Peak Precipitation Research Articles

Detection and Tracking of Tropical Convective Storms Based on Globally Gridded Precipitation Measurements: Algorithm and Survey over the Tropics

AbstractThis paper is the first attempt to document a simple convection-tracking method based on the IMERG precipitation product to generate an IMERG-based Convection Tracking (IMERG-CT) dataset. Up to now, precipitation datasets have been Eulerian accumulations. Now with IMERG-CT, we can estimate total rainfall based on Lagrangian accumulations, which is a very important step in diagnosing cloud-precipitation process following the evolution of air masses. Convection-tracking algorithms have traditionally been developed on the basis of brightness temperature (Tb) from satellite infrared (IR) retrievals. However, vigorous rainfall can be produced by warm-topped systems in a moist environment; this situation cannot be captured by traditional IR-based tracking but is observed in IMERG-CT. Therefore, an advantage of IMERG-CT is its ability to include the previously missing information of shallow clouds that grow into convective storms, which provides us more-complete life cycle records of convective storms than traditional IR-based tracking does. This study also demonstrates the utility of IMERG-CT through investigating various properties of convective systems in terms of the evolution before and after peak precipitation rate and amount. For example, composite analysis reveals a link between evolution of precipitation and convective development: the signature of stratiform anvils remaining after the storm has produced the maximum rainfall, as average Tb stays almost constant for 5 h after the peak of precipitation. Our study highlights the importance of joint analysis of cloud and precipitation data in time sequence, which helps to elucidate the underlying dynamic processes producing tropical rainfall and its resultant effects on the atmospheric thermodynamics.

Transient Aggregation of Convection: Observed Behavior and Underlying Processes

AbstractConvective self-aggregation is among the most striking features emerging from radiative–convective equilibrium simulations, but its relevance to convective disturbances observed in the real atmosphere remains under debate. This work seeks the observational signals of convective aggregation intrinsic to the life cycle of cloud clusters. To this end, composite time series of the Simple Convective Aggregation Index (SCAI), a metric of aggregation, and other variables from satellite measurements are constructed around the temporal maxima of precipitation. All the parameters analyzed are large-scale means over 10° × 10° domains. The composite evolution for heavy precipitation regimes shows that cloud clusters are gathered into fewer members during a period of ±12 h as precipitation picks up. The high-cloud cover per cluster expands as the number of clusters drops, suggesting a transient occurrence of convective aggregation. The sign of the transient aggregation is less evident or entirely absent in light precipitation regimes. An energy budget analysis is performed in search of the physical processes underlying the transient aggregation. The column moist static energy (MSE) accumulates before the precipitation peak and dissipates after, accounted for primarily by the horizontal MSE advection. The domain-averaged column radiative cooling is greater in a more aggregated composite than in a less aggregated one, although the role of radiative–convective feedback behind this remains unclear.

Impact of EMIC‐Wave Driven Electron Precipitation on the Radiation Belts and the Atmosphere

AbstractIn recent years, there has been a growing body of direct experimental evidence demonstrating electromagnetic ion cyclotron (EMIC) waves driving energetic electron precipitation (EEP) at unexpectedly low, sub‐MeV energies—as low as only a few hundred keV. EMIC‐wave driven scattering at these energies has important ramifications for our understanding of not only radiation belt electron dynamics, but also the importance of EMIC‐driven EEP to the chemical balance of the Earth's atmosphere. In this study, we use three experimentally derived EMIC‐driven EEP flux spectra to investigate the impact of this precipitation on trapped radiation belt fluxes. In doing so, we resolve an apparent contradiction with earlier results derived from trapped electron flux populations that suggested EMIC waves only caused significant scattering at ultrarelativistic energies. We show that strong sub‐MeV EEP measurements are not necessarily mutually exclusive with a strongly relativistic‐only trapped flux response, as the sub‐MEV peak precipitation is comparatively much smaller than the trapped population at those energies. Using a further six EEP spectra, we also demonstrate that EMIC‐driven EEP can generate significant ionization of the Earth's atmosphere above 40 km, leading to the loss of mesospheric ozone. We find poor correlation between EMIC‐driven EEP fluxes and geomagnetic activity proxies, such that EMIC‐driven EEP is likely to be poorly specified in the forcing factors of modern coupled‐climate models.

