with the strengthening of local Hadley circulation over the BOB. The increasing Sea Surface Temperature (SST) over the Indian Ocean is playing an important role to the observed changes in convections and cloudiness over the BOB. In addition, the strengthening of the westerly enhances the wind speed and transports more moisture to the North Indian Ocean. In addition, warming of the SST and large-scale moisture convergence cause the rainfall to increase and the latent heat released in the process strengthens the divergent wind at 200 hPa. This strengthens the local Hadley circulation over the BOB, which may affect the monsoon strength over the Indian subcontinent, by influencing the onset of the Indian summer monsoon. This study will improve the understanding by monitoring the changes in the atmospheric processes over the BOB and the Indian subcontinent, which may result in better predictability of the Indian summer monsoon.

A possible mechanism for the occurrence of wintertime extreme precipitation events over South China

This study examines the formation of circulation patterns favorable to wintertime extreme precipitation events over South China between 1979 and 2013. During these extreme precipitation events, a barotropic wave train having seven centers of action was observed to extend from the Arabian Sea to the west coast of the North American continent with a maximum amplitude at 300 hPa. A center of action located over South China, comprised of cyclonic anomalies, favored powerful updrafts and large-scale moisture convergence over South China. About 77% of wintertime extreme precipitation events in South China were preceded by European blocking events. The formation mechanism for the 141 precipitation events with pre-existing European blocking highs is presented. Rossby wave energy propagation associated with the positive phase of circumglobal teleconnection enabled the European blocking event with a lead time of $$\sim$$ 10 days to generate cyclonic anomalies over South China. Moreover, significant warm anomalies were present over South China before the onset of these blocking-related extreme precipitation events. Increases in atmospheric moisture holding capacity associated with these warm anomalies enabled a buildup of precipitable water via moisture fluxes into South China through the western and southern boundaries. Onset of the extreme precipitation events was then triggered by the intrusion of cold temperature anomalies from the north, which lifted warm moist air upward from the surface and lowered the moisture holding capacity, producing large amounts of precipitation.

A process-level attribution of the annual cycle of surface temperature over the Maritime Continent

The annual cycle of the surface temperature over the Maritime Continent (MC) is characterized by two periods of rapid warming in March–April and September–October, respectively, and a period of rapid cooling in June–July. Based upon an analysis of energy balance within individual atmosphere–surface columns, the seasonal variations of surface temperature in the MC are partitioned into partial temperature changes associated with various radiative and non-radiative (dynamical) processes. The seasonal variations in direct solar forcing and surface latent heat flux show the largest positive contributions to the annual cycle of MC surface temperature while the changes in oceanic dynamics (including ocean heat content change) work against the temperature changes related to the annual cycle. The rapid warming in March–April is mainly a result of the changes in atmospheric quick processes and ocean–atmosphere coupling such as water vapor, surface latent heat flux, clouds, and atmospheric dynamics while the contributions from direct solar forcing and oceanic dynamics are negative. This feature is in contrast to that associated with the warming in September–October, which is driven mainly by the changes in solar forcing with a certain amount of contributions from water vapor and latent heat flux change. More contribution from atmospheric quick processes and ocean–atmosphere coupling in March–April coincides with the sudden northward movement of deep convection belt, while less contribution from these quick processes and coupling is accompanied with the convection belt slowly moving southward. The main contributors to the rapid cooling in June–July are the same as those to the rapid warming in March–April, and the cooling is also negatively contributed by direct solar forcing and oceanic dynamics. The changes in water vapor in all three periods contribute positively to the change in total temperature and they are associated with the change in the location of the center of large-scale moisture convergence during the onset and demise stages of the East Asian summer monsoon.

Orographically Anchored El Niño Effect on Summer Rainfall in Central China

Year-to-year variations in summer precipitation have great socioeconomic impacts on China. Historical rainfall variability over China is investigated using a newly released high-resolution dataset. The results reveal summer-mean rainfall anomalies associated with ENSO that are anchored by mountains in central China east of the Tibetan Plateau. These orographically anchored hot spots of ENSO influence are poorly represented in coarse-resolution datasets so far in use. In post–El Niño summers, an anomalous anticyclone forms over the tropical northwest Pacific, and the anomalous southwesterlies on the northwest flank cause rainfall to increase in mountainous central China through orographic lift. At upper levels, the winds induce additional adiabatic updraft by increasing the eastward advection of warm air from Tibet. In post–El Niño summers, large-scale moisture convergence induces rainfall anomalies elsewhere over flat eastern China, which move northward from June to August and amount to little in the seasonal mean.

Super-Clausius–Clapeyron Scaling of Extreme Hourly Convective Precipitation and Its Relation to Large-Scale Atmospheric Conditions

AbstractPresent-day precipitation–temperature scaling relations indicate that hourly precipitation extremes may have a response to warming exceeding the Clausius–Clapeyron (CC) relation; for the Netherlands the dependency on surface dewpoint temperature follows 2 times the CC relation (2CC). The authors’ hypothesis—as supported by a simple physical argument presented here—is that this 2CC behavior arises from the physics of convective clouds. To further investigate this, the large-scale atmospheric conditions accompanying summertime afternoon precipitation events are analyzed using surface observations combined with a regional reanalysis. Events are precipitation measurements clustered in time and space. The hourly peak intensities of these events again reveal a 2CC scaling with the surface dewpoint temperature. The temperature excess of moist updrafts initialized at the surface and the maximum cloud depth are clear functions of surface dewpoint, confirming the key role of surface humidity on convective activity. Almost no differences in relative humidity and the dry temperature lapse rate were found across the dewpoint temperature range, supporting the theory that 2CC scaling is mainly due to the response of convection to increases in near-surface humidity, while other atmospheric conditions remain similar. Additionally, hourly precipitation extremes are on average accompanied by substantial large-scale upward motions and therefore large-scale moisture convergence, which appears to accelerate with surface dewpoint. Consequently, most hourly extremes occur in precipitation events with considerable spatial extent. Importantly, this event size appears to increase rapidly at the highest dewpoint temperature range, suggesting potentially strong impacts of climatic warming.

Effects of air-sea coupling over the North Sea and the Baltic Sea on simulated summer precipitation over Central Europe

This study introduces a new approach to investigate the potential effects of air-sea coupling on simulated precipitation inland over Central Europe. We present an inter-comparison of two regional climate models (RCMs), namely, the COSMO-CLM (hereafter CCLM) and RCA4 models, which are configured for the EURO-CORDEX domain in the coupled and atmosphere-only modes. Two versions of the CCLM model, namely, 4.8 and 5.0, join the inter-comparison being almost two different models while providing pronouncedly different summer precipitation simulations because of many changes in the dynamics and physics of CCLM in version 5.0. The coupling effect on the prominent summer dry bias over Central Europe is analysed using seasonal (JJA) mean statistics for the 30-year period from 1979 to 2009, with a focus on extreme precipitation under specific weather regimes. The weather regimes are compared between the coupled and uncoupled simulations to better understand the mechanism of the coupling effects. The comparisons of the coupled systems with the atmosphere-only models show that coupling clearly reduces the dry bias over Central Europe for CCLM 4.8, which has a large dry summer bias, but not for CCLM 5.0 and RCA4, which have smaller dry biases. This result implies that if the atmosphere-only model already yields reasonable summer precipitation over Central Europe, not much room for improvement exists that can be caused by the air-sea coupling over the North Sea and the Baltic Sea. However, if the atmosphere-only model shows a pronounced summer dry bias because of a lack of moisture transport from the seas into the region, the considered coupling may create an improved simulation of summer precipitation over Central Europe, such as for CCLM 4.8. For the latter, the benefit of coupling varies over the considered timescales. The precipitation simulations that are generated by the coupled system COSTRICE 4.8 and the atmosphere-only CCLM 4.8 are mostly identical for the summer mean. However, the COSTRICE simulations are generally more accurate than the atmosphere-only CCLM simulations if extreme precipitation is considered, particularly under Northerly Circulation conditions, in which the airflow from the North Atlantic Ocean passes the North Sea in the coupling domain. The air-sea feedback (e.g., wind, evaporation and sea surface temperature) and land-sea interactions are better reproduced with the COSTRICE model system than the atmosphere-only CCLM and lead to an improved simulation of large-scale moisture convergence from the sea to land and, consequently, increased heavy precipitation over Central Europe.

Scale Dependence of Land-Atmosphere Interactions in Wet and Dry Regions as Simulated with NU-WRF over the Southwestern and South-Central United States.

Large-scale forcing and land-atmosphere interactions on precipitation are investigated with NASA-Unified WRF (NU-WRF) simulations during fast transitions of ENSO phases from spring to early summer of 2010 and 2011. The model is found to capture major precipitation episodes in the 3-month simulations without resorting to nudging. However, the mean intensity of the simulated precipitation is underestimated by 46% and 57% compared with the observations in dry and wet regions in the southwestern and south-central United States, respectively. Sensitivity studies show that large-scale atmospheric forcing plays a major role in producing regional precipitation. A methodology to account for moisture contributions to individual precipitation events, as well as total precipitation, is presented under the same moisture budget framework. The analysis shows that the relative contributions of local evaporation and large-scale moisture convergence depend on the dry/wet regions and are a function of temporal and spatial scales. While the ratio of local and large-scale moisture contributions vary with domain size and weather system, evaporation provides a major moisture source in the dry region and during light rain events, which leads to greater sensitivity to soil moisture in the dry region and during light rain events. The feedback of land surface processes to large-scale forcing is well simulated, as indicated by changes in atmospheric circulation and moisture convergence. Overall, the results reveal an asymmetrical response of precipitation events to soil moisture, with higher sensitivity under dry than wet conditions. Drier soil moisture tends to suppress further existing below-normal precipitation conditions via a positive soil moisture-land surface flux feedback that could worsen drought conditions in the southwestern United States.

Effects of Irrigation on Summer Precipitation over the United States

Abstract Irrigation’s effects on precipitation during an exceptionally dry summer (June–August 2012) in the United States were quantified by incorporating a novel dynamic irrigation scheme into the Weather Research and Forecasting (WRF) Model. The scheme is designed to represent a typical application strategy for farmlands across the conterminous United States (CONUS) and a satellite-derived irrigation map was incorporated into the WRF-Noah-Mosaic module to realistically trigger the irrigation. Results show that this new irrigation approach can dynamically generate irrigation water amounts that are in close agreement with the actual irrigation water amounts across the high plains (HP), where the prescribed scheme best matches real-world irrigation practices. Surface energy and water budgets have been substantially altered by irrigation, leading to modified large-scale atmospheric circulations. In the studied dry summer, irrigation was found to strengthen the dominant interior high pressure system over the southern and central United States and deepen the trough over the upper Midwest. For the HP and central United States, the rainfall amount is slightly reduced over irrigated areas, likely as a result of a reduction in both local convection and large-scale moisture convergence resulting from interactions and feedbacks between the land surface and atmosphere. In areas downwind of heavily irrigated regions, precipitation is enhanced, resulting in a 20%–100% reduction in the dry biases (relative to the observations) simulated over a large portion of the downwind areas without irrigation in the model. The introduction of irrigation reduces the overall mean biases and root-mean-square errors in the simulated daily precipitation over the CONUS.

Cloud-Resolving Large-Eddy Simulation of Tropical Convective Development and Surface Fluxes

Abstract Cloud-resolving large-eddy simulations (LES) on a 500 km × 500 km periodic domain coupled to a thermodynamic ocean mixed layer are used to study the effect of large-scale moisture convergence M on the convective population and heat and moisture budgets of the tropical atmosphere, for several simulations with M representative of the suppressed, transitional, and active phases of the Madden–Julian oscillation (MJO). For a limited-area model without an imposed vertical velocity, M controls the overall vertical temperature structure. Moisture convergence equivalent to ~200 W m−2 (9 mm day−1) maintains the observed temperature profile above 5 km. Increased convective heating for simulations with higher M is partially offset by greater infrared cooling, suggesting a potential negative feedback that helps maintain the weak temperature gradient conditions observed in the tropics. Surface evaporation decreases as large-scale moisture convergence increases, and is only a minor component of the overall water budget for convective conditions representing the active phase of the MJO. Cold pools generated by evaporation of precipitation under convective conditions are gusty, with roughly double the wind stress of their surroundings. Consistent with observations, enhanced surface evaporation due to cold pool gusts is up to 40% of the mean, but has a small effect on the total moisture budget compared to the imposed large-scale moisture convergence.

Interactions between cumulus convection and its environment as revealed by the MC3E sounding array

This study attempts to understand interactions between midlatitude convective systems and their environments through a heat and moisture budget analysis using the sounding data collected from the Midlatitude Continental Convective Clouds Experiment (MC3E) in central Oklahoma. Distinct large-scale structures and diabatic heating and drying profiles are presented for cases of weaker and elevated thunderstorms as well as intense squall line and supercell thunderstorm events during the campaign. The elevated cell events were nocturnal convective systems occurring in an environment having low convective available potential energy (CAPE) and a very dry boundary layer. In contrast, deeper convective events happened during the morning into early afternoon within an environment associated with large CAPE and a near-saturated boundary layer. As the systems reached maturity, the diagnosed diabatic heating in the latter deep convective cases was much stronger and of greater vertical extent than the former. Both groups showed considerable diabatic cooling in the lower troposphere, associated with the evaporation of precipitation and low-level clouds. The horizontal advection of moisture also played a dominant role in moistening the lower troposphere, particularly for the deeper convective events, wherein the near surface southeasterly flow allows persistent low-level moisture return from the Gulf of Mexico to support convection. The moisture convergence often was present before these systems develop, suggesting a strong correlation between the large-scale moisture convergence and convection. Sensitivity tests indicated that the uncertainty in the surface precipitation and the size of analysis domain mainly affected the magnitude of these analyzed fields rather than their vertical structures.

An Idealized Prototype for Large-Scale Land–Atmosphere Coupling

AbstractA process-based, semianalytic prototype model for understanding large-scale land–atmosphere coupling is developed here. The metric for quantifying the coupling is the sensitivity of precipitation P to soil moisture W, . For a range of prototype parameters typical of conditions found over tropical or summertime continents, the sensitivity measure exhibits a broad minimum at intermediate soil moisture values. This minimum is attributed to a trade-off between evaporation (or evapotranspiration) E and large-scale moisture convergence across the range of soil moisture states. For water-limited, low soil moisture conditions, is dominated by evaporative sensitivity , reflecting high potential evaporation Ep arising from relatively warm surface conditions and a moisture-deficient atmospheric column under dry surface conditions. By contrast, under high soil moisture (or energy limited) conditions, becomes slightly negative as Ep decreases. However, because convergence and precipitation increase strongly with decreasing (drying) moisture advection, while soil moisture slowly saturates, is large. Variation of key parameters is shown to impact the magnitude of , for example, increasing the time scale for deep convective adjustment lowers at a given W, especially on the moist side of the profile where convergence dominates. While the prototype’s applicability for direct quantitative comparison with either observations or models is clearly limited, it nonetheless demonstrates how the complex interplay of surface turbulent and column radiative fluxes, deep convection, and horizontal and vertical moisture transport influences the coupling of the land surface and atmosphere that may be expected to occur in either more realistic models or observations.

Stochastic and deterministic multicloud parameterizations for tropical convection

Despite recent advances in supercomputing, current general circulation models poorly represent the variability associated with organized tropical convection. In a recent study, the authors have shown, in the context of a paradigm two baroclinic mode system, that a stochastic multicloud convective parameterization based on three cloud types (congestus, deep and stratiform) can be used to improve the variability and the dynamical structure of tropical convection. Here, the stochastic multicloud model is modified with a lag type stratiform closure and augmented with an explicit mechanism for congestus detrainment moistening. These modifications improve the representation of intermittent coherent structures such as synoptic and mesoscale convective systems. Moreover, the new stratiform-lag closure allows for increased robustness of the coherent features of the model with respect to the amount of stochastic noise and leading to a multi-scale organization of slowly moving waves envelopes in which short-lived and chaotic convective events persist. Congestus cloud decks dominate the suppressed-dry phase of the wave envelopes. The simulations with the new closure have a higher amount of stochastic noise and result in a Walker type circulation with realistic mean and coherent variability which surpasses results of previous deterministic and stochastic multicloud models in the same parameter regime. Further, deterministic mean field limit equations (DMFLE) for the stochastic multicloud model are considered. Aside from providing a link to the deterministic multicloud parameterization, the DMFLE allow a judicious way of determining the amount of deterministic and stochastic “chaos” in the system. It is shown that with the old stratiform heating closure, the stochastic process accounts for most of the chaotic behavior. The simulations with the new stratiform heating closure exhibit a mixture of stochastic and deterministic chaos. The highly chaotic dynamics in the simulations with congestus detrainment mechanism is due to the strongly nonlinear and numerically stiff deterministic dynamics. In the latter two cases, the DMFLE can be viewed as a “standalone” parameterization, which is capable of capturing some dynamical features of the stochastic parameterization. Furthermore, it is shown that, in spatially extended simulations, the stochastic multicloud model can capture qualitatively two local statistical features of the observations: long and short auto-correlation times of moisture and precipitation, respectively and the approximate power-law in the probability density of precipitation event size for large precipitation events. The latter feature is not reproduced in the column simulations. This fact underscores the importance of gravity waves and large scale moisture convergence.

Investigating the Mechanisms of Diurnal Rainfall Variability Using a Regional Climate Model

Abstract This study investigates the ability of a regional climate model (RCM) to simulate the diurnal cycle of precipitation over southeast Australia, to provide a basis for understanding the mechanisms that drive diurnal variability. When compared with 195 observation gauges, the RCM tends to simulate too many occurrences and too little intensity for precipitation events at the 3-hourly time scale. However, the overall precipitation amounts are well simulated and the diurnal variability in occurrences and intensities are generally well reproduced, particularly in spring and summer. In terms of precipitation amounts, the RCM overestimated the diurnal cycle during the warmer months but was reasonably accurate during winter. The timing of the maxima and minima was found to match the observed timings well. The spatial pattern of diurnal variability in the Weather Research and Forecasting model outputs was remarkably similar to the observed record, capturing many features of regional variability. The RCM diurnal cycle was dominated by the convective (subgrid scale) precipitation. In the RCM the diurnal cycle of convective precipitation over land corresponds well to atmospheric instability and thermally triggered convection over large areas, and also to the large-scale moisture convergence at 700 hPa along the east coast, with the strongest diurnal cycles present where these three mechanisms are in phase.

The MJO and Convectively Coupled Waves in a Coarse-Resolution GCM with a Simple Multicloud Parameterization

Abstract The adequate representation of the dominant intraseasonal and synoptic-scale variability in the tropics, characterized by the Madden–Julian oscillation (MJO) and convectively coupled waves, is still problematic in current operational general circulation models (GCMs). Here results are presented using the next-generation NCAR GCM—the High-Order Methods Modeling Environment (HOMME)—as a dry dynamical core at a coarse resolution of about 167 km, coupled to a simple multicloud parameterization. The coupling is performed through a judicious choice of heating vertical profiles for the three cloud types—congestus, deep, and stratiform—that characterize organized tropical convection. Important control parameters that affect the types of waves that emerge are the background vertical gradient of the moisture and the stratiform fraction in the multicloud parameterization, which set the strength of large-scale moisture convergence and unsaturated downdrafts in the wake of deep convection, respectively. Three numerical simulations using different moisture gradients and different stratiform fractions are considered. The first experiment uses a large moisture gradient and a small stratiform fraction and provides an MJO-like example. It results in an intraseasonal oscillation of zonal wavenumber 2, moving eastward at a constant speed of roughly 5 m s−1. The second uses a weaker background moisture gradient and a large stratiform fraction and yields convectively coupled Rossby, Kelvin, and two-day waves, embedded in and interacting with each other; and the third experiment combines the small stratiform fraction and the weak background moisture gradient to yield a planetary-scale (wavenumber 1) second baroclinic Kelvin wave. While the first two experiments provide two benchmark examples that reproduce several key features of the observational record, the third is more of a demonstration of a bad MJO model solution that exhibits very unrealistic features.

Impact of land surface processes on the South American warm season climate

The present study demonstrates that (1) the simulation of the South American warm season (December–February) climate by an atmospheric general circulation model (AGCM) is sensitive to the representation of land surface processes, (2) the sensitivity is not confined to the “hot spot” in Amazonia, and (3) upgrading the representation of those processes can produce a significant improvement in AGCM performance. The reasons for sensitivity and higher success are investigated based on comparisons between observational datasets and simulations by the AGCM coupled to either a simple land scheme that specifies soil moisture availability or to the Simplified Simple Biosphere Model (SSiB) that allows for consideration of soil and vegetation biophysical process. The context for the study is the UCLA AGCM. The most notable simulation improvements are along the lee of the Andes in the lower troposphere, where poleward flow transports abundant moisture from the Amazon basin to high latitudes, and in the monsoon region where the intensity and pattern of precipitation and upper level ice water content are more realistic. It is argued that a better depiction of the Chaco Low, which is controlled by local effects of land surface processes, decisively contributes to the superior model performance with low-level flows in central South America. The better representation of the atmospheric column static stability and large-scale moisture convergence in tropical South America contribute to more realistic precipitation over the monsoon region. The overall simulation improvement is, therefore, due to a combination of different regional processes. This finding is supported by idealized AGCM experiments.

A Numerical Study to Investigate the Relationship between Moisture Convergence Patterns and Orography in Central Mexico*

Abstract This study examines small-scale orographic effects on atmospheric moisture convergence at the ridge–valley scale in the Grande de Santiago River basin in central Mexico during a major monsoon storm on 13–14 August 1999. The simulation was performed using a coupled land–cloud resolving model on three nested grids (at 12-, 3-, and 1-km resolutions). The specific objective is to investigate the physical mechanisms that explain the regional space–time organization of orographic precipitation and cloudiness identified in the region from satellite data. The overarching goals of the research were 1) to characterize the effects of landform and topography-flow geometry relationships on the spatial distribution of precipitation and clouds, especially with regard to the role of mountain winds and lateral drainage flows, and 2) to assess the influence of land–atmosphere interactions (specifically latent and sensible heat fluxes) on moisture convergence patterns during monsoon storms. The model results indicate that large-scale moisture convergence dominates the distribution of total water in the troposphere during monsoon storms, which is modulated by topographically induced gravity waves and thermodynamic gradients associated with the land–sea contrast in the coastal zone. In the Grande de Santiago River basin, mountain–plain differences in thermodynamic response control mesoscale moisture convergence patterns leading to nocturnal buildup in the valleys. At the ridge–valley scale, strong convergence and strong winds (up to 15 m s−1) occur on the lee side of ridges oriented perpendicularly to the impinging synoptic flow with the development of transient flow conditions (from supercritical to subcritical) in the valley, independently of the time of day. In turn, this localized hydraulic-jump-like circulation drives strong return winds (i.e., cold outflows in the downvalley direction) that converge in the central lowlands preceding nighttime rainfall, lifting warm moist air at the mouth of the valley, thus initiating nocturnal convection. Simulated moisture convergence patterns are clustered along the ridges and against the foot slopes at the outlet of the north–south oriented catchments consistent with the space–time distribution of satellite observations of precipitation and clouds in the region overall, and with precipitation features detected during the simulated event in particular.

Regional Simulation of Intraseasonal Variations in the Summertime Hydrologic Cycle over the Southwestern United States

Abstract Using results taken from a finescale (25 km) regional modeling simulation for the summer of 1999, intraseasonal variations in the climatological summertime hydrologic cycle over the southwestern United States are described for two previously identified spatiotemporal precipitation patterns. Over the western portion of the Rocky Mountain plateau, centered on eastern Utah and western Colorado, columnar moisture divergence associated with precipitation is balanced by a combination of seasonal-mean convective moisture convergence and anomalous upper-air (>4 km) large-scale moisture convergence. The actual precipitating events themselves are predicated upon the anomalous upper-level advection of water vapor into the precipitating region; absent this large-scale advection at upper levels, vertical diffusion of moisture into the atmosphere balances large-scale divergence at midlevels, with little precipitation occurring. The anomalous large-scale advection during precipitating events is due primarily to...

The Role of Surface Heat and Moisture Fluxes Associated with Large-Scale Ocean Current Meanders in Maritime Cyclogenesis

Abstract A structurally simple analytic quasi-geostrophic model is used to investigate the role of diabatic process resulting from surface fluxes of heat and moisture associated with ocean current meanders in midlatitude maritime cyclogenesis. The combined effect of sensible heat transfer and convective latent heating resulting both from surface water vapor fluxes and large-scale moisture convergence can contribute both directly and indirectly to cyclogenesis, the extent of the contribution being dependent on a number of cooperative processes interacting within the cyclone. Latent and sensible heating associated with shallow convection in a moist adiabatic environment in which the large scale convergence supplies ample moisture to the cumulus clouds represent the conditions under which the greatest enhancement of adiabatic model cyclogenesis occurs; but cyclogenesis induced by differential vorticity advection is 90° out of phase (with respect to the Large-scale tropospheric thermal wave) with direct diaba...