Moisture Budget Research Articles

Teleconnections between the Atlantic Multidecadal Oscillation and eastern China summer precipitation during the Medieval Climate Anomaly and Little Ice Age

The teleconnection between the Atlantic Multidecadal Oscillation (AMO) and eastern China summer precipitation (ECSP) for the modern period has been emphasized in the past few decades. We test its stability during the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) based on the composite results of the median of six models under the Paleoclimate Modeling Intercomparison Project Phase3 (PMIP3) framework. During the MCA, ECSP generally increases in positive AMO phases relative to negative phases, similar to the modern teleconnection, while during the LIA, the precipitation tends to reduce over eastern China, especially in the Yangtze River basin. Decomposing the precipitation change on the basis of a diagnostic moisture budget manifests that the different AMO-related precipitation changes stem from distinct effects of circulation-induced moisture convergence during the two periods. Compared with negative AMO phases, positive AMO phases during the MCA show an anomalous lower-level cyclone and upper-level anticyclone over eastern China that facilitate the upward motion anomaly and precipitation excess. During the LIA, a barotropic anticyclone centers in Northeast China and weakens the high-level westerlies over eastern China; this favors descending and upper-level divergence anomalies and leads to precipitation decreases. The distinct convergence changes are determined by differing propagation paths of the AMO-induced teleconnection wave train during the two periods.

Improved Simulation of Monsoon Depressions and Heavy Rains From Direct and Indirect Initialization of Soil Moisture Over India

AbstractThis study investigates the impact of direct versus indirect initialization of soil moisture (SM) and soil temperature (ST) on monsoon depressions (MDs) and heavy rainfall simulations over India. SM/ST products obtained from high‐resolution, land data assimilation system (LDAS) are used in the direct initialization of land surface conditions in the ARW modeling system. In the indirect method, the initial SM is sequentially adjusted through the flux‐adjusting surface data assimilation system (FASDAS). These two approaches are compared with a control experiment (CNTL) involving climatological SM/ST conditions for eight MDs at 4‐km horizontal resolution. The surface fields simulated by the LDAS run showed the highest agreement, followed by FASDAS for relatively dry June cases, but the error is high (~15–30%) for the relatively wet August cases. The moisture budget indicates that moisture convergence and local influence contributed more to rainfall. The surface‐rainfall feedback analysis reveals that surface conditions and evaporation have a dominant impact on the rainfall simulation and these couplings are notable in LDAS runs. The contiguous rain area (CRA) method indicates better performance of LDAS for very heavy rainfall distribution, and the location (ETS > 0.2), compared to FASDAS and CNTL. The pattern error contributes the maximum to the total rainfall error, and the displacement error is more in August cases' rainfall than that in June cases. Overall analyses indicated that the role of land conditions is significantly high in the drier month (June) than a wet month (August), and direct initialization of SM/ST fields yielded improved MD and heavy rain simulations.

Mechanisms of Winter Precipitation Variability in the European–Mediterranean Region Associated with the North Atlantic Oscillation

AbstractThe physical mechanisms whereby the mean and transient circulation anomalies associated with the North Atlantic Oscillation (NAO) drive winter mean precipitation anomalies across the North Atlantic Ocean, Europe, and the Mediterranean Sea region are investigated using the European Centre for Medium-Range Weather Forecasts interim reanalysis. A moisture budget decomposition is used to identify the contribution of the anomalies in evaporation, the mean flow, storm tracks and the role of moisture convergence and advection. Over the eastern North Atlantic, Europe, and the Mediterranean, precipitation anomalies are primarily driven by the mean flow anomalies with, for a positive NAO, anomalous moist advection causing enhanced precipitation in the northern British Isles and Scandinavia and anomalous mean flow moisture divergence causing drying over continental Europe and the Mediterranean region. Transient eddy moisture fluxes work primarily to oppose the anomalies in precipitation minus evaporation generated by the mean flow, but shifts in storm-track location and intensity help to explain regional details of the precipitation anomaly pattern. The extreme seasonal precipitation anomalies that occurred during the two winters with the most positive (1988/89) and negative (2009/10) NAO indices are also explained by NAO-associated mean flow moisture convergence anomalies.

Relative Importance of Greenhouse Gases, Sulfate, Organic Carbon, and Black Carbon Aerosol for South Asian Monsoon Rainfall Changes

AbstractThe contribution of individual aerosol species and greenhouse gases to precipitation changes during the South Asian summer monsoon is uncertain. Mechanisms driving responses to anthropogenic forcings need further characterization. We use an atmosphere‐only climate model to simulate the fast response of the summer monsoon to different anthropogenic aerosol types and to anthropogenic greenhouse gases. Without normalization, sulfate is the largest driver of precipitation change between 1850 and 2000, followed by black carbon and greenhouse gases. Normalized by radiative forcing, the most effective driver is black carbon. The precipitation and moisture budget responses to combinations of aerosol species perturbed together scale as a linear superposition of their individual responses. We use both a circulation‐based and moisture budget‐based argument to identify mechanisms of aerosol and greenhouse gas induced changes to precipitation and find that in all cases the dynamic contribution is the dominant driver to precipitation change in the monsoon region.

Impacts of TIPEX‐III Rawinsondes on the Dynamics and Thermodynamics Over the Eastern Tibetan Plateau in the Boreal Summer

AbstractObservations and simulations have shown that the Tibetan Plateau (TP) plays an important role in the climate and weather of East Asia. Uncertainties in reanalysis data sets have been documented by comparing them with independent rawinsonde data, but the impacts of incorporating this data into a reanalysis data set on the dynamical and thermodynamical features over the TP have not been assessed. Observations from the recent Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX‐III) can be used to evaluate the Eastern TP ERA‐Interim reanalysis data set (ERA). In this study, the Barnes objective analysis scheme is used to incorporate the rawinsonde data into the ERA to acquire the gridded data (OBJ) for examining heat and moisture budgets. It is shown that the OBJ budget‐estimated rainfall intensity is better correlated with the observations than the ERA budget‐estimated rainfall intensity. The ERA overestimates weak rainfall but underestimates heavy rainfall. The incorporation of special rawinsonde into the OBJ impacts the dynamical and thermodynamical characteristics both spatially and temporally. The downward motion and cooling are enhanced over the rainless regions/periods, while the upward motion and heating are enhanced over the rainy regions/periods. The rawinsonde effect is stronger with the increasing rainfall intensity. The differences of spatial distributions and temporal variations in heating between the OBJ and ERA are primarily determined by the vertical advection terms in heat and moisture budgets. The OBJ‐derived large‐scale forcing is essential for cloud‐resolving model simulations of TP cloud systems. The above conclusions can also be applied to the ERA5.

North Atlantic Oscillation Effect on Interannual Variability in Winter Precipitation over the Gulf Stream

AbstractIn winter, the warm water of the Gulf Stream anchors a salient precipitation band. Previous studies suggested a close relationship between the sea surface temperature (SST) front and the precipitation band through sea level pressure (SLP) adjustment. This study uses 17 years of high-resolution precipitation observations to reveal that the variation in wintertime precipitation over the Gulf Stream is related to the North Atlantic Oscillation (NAO) at the interannual time scale. The moisture budget analysis shows that the climatological precipitation band is supported by the large evaporation from the Florida Current, mean flow, and synoptic moisture convergence within the boundary layer, with a negative contribution from mean-flow moisture advection by the prevailing northwesterlies. For interannual variability, by contrast, the negative contribution of mean-flow moisture advection significantly decreases due to anomalous southeasterlies west of the intensified Azores high at the positive NAO phase. The contributions from mean-flow moisture advection and mean and synoptic convergence vary greatly along the Gulf Stream. In addition, mean-flow and synoptic moisture convergences positively contribute to the precipitation band both in climatology and at the interannual time scale, indicative of a positive feedback between precipitation and boundary layer convergence. Our analysis suggests that the SLP adjustment mechanism across the SST front is still at work in interannual variability, and the variation of synoptic activities over the Gulf Stream plays an important role in modulating the frontal precipitation. By relating the frontal precipitation to the NAO, this study bridges small-scale air–sea interaction and large-scale atmospheric circulation.

Impacts of Insolation and Soil Moisture on the Seasonality of Interactions Between the Madden‐Julian Oscillation and Maritime Continent

AbstractThis study investigates the seasonality of the interaction of the Madden‐Julian Oscillation (MJO) with the Maritime Continent (MC). Beside their seasonal east‐west migration with the monsoons, observations show that the MJO amplitude and precipitation over the MC islands exhibit semiannual variability, with apparent strengthening of MJO signal during March and September, when solar insolation is strongest over the MC region. Furthermore, during the high‐insolation months, soil moisture shows wetter conditions during the early phases of the MJO. Motivated by these results, a series of regional convection permitting simulations are performed for the November 2014 MJO event as a case study under the following conditions: (i) increased solar insolation to mimic the local summer, (ii) reduced initial soil moisture, and (iii) the two conditions combined to isolate their local effects from their impacts on the large‐scale circulation. Results show that increased insolation increases precipitation including that associated with MJO moisture convergence over the MC region. On the other hand, decreased initial soil moisture slightly increases precipitation over the MC islands and reduces it over the surrounding waters, but such effect is short lived as the increased precipitation gradually restores the soil moisture to wetter conditions. Using a moisture budget analysis that isolates the MJO and non‐MJO signals and additional idealized simulations, the MJO response to high insolation is demonstrated to be related to an increase in the basic state moisture over the MC region rather than a direct response of the MJO to the insolation.

The atmospheric moisture budget in the Mediterranean: Mechanisms for seasonal changes in the Last Glacial Maximum and future warming scenario

We have investigated the sensitivity of the Mediterranean atmospheric moisture budget (P-E) to large global mean temperature changes and the mechanisms for its seasonal changes. Model simulations of the Last Glacial Maximum (LGM) and Representative Concentration Pathway rcp8.5 scenario have been compared to pre-Industrial (PI) conditions. We have decomposed (P-E) in thermodynamic, dynamic, transient eddy and surface term. The decomposition revealed that mechanisms for (P-E) changes in the Mediterranean do not follow a simple scaling with temperature from the cold LGM to the warm rcp8.5 and they cannot be described by the thermodynamic contribution associated with the “wet-get-wetter, dry-get-drier” paradigm. Winter changes are caused by the anomalous mean atmospheric circulation. In summer, the thermodynamics provides the largest contribution, but its effect is counteracted by other mechanisms. The Mediterranean response to climate change differs from the averaged behavior of other regions in the same latitudinal belt. The complex spatial interplay of the atmospheric moisture budget contributions results in reduced precipitation in the rcp8.5 and reduced evaporation in the LGM relative to PI. This analysis shows that past (LGM) and future (rcp8.5) atmospheric moisture budget changes have very different basic mechanisms, which cannot be simply linked to changes of global mean temperature.

Impacts of Storm Track Variations on Wintertime Extreme Precipitation and Moisture Budgets over the Ohio Valley and Northwestern United States

AbstractPrevious studies have shown that variations in extratropical cyclone activity significantly affect the frequency of extreme precipitation events over the Ohio Valley and northwestern United States. In this study, we examine the similarities and differences between the dynamics governing these events in these two regions. In the Ohio Valley, extreme precipitation events are associated with midlatitude synoptic-scale convergence northeast of cyclones and a southwestward oriented ridge near the Atlantic coast that drives strong water vapor transport from the Gulf of Mexico into the Ohio Valley. In the northwestern United States, extreme precipitation events are associated with a cyclonic and anticyclonic circulation pair aligned northwest to southeast, which together drive a long and strong moisture transport corridor from the lower latitude of the central Pacific Ocean toward the northwestern United States. Moisture budget analysis shows that moisture convergence due to dynamical convergence dominates in the Ohio Valley, whereas moisture advection dominates over the Pacific Northwest. Differences between the cases in the same region are examined by an empirical orthogonal function (EOF) analysis conducted on the vertically integrated moisture flux. Different EOFs highlight shifts in spatial location, orientation, and intensity of the moisture flux but demonstrate consistent roles of dynamics in the two regions. Composites based on these EOFs highlight the range of likely synoptic scenarios that can give rise to precipitation extremes over these two regions.

Investigating the Spatio-Temporal dynamics in the soil water storage in Alberta’s Agricultural region

Anomalous atmospheric conditions which, for the most part, manifest in the form of heavy precipitation events leading to the generation of pluvial or fluvial flooding in some years, or as extended extreme dry spells resulting in the development of severe drought conditions in other years has significant implications for subsurface moisture budgets in regional and local catchments in the Canadian Prairies. The myriad implications of these frequent and extended dry spells over Alberta’s Agricultural Region can be far reaching economically, socially and otherwise, thereby exerting substantial negative toll on agricultural productivity (crop cultivation, livestock husbandry, and pasture management) within the Province. Soil moisture dynamics both in spatial and temporal scales has critical implications for agricultural water management especially under semi-arid climate conditions that are typical of the local- and regional-scale catchments in southern Alberta. This study is focused on the assessment of the observations of the soil water storage, soil temperature and meteorological variables (precipitation and air temperature) acquired from 39 monitoring stations spanning a 12-year period with the principal objective of examining the spatial and temporal dynamics in the shallow and deep soil water storage processes across the study domain. This study revealed that the moisture anomalies in the deep soil moisture store exhibit the longest hydrologic memory relative to those in the shallow and intermediate soil moisture storages as well as those in the meteorological variables across the study domain. The anomalies computed for the soil moisture measurements undertaken in the deep soil moisture store revealed frequency features that are dominant at seasonal cycle (9-month) and at a higher periodicity of 32-month cycle. This low-frequency feature (32-month cycle) is evidently non-existent in the soil water storage measured at the shallower soil depths nor in the meteorological variables evaluated in this study. This implies that the low-frequency feature of the soil water storage wavelet spectra increases with increasing soil depth across the Agricultural Region; indicating that at deeper soil depths, the largest variances are shifted to lower frequencies. This observed dominant long-memory feature in the deep soil moisture store could have important implications for characterizing the development of temperature extremes as well as for the evaluation of the severity of the recurrent drought outbreaks over regional watersheds in Alberta and potentially for other Canadian Prairie catchments.

Impacts of the stochastic multicloud parameterization on the simulation of Western North Pacific summer rainfall

This study investigated the sensitivity of the western North Pacific (WNP) summer precipitation to the convection schemes and discussed the associated dynamical processes. Two convection schemes were compared: one is the default mass-flux convection scheme used in the state-of-the-art ECHAM6.3 atmosphere model and the other incorporates the Stochastic Multicloud Model (SMCM) into ECHAM6.3. Incorporation of the SMCM reduces the bias of cloud cover and shortwave and longwave radiation by regulating the shortwave and longwave cloud radiative forcing over the WNP. Compared to the default model, the modified model with the SMCM alleviates the dry bias in the WNP, which is associated with enhanced ascending motion. The moist static energy balance revealed that improved simulation of precipitation in the modified model is contributed by enhanced horizontal advection of moist enthalpy and increased net energy in the atmosphere, which is attributed to increased total cloud cover, over the WNP. Additionally, intensified latent energy advection over the WNP dominates enhanced horizontal advection of moist enthalpy in the modified model. On the other hand, the moisture budget analysis of the WNP demonstrated that strengthened convergence of moisture flux in the modified model plays the most influential role in reducing precipitation bias. Further analysis unraveled that enhanced zonal-mean moisture transported by the stationary eddy zonal flow convergence in the WNP dominates intensified zonal moisture convergence, thus increased horizontal convergence of moist flux in the modified model.

The changes in ENSO-induced tropical Pacific precipitation variability in the past warm and cold climates from the EC-Earth simulations

The El Niño-Southern Oscillation (ENSO) is one of the most significant climate variability signals. Studying the changes in ENSO-induced precipitation variability (ENSO precipitation) in the past climate offers a possibility to a better understanding of how they may change under future climate conditions. This study uses simulations performed with the European community Earth-System Model (EC-Earth) to investigate the relative contributions of dynamic effect (the circulation anomalies together with the climatological specific humidity) and thermodynamic effect (the specific humidity anomalies together with the climatological circulation) on the changes in ENSO precipitation in the past warm and cold climates, represented by the Pliocene and the Last Glacial Maximum (LGM), respectively. The results show that the changes in ENSO precipitation are intensified (weakened) over the tropical western Pacific but weakened (intensified) over the tropical central Pacific in Pliocene (LGM) compared with the pre-industrial (PI) simulation. Based on the decomposed moisture budget equation, these changes in ENSO precipitation patterns are highly related to the dynamic effect. The mechanism can be understood as follows: the zonal gradient of the mean sea surface temperature (SST) over the tropical Indo-Pacific is increased (reduced) during the Pliocene (LGM), leading to the strengthening (weakening) of Pacific Walker Circulation as well as a westward (eastward) shift. In the Pliocene, the westward shift of Walker Circulation results in an increased (decreased) ENSO-induced low-level vertical velocity variability in the tropical western Pacific (central Pacific), and, in turn, favoring convergent (divergent) moisture transport through a dynamic process, and then causing intensified (weakened) ENSO precipitation there. The opposite mechanism exists in LGM. These results suggest that changes in the zonal SST gradient over tropical Indo-Pacific under different climate conditions determine the changes in ENSO precipitation through a dynamic process.

A tale of two futures: contrasting scenarios of future precipitation for West Africa from an ensemble of regional climate models

The results of a large ensemble of regional climate models lead to two contrasting but plausible scenarios for the precipitation change over West Africa, one where mean precipitation is projected to decrease significantly over the Gulf of Guinea in spring and the Sahel in summer, and the other where summer precipitation over both regions is projected to increase.Dry and wet models show similar patterns of the dynamic and thermodynamic terms of the moisture budget, although their magnitudes are larger in the dry models. The largest discrepancies are found in the strength of the land-atmosphere coupling, with dry models showing a marked decrease in soil moisture and evapotranspiration.Some changes in precipitation characteristics are consistent for both sets of models. In particular, precipitation frequency is projected to decrease in spring over the Gulf of Guinea and in summer over the Sahel, but precipitation is projected to become more intense.

The dynamic and thermodynamic processes dominating the reduction of global land monsoon precipitation driven by anthropogenic aerosols emission

Changes in monsoon precipitation have profound social and economic impacts as more than two-thirds of the world’s population lives in monsoon regions. Observations show a significant reduction in global land monsoon precipitation during the second half of the 20th century. Understanding the cause of this change, especially possible anthropogenic origins, is important. Here, we compare observed changes in global land monsoon precipitation during 1948–2005 with those simulated by 5 global climate models participating in the Coupled Model Inter-comparison Project-phase 5 (CMIP5) under different external forcings. We show that the observed drying trend is consistent with the model simulated response to anthropogenic forcing and to anthropogenic aerosol forcing in particular. We apply the optimal fingerprinting method to quantify anthropogenic influences on precipitation and find that anthropogenic aerosols may have contributed to 102% (62–144% for the 5–95% confidence interval) of the observed decrease in global land monsoon precipitation. A moisture budget analysis indicates that the reduction in precipitation results from reduced vertical moisture advection in response to aerosol forcing. Since much of the monsoon regions, such as India and China, have been experiencing rapid developments with increasing aerosol emissions in the past decedes, our results imply a further reduction in monsoon precipitation in these regions in the future if effective mitigations to reduce aerosol emissions are not deployed. The observed decline of aerosol emission in China since 2006 helps to alleviate the reducing trend of monsoon precipiptaion.

Influence of Indian Ocean SST regionality on the East African short rains

Three sub-regions of the Indian Ocean in which SSTs significantly influence the equatorial East African short rains on interannual timescales are identified, and the physical processes of this influence are studied using regional climate model simulations from the Weather Research and Forecasting model (WRF). Five 20-year ensemble integrations are generated to represent a control climate and to simulate the individual and combined effects of SSTAs in the influential regions. SSTAs in the western Indian Ocean exert a stronger influence on the equatorial East African short rains than central and eastern Indian Ocean SSTAs both in terms of the coverage of significantly-changed precipitation and the magnitude of the precipitation response. Positive western Indian Ocean SSTAs significantly increase the short rains over 95% of the equatorial East Africa domain (30°–40°E, 5°S–5°N), while only 30% of the region responds to central and eastern Indian Ocean SSTAs. Evidence of an influential Indian Ocean dipole mode does not emerge from the analysis. The mechanisms of this influence are diagnosed using atmospheric moisture budget and moist static energy analyses, with reference to Kelvin and Rossby wave generation as in the Gill model, but in the presence of complicated topography and nonzero background flows. Wind convergence anomalies in a moist environment primarily support precipitation anomalies in all cases, while changes in atmospheric instability are largely controlled by low-level moisture. Central and eastern Indian Ocean SSTAs change circulations and precipitation locally, but the remote influence on East Africa is weaker than that of the western Indian Ocean SSTAs.

Recent Changes in Heavy Precipitation Events in Northern Central China and Associated Atmospheric Circulation

Northern Central China (NC China) is a seriously arid region. Precipitation variations are vital for environmental protection and socio-economical development. This study investigates the recent changes in summer heavy precipitation (HP) events over NC China and associated atmospheric circulation anomalies. Compared with the period of 1986–2002, the HP amount and frequency both significantly increase during 2003–2016, contributing to the shift to increase in total summer precipitation amount. After the early 2000s, the Northwest Pacific subtropical high becomes intensified and shifts westward and northward, leading to significant moisture convergence anomalies over NC China and divergence anomalies over Northwest Pacific. Therefore, the net moisture budget dramatically increases since the early 2000s. Further results show that the strengthened net moisture influx across the southern boundary contributes dominantly to the increased net moisture budget, especially at the lower level. Despite a relatively small magnitude, the intensified westerly current across western boundary makes a dominant contribution at middle and upper layers. Additionally, the strengthened westerly and easterly anomalies occupy northern and southern China, respectively, along with the eastward expansion of westerly jet stream region. Thus, the lower-level convergence and upper-level divergence both intensify, and further trigger enhanced ascending movement. These conditions jointly contribute to the inter-decadal change in HP events over NC China after the early 2000s.

Intensity Change of Typhoon Nancy (1961) during Landfall in a Moist Environment over Japan: A Numerical Simulation with Spectral Nudging

Abstract Intensity change of tropical cyclones (TCs) as they make landfall is closely linked to sustained periods of high surface winds and heavy precipitation. Few studies have investigated the intensity change of intense TCs that make landfall in middle latitudes such as Japan because few intense typhoons make landfall in middle latitudes. In this study, a numerical simulation of intense Typhoon Nancy (1961) was used to understand the intensity change that occurred when Nancy made landfall in Japan. A spectral nudging technique was introduced to reduce track errors in the simulation. During landfall, the simulated storm exhibited the salient asymmetric structure and rapid eyewall contraction. A tangential wind budget indicated that the maximum wind speed decreased concurrent with an increase in surface friction and advection associated with low-level asymmetric flows. Detailed evolution of the storm’s warm core was analyzed with a potential temperature budget. In the upper part of the warm core centered at a 12-km height, cooling due to ventilation by asymmetric flows and longwave radiation overcame heating due to condensation and shortwave radiation during the contraction of eyewall clouds. In the lower part of the warm core, adiabatic cooling more than offset warm-air intrusions associated with asymmetric flows and condensational heating. The condensation was supplied by an abundance of moisture due to evaporation from the ocean in the well-developed typhoon based on a moisture budget. Sensitivity experiments revealed that environmental baroclinicity in the midlatitudes, orography, and radiative processes made minor contributions to the weakening. The weakening was instead controlled by inner-core dynamics and interactions with land surfaces.

Mechanisms for Spatially Inhomogeneous Changes in East Asian Summer Monsoon Precipitation during the Mid-Holocene

AbstractThe East Asian summer monsoon (EASM) intensified during the early to mid-Holocene relative to the present primarily due to orbital forcing. However, on the regional scale, changes in the monsoonal precipitation exhibit considerable spatial disparity, and the underlying mechanisms remain unresolved. In this study, the dynamic processes responsible for the difference of the EASM precipitation between the mid-Holocene and preindustrial period are systematically examined using the CMIP5 multimodel simulations. The moisture budget diagnostic identifies vertical motion as the key factor determining the cross-like precipitation pattern in East Asia. Relative to the preindustrial period, the mid-Holocene anomalous ascending motion corresponds well with the excessive precipitation over northern and southern China, and vice versa for west-central China, the Korean peninsula, Japan, and its marginal seas. In the framework of the moist static energy budget, the increased insolation and the attendant intensification of land–sea thermal contrast give rise to anomalous ascending motions, while descending motions are fundamentally forced by the decreased latitudinal insolation gradient. In particular, thermodynamic changes, namely, the reduced pole–equator temperature and humidity gradients, account for the downward motions over the northwestern Pacific. Dynamic changes, namely, the weakened westerlies, play a leading role in suppressing updrafts in west-central China. This study highlights that the orbital-scale monsoonal precipitation changes are not solely determined by local radiative forcing as repeatedly emphasized before. The latitudinal uneven distribution of insolation is crucial to explain the spatial inhomogeneity in the EASM precipitation changes during the Holocene.

Assessing the performance of cloud microphysical parameterization over the Indian region: Simulation of monsoon depressions and validation with INCOMPASS observations

Abstract This study validates the performance of four different cloud microphysics parameterization (CMP) with the INCOMPASS aircraft observations during monsoon 2016 and assesses its impact on simulations of two monsoon depressions (MDs) using the Weather Research and Forecasting (WRF) model. The simulations are carried out with a lead time up to 96 h. It is found that the Aerosol Aware Thompson (AAT) scheme showed better result in terms of wind and the WRF Double Moment Six Class microphysical scheme (WDM6) showed better correlations for temperature and dew point temperature compared to aircraft measurements. It is noted that the choice of CMP significantly impacts the key characteristics of the MDs such as rainfall, wind, temperature, hydrometeors and associated convective processes (e.g. moist static energy, moisture convergence). In general, CMPs have overestimated the rainfall compared to satellite estimates Tropical Rainfall Measuring Mission (TRMM), with WDM6 producing the least errors. Therefore, inter-comparisons of simulations of CMPs are carried out using WDM6 as the benchmark. Inter-comparison results suggest that there is a substantial reduction in rainfall for the Morrison due to drier lower and middle troposphere leading to subdued convective activity compared to others. Further, WDM6 has produced the least errors in the distribution of frozen hydrometer compared to ERA5. By examining the water budget, it is found that moisture convergence is the major driver for the rainfall, and the magnitude of moisture convergence is strongly affected by the choice of CMPs. Additionally, the local and advection terms of the moisture budget equation provide minimal contributions towards rainfall generation.

Diurnal Cycle of the Asian Summer Monsoon: Air Pump of the Second Kind

AbstractDiurnal variations of rainfall and winds are pronounced over the Asian summer monsoon region, but their activities under different monsoon conditions are not clarified. Here, the diurnal cycle of monsoon flow and its influence are examined using 20-yr satellite rainfall and reanalysis data. A total of 1840 summer days are partitioned into four dynamic groups of strong or weak background flows with large or small diurnal amplitudes of low-level meridional wind. Large-scale southerly wind is found to be strongest after midnight, with a large diurnal amplitude on strong monsoon days over central-north India and southeast China. Such a nocturnal speed-up is closely associated with the Blackadar boundary layer inertial oscillation due to the diurnal heating over low-lying landmass. It acts like a large air pump that injects moisture poleward at night and strengthens monsoonal circulation with anomalous rising motion at the northern rainband of the Asian monsoon. In particular, monsoon southerlies with large nighttime speed-up converge with downslope winds from the Himalayas or northerly anomaly from midlatitudes. Enhanced water vapor convergence facilitates the growth of organized convection, producing substantial rainfall at the Himalayan foothills in predawn hours and at the mei-yu–baiu zone from predawn to noon. When monsoon flow undergoes a small diurnal cycle, rainfall is instead displaced south and mostly recorded in daytime. Both the daily mean and morning peak of rainfall are suppressed on land under weak monsoon southerlies. Moreover, the monsoon diurnal cycle exhibits evident intraseasonal/interannual variations and contributes to rainfall variability. The results highlight that monsoon flow couples with subdaily forcings to strongly regulate the detailed patterns of rainfall and moisture budget over the Asian monsoon regions.