Disintegration of uncertainties associated with real-time multi-satellite precipitation products in diverse topographic and climatic area in Pakistan

Satellite-based Precipitation Estimates (SPEs) have gained importance due to enhanced spatial and temporal resolution, particularly in Indus basin, where raingauge network has fewer observation stations and drainage area is laying in many countries. Formulation of SPEs is based on indirect mechanism, therefore, assessment and correction of associated uncertainties is required. In the present study, disintegration of uncertainties associated with four prominent real time SPEs, IMERG, TMPA, CMORPH and PERSIANN has been conducted at grid level, regional scale, and summarized in terms of regions as well as whole study area basis. The bias has been disintegrated into hit, missed, false biases, and Root Mean Square Error (RMSE) into systematic and random errors. A comparison among gauge- and satellite-based precipitation estimates at annual scale, showed promising result, encouraging use of real time SPEs in the study area. On grid basis, at daily scale, from box plots, the median values of total bias (−0.5 to 0.5 mm) of the used SPEs were also encouraging although some under/over estimations were noted in terms of hit bias (−0.15 to 0.05 mm/day). Relatively higher values of missed (0.3 to 0.5 mm/day) and false (0.5 to 0.7 mm/day) biases were observed. The detected average daily RMSE, systematic errors, and random errors were also comparatively higher. Regional-scale spatial distribution of uncertainties revealed lower values of uncertainties in plain areas, depicting the better performance of satellite-based products in these areas. However, in areas of high altitude (>4000 m), due to complex topography and climatic conditions (orographic precipitation and glaciated peaks) higher values of biases and errors were observed. Topographic barriers and point scale gauge data could also be a cause of poor performance of SPEs in these areas, where precipitation is more on ridges and less in valleys where gauge stations are usually located. Precipitation system’s size and intensity can also be a reason of higher biases, because Microwave Imager underestimate precipitation in small systems ( 2000 km2). At present, use of bias correction techniques at daily time scale is compulsory to utilize real time SPEs in estimation of floods in the study area. Inter comparison of satellite products indicated that IMERG gave better results than the others with the lowest values of systematic errors, missed and false biases.

The Spatiotemporal Evolution of Storm Pulse Particulate Organic Carbon in a Low Gradient, Agriculturally Dominated Watershed

Streams and rivers integrate and transport particulate organic carbon (POC) from an array of aquatic and terrestrial sources. Storm events greatly accelerate the transport of POC. The sequences by which individual POC inputs are mobilized and transported are not well-documented but are predicted to be temporally transient and spatially dependent because of changes in forcing functions, such as precipitation, discharge, and watershed morphology. In this study, the 3rd−4th order agricultural stream network, Clear Creek in Iowa, U.S.A., was sampled at a nested series of stations through storm events to determine how suspended POC changes over time and with distance downstream. Carbon and nitrogen stable isotope ratios were used to identify changes in POC. A temporal sequence of inputs was identified: in-channel algal production prior to heavy precipitation, row crop surface soils mobilized during peak precipitation, and material associated with the peak hydrograph that is hypothesized to be an integrated product from upstream. Tile drains delivered relatively 13C- and 15N-depleted particulate organic carbon that is a small contribution to the total POC inventory in the return to baseflow. The storm POC signal evolved with passage downstream, the principal transformation being the diminution of the early flush surface soil peak in response to a loss of connectivity between the hillslope and channel. Bank erosion is hypothesized to become increasingly important as the signal propagates downstream. The longitudinal evolution of the POC signal has implications for C-budgets associated with soil erosion and for interpreting the organic geochemical sedimentary record.

From Hydrometeorology to River Water Quality: Can a Deep Learning Model Predict Dissolved Oxygen at the Continental Scale?

Dissolved oxygen (DO) reflects river metabolic pulses and is an essential water quality measure. Our capabilities of forecasting DO however remain elusive. Water quality data, specifically DO data here, often have large gaps and sparse areal and temporal coverage. Earth surface and hydrometeorology data, on the other hand, have become largely available. Here we ask: can a Long Short-Term Memory (LSTM) model learn about river DO dynamics from sparse DO and intensive (daily) hydrometeorology data? We used CAMELS-chem, a new data set with DO concentrations from 236 minimally disturbed watersheds across the U.S. The model generally learns the theory of DO solubility and captures its decreasing trend with increasing water temperature. It exhibits the potential of predicting DO in "chemically ungauged basins", defined as basins without any measurements of DO and broadly water quality in general. The model however misses some DO peaks and troughs when in-stream biogeochemical processes become important. Surprisingly, the model does not perform better where more data are available. Instead, it performs better in basins with low variations of streamflow and DO, high runoff-ratio (>0.45), and winter precipitation peaks. Results here suggest that more data collections at DO peaks and troughs and in sparsely monitored areas are essential to overcome the issue of data scarcity, an outstanding challenge in the water quality community.

Added value of kilometer-scale modeling over the third pole region: a CORDEX-CPTP pilot study

High-accuracy meteorological datasets are urgently required for understanding hydrological processes across the Third Pole (Qinghai-Tibetan Plateau, or TP), where meteorological stations are sparse. Low-resolution weather and climate simulations have significant errors in this region due to their inability to resolve meso-microscale processes associated with the complex terrain and convective clouds. This work presents a contribution to CORDEX Convection-Permitting Third Pole (CPTP) using dynamical downscaling of the latest global reanalysis data produced by the European Centre for Medium-Range Weather Forecasts (i.e., ERA5) at very high resolution (approximately 0.033°) based on the Weather Research and Forecasting (WRF) model. The results show that the kilometer-scale horizontal grid spacing simulation (WRF3) outperforms the ERA5 and the High Asia Refined regional reanalysis (HAR v2) in terms of smaller biases and root mean square errors, as well as higher spatial pattern correlation coefficients for 10-m wind speed and precipitation. Furthermore, WRF3 more realistically reproduces observed night-time precipitation peaks in the interior TP, while ERA5 and HAR v2 show erroneous afternoon precipitation peaks. Therefore, the added values achieved by resolving detailed physical processes when increasing grid spacing are considerable. This work demonstrates the potential for developing a reliable high resolution meteorological dataset required for research in this unique region.

Provenance and Weathering of Clays Delivered to the Bay of Bengal During the Middle Miocene: Linkages to Tectonics and Monsoonal Climate

AbstractTectonics and regional monsoon strength control weathering and erosion regimes of the watersheds feeding into the Bay of Bengal, which are important contributors to global climate evolution via carbon cycle feedbacks. The detailed mechanisms controlling the input of terrigenous clay to the Bay of Bengal on tectonic to orbital timescales are, however, not yet well understood. We produced orbital‐scale resolution geochemical records for International Ocean Discovery Program Site U1443 (southern Bay of Bengal) across five key climatic intervals of the middle to late Miocene (15.8–9.5 Ma). Our new radiogenic Sr, Nd, and Pb isotope time series of clays transported to the Ninetyeast Ridge suggest that the individual contributions from different erosional sources overall remained remarkably consistent during the Miocene despite major tectonic reorganizations in the Himalayas. On orbital timescales, however, high‐resolution data from the five investigated intervals show marked fluctuations of all three isotope systems. Interestingly, the variability was much higher within the Miocene Climatic Optimum (around 16–15 Ma) and across the major global cooling (~13.9–13.8 Ma) until ~13.5 Ma, than during younger time intervals. This change is attributed to a major restriction on the supply of High Himalayan erosion products due to migration of the peak precipitation area toward the frontal domains of the Himalayas and the Indo‐Burman Ranges. The transient excursions of the radiogenic isotope signals on orbital timescales most likely reflect climatically driven shifts in monsoon strength.

An Analysis of the Performance of the Houston Lightning Mapping Array During an Intense Period of Convection During Tropical Storm Harvey

AbstractExtreme lightning activity (∼300,000 flashes) was observed by the Houston Lightning Mapping Array (HLMA) network during Tropical Storm Harvey on August 26, 2017–August 27, 2017. There was an overall weak correlation between the lightning flash and precipitation rates (R = 0.14; p ∼ 0.17) likely due to feeder bands transitioning between deep and shallow convection. The peak precipitation rate (14.2 mm hr−1) lagged the peak lightning flash rate (∼18,000 flashes [30 min]−1) by 7 h. Adjusting for the lag, the correlation increased to 0.71 (p < 0.01) indicating a relationship between the uptick in lightning activity in the convective feeder bands and the resulting heavy precipitation during episodes of shallow convection. Peak lightning flash extent densities of 12.4 km−2 min−1 were spatially collocated with precipitation rates exceeding 9 mm hr−1. NEXRAD reflectivity data featured 30 dBZ echo heights in excess of 10 km that were collocated with peak lightning flash extent densities exceeding 3 km−2 min−1. The NEXRAD specific phase (kdp) and differential phase (Zdr) dual‐polarization products were highest (≥2° km−1 and dB) below the melting layer (4.6 km) indicating heavy precipitation and strong updrafts. Negative kdp and Zdr values (≤−0.15° km−1 and dB) collocated with the peak lightning flash extent density altitudes indicated ice particle generation which likely augmented the charge separation process. This is likely the first time a major tropical cyclone has occurred within the confines of a lightning mapping array. The lightning data, when used synergistically with radar and precipitation data, can adequately quantify tropical convection exhibiting an elevated mixed‐phase depth.

DRAINMOD simulation of paddy field drainage strategies and adaptation to future climate change in lower reaches of the Yangtze river basin*

AbstractThe DRAINMOD model was used to simulate the current and future water balance in the Yanyun Irrigation District, China. Projections of future climate in the district (2021:2050) show that annual maximum and minimum temperatures will increase by [1.09 and 1.15 °C] under the representative concentration pathway RCP4.5 and [1.34 and 1.39 °C] under RCP8.5, besides more frequency of one‐day peak precipitation events. DRAINMOD simulations revealed that future annual runoff will increase by 33.3% (RCP4.5) and 36.9% (RCP8.5), while groundwater tables will drop by 6% (RCP4.5) and 14.5% (RCP8.5). Future hydrology has been resimulated, but with increasing the capacity of surface depressions in an attempt to minimize adverse impacts of climate change, and results have been checked twice: under proper and improper subsurface drainage practices. Under proper subsurface drainage practices, it was revealed that future runoff will decrease by 25.3% (RCP4.5) and 23.6% (RCP8.5), which will cause a rise in groundwater tables to desired levels, while in contrast, improper subsurface drainage practices will increase future runoff by 13.7% (RCP4.5) and 2.8% (RCP8.5), all compared to future values before adaptation. Therefore, proper drainage strategies are a vital tool that helps to cope with climate change in agricultural lands and mitigate future runoff rates.

Assessment of CMIP6 in simulating precipitation over arid Central Asia

Precipitation outputs from 30 Global Circulation Models (GCMs) of the Coupled Model Inter-comparison Project Phase 6 (CMIP6) were evaluated from 1951 to 2014 over six climate zones in arid Central Asia (ACA) using the Climate Research Unit TS 4.04 (CRU) precipitation datasets as reference. An evaluation framework was constructed taking into account metrics of annual precipitation patterns, annual cycle precipitation statistics, categorical validation and long-term precipitation trend. The performance of GCMs vary from region to region. Compared to CRU, the 30 selected GCMs present considerable wet bias in the Southern Xinjiang (SX) and Hexi Corridor (HC) regions which can be larger than 100%. The simulated precipitation from most GCMs shows a consistent annual cycle shape and closer with that of observations in the western ACA, but fails to capture precipitation peak in NK and NX regions. Compared to NK, CD, TM and NX, all models and the full model ensemble have limited ability in reproducing the Probability Density Function (PDF) of observations over SX and HC. Most GCMs show limited competence to reproduce the long-term precipitation trend with high wet bias over the Hexi Corridor. Based on the comprehensive performance ranking, it is discovered that CESM2, CESM2-FV2, CESM2-WACCM and CESM2-WACCM-FV2 from NCAR, ACCESS-CM2 from CSIRO and CanESM5 from Canadian Centre for Climate Modeling demonstrate better performances across ACA. The ensemble of 30 selected GCMs has limited ability to accurately simulate precipitation according to the above four types of metrics. The results of this paper may provide scientific guidance to CMIP6 end-users when selecting the most suitable GCMs for their specific applications over ACA.

Initiation of an Elevated Mesoscale Convective System With the Influence of Complex Terrain During Meiyu Season

AbstractA mesoscale convective system (MCS) was initiated on the lee side of the Wuling Mountain in central‐east China in the late afternoon (1700 LT) on June 27, 2016, and led to an early morning precipitation peak. We investigated the convective initiation (CI) mechanism of this MCS using radars and automatic weather station observations, WRF model simulation, and analysis from the radar data assimilation system Variational Doppler Radar Analysis System. It was found that the convection initiation was elevated as indicated by an elevated convergence zone and updraft located above 1,625 m from the ground. A zero convective inhibition layer was located at 850 hPa, over a stable boundary layer (BL) resulting from the daytime rainfall in the plain region, and provided a favorable condition for the elevated CI. Our study suggested that the transition of mountain‐plain thermal wind near sunset was the trigger for the convective initiation. An early afternoon mountain‐plain thermal circulation was disrupted when the east‐facing lee side slope was in the shadow of sunshine after 4 p.m. As a result, a local‐scale northwest downslope wind accelerated and led to a downstream convergence above the stable BL, where the elevated convection was initiated near 1,625 m above ground level. By combining the model simulation, the analysis from data assimilation, and the observations, this study is the first to provide a detailed analysis of CI mechanism of elevated convection in a complex topography region during the Meiyu season in China.

WRF 모형에서의 적운모수화 사용 여부에 따른 여름철 남한 지역 집중호우 모의 민감도 분석

In this study, the impact of cumulus parameterization usage in Weather Research and Forecasting (WRF) model on reproducing summer precipitation in South Korea is evaluated. Two sensitivity experiments are set up with using cumulus parameterization (ON experiment) and without using cumulus parameterization, which is called Convection Permitting Model (OFF experiment). For the both ON and OFF experiments, the horizontal grid resolution is 2.5km, and initial and lateral boundary conditions are derived from ERA5 reanalysis data. Overall, both of the two experiments can capture the spatial distribution of 2014 summer mean and extreme precipitation but show dry biases in the southern region of Korean Peninsula. Occurrence percentage analyses for different precipitation intensity reveal that OFF experiments show better performance than ON experiment for extreme precipitation. In the case of heavy rainfall over Gyeongnam region for 25 August 2014, OFF experiment shows similar characteristic of rainfall to the observations, although it simulates earlier precipitation peak. On the other hand, ON experiment underestimates the amount of precipitation. Also, vertical distribution of equivalent potential temperature and strong southerly wind which play an important role in developing heavy rainfall on 25 August 2014 are better simulated in OFF experiment.

SHARPEN: A Scheme to Restore the Distribution of Averaged Precipitation Fields

AbstractA key strategy in obtaining complete global coverage of high-resolution precipitation is to combine observations from multiple fields, such as the intermittent passive microwave observations, precipitation propagated in time using motion vectors, and geosynchronous infrared observations. These separate precipitation fields can be combined through weighted averaging, which produces estimates that are generally superior to the individual parent fields. However, the process of averaging changes the distribution of the precipitation values, leading to an increase in precipitating area and decrease in the values of high precipitation rates, a phenomenon observed in IMERG. To mitigate this issue, we introduce a new scheme called SHARPEN, which recovers the distribution of the averaged precipitation field based on the idea of quantile mapping applied to the local environment. When implemented in IMERG, precipitation estimates from SHARPEN exhibit a distribution that resembles that of the original instantaneous observations, with matching precipitating area and peak precipitation rates. Case studies demonstrate its improved ability in bridging between the parent precipitation fields. Evaluation against ground observations reveals a distinct improvement in precipitation detection skill, but also a slightly reduced correlation likely because of a sharper precipitation field. The increased computational demand of SHARPEN can be mitigated by striding over multiple grid boxes, which has only marginal impacts on the accuracy of the estimates. SHARPEN can be applied to any precipitation algorithm that produces an average from multiple input precipitation fields and is being considered for implementation in IMERG V07.

Microstructures, Aging Behavior and Thermal Expansion of 7075 Al–Silica Particle Waste Composite Foams Produced with Recycled Aluminum Cans

7075 Al–SiO2 composite foams were prepared by direct melt foaming method. The microstructures of the composite foams showed uniformly distributed particles in the composite foams. Aging studies showed that the hardness values of as-quenched composite foams were higher than that of 7075 Al foam and all the foam samples revealed two clear aging peaks. The aging behavior of the 7075 Al foam was different in kinetic and magnitude from of the 7075 Al–SiO2 composites, where the levels of peak hardness attained in the two peaks of 7075 Al foam were higher than those of the composite foams. Also, the aging times to attain the two peaks of the 7075 Al foam were shorter than those of the composite foams. DTA analyses indicated that the precipitation sequences of the composite foams were similar to that of the 7075 Al foam. However, the precipitation peaks of the composite foams were shifted to higher temperatures and the sizes of the hardening precipitate formation peaks in the composite foams were decreased, indicating that the formation of hardening precipitate zones was suppressed by the incorporation of the SiO2 particles or increasing the content of the foaming agent. The CTE of the 7075 Al foam was higher than that of the 7075 Al–15 vol % SiO2 composite foam.

Risk of hydrological failure under the compound effects of instant flow and precipitation peaks under climate change: A case study of Mountain Island Dam, North Carolina

The risk of flooding as a result of infrastructure failures among aging dams in the United States continues to increase dramatically, which lead to the severe consequences in socio-economic and environmental terms. Environmental impacts regarding the failure of hydrological facilities, including the loss of sustainable habitats and clean water previously provided by reservoir, have raised concerns. In this study, the potential risks to hydrological infrastructure within the context of the 100-year joint probability of maximum rainfall and peak water stage were examined over the Mountain Island Dam (North Carolina, USA) for the period of 1950–2099. We found that nearly 92% of annual maximum extreme rainfall events were closely related to the passage of hurricanes over the dam’s watershed over the 25 years of the validation period. The results show that the projected annual maximum precipitation is likely to intensify under the RCP8.5 climate scenario and the frequency of the annual precipitation maxima exceeding 100 mm/day is projected to increase in the future. Noticeably, it is found that even weaker storm events can contribute a significant amount of rainfall at inland locations. The probability of occurrence of a hurricane like Gustav is higher than that of a 100-year storm rainfall event under the RCP8.5 climate change scenario, this type of storm rainfall will more likely occur near the Mountain Island Dam in the future. Inspectors should focus more on observing water level fluctuation and rainfall volume variation at station two under the same rainstorm - water level situations, because it is more sensitive to the once-in-a-century co-occurrence event than other stations. A HEC-RAS (The Hydrologic Engineering Center (HEC) developed the River Analysis System (RAS) to aid hydraulic engineers in channel flow analysis and floodplain determination) simulation demonstrated that the occurrence probability of hurricane Gustav induced rainfall is more than that of 100-year rainfall events and this type of storm rainfall would highly likely occur near the Mountain Island Dam again under the future warming conditions by the conditional prediction of Copula function. Insights gained suggest that it is critical to evaluate the risk of dam failures regarding either extreme weather conditions or potential environmental concerns under the climate change scenarios, especially where human lives are threatened.

Surface Expressions of Atmospheric Thermal Tides in the Tropical Atlantic and Their Impact on Open‐Ocean Precipitation

AbstractDiurnal and semidiurnal variations of atmospheric pressure and surface winds are fundamental to the Earth‐Sun system. Past research in the tropical Pacific shows semidiurnal and diurnal patterns in the zonal and meridional wind anomalies, respectively. While the semidiurnal zonal wind pattern is consistent with atmospheric thermal tidal forcing, it is not yet certain what drives diurnal meridional wind variability. This study examines the diurnal cycle of meridional winds in the tropical Atlantic Ocean across four different seasons and the extent to which they impact the diurnal evolution of open‐ocean precipitation in boreal summer. Comparisons of direct observations from long‐term moored buoys to an atmospheric reanalysis (MERRA‐2) show that MERRA‐2 reproduces diurnal and semidiurnal pressure and wind variations. The MERRA‐2 atmospheric thermal tides are decomposed into migrating and nonmigrating tidal components, and the linear terms in the zonal and meridional momentum equations are calculated. There is an approximate balance between the acceleration and pressure gradient terms for the zonal migrating momentum budget in the open ocean, with larger residuals or imbalances between those terms for the other budgets. The meridional nonmigrating budget shows largest residuals near the African coastline, indicating that nonlinear processes like the evening initiation of convection are important over land. This evening convection drives the meridional component of surface convergence which is translated offshore by the nonmigrating tide in the form of gravity waves, potentially modified by the African Easterly Jet. During boreal summer, this convergence helps to induce a morning peak of precipitation over the open ocean.

Atmospheric moisture sources associated with extreme precipitation during the peak precipitation month

Understanding moisture transport is crucial for understanding continental precipitation and extreme precipitation events, which are expected to become substantially more frequent under global warming conditions. In this work, the influence of major global moisture sources on extreme continental precipitation during the peak precipitation month is estimated and compared with climatological patterns. The results show a general change in the pattern of contribution of the primary source of precipitation (the source with the highest contribution) during extreme precipitation events. Most primary sources show a general reduction in their area of influence and a notable reduction in their contribution to the total precipitation.

Dynamic and Energetic Constraints on the Modality and Position of the Intertropical Convergence Zone in an Aquaplanet

AbstractThe tropical zonal-mean precipitation distribution varies between having single or double peaks, which are associated with intertropical convergence zones (ITCZs). Here, the effect of this meridional modality on the sensitivity of the ITCZ to hemispherically asymmetric heating is studied using an idealized GCM with parameterized Ekman ocean energy transport (OET). In the idealized GCM, transitions from unimodal to bimodal distributions are driven by equatorial ocean upwelling and cooling, which inhibits equatorial precipitation. For sufficiently strong equatorial cooling, the tropical circulation bifurcates to anti-Hadley circulation in the deep tropics, with a descending branch near the equator and off-equatorial double ITCZs. The intensity and extent of the anti-Hadley circulation is limited by a negative feedback: westerly geostrophic surface wind tendency in its poleward-flowing lower branches balances the easterly stress (and hence equatorial upwelling) required for its maintenance. For weak ocean stratification, which goes along with unimodal or weak bimodal tropical precipitation distribution, OET damps shifts of the tropical precipitation centroid but amplifies shifts of precipitation peaks. For strong ocean stratification, which goes along with pronounced double ITCZs, asymmetric heating leads to relative intensification of the precipitation peak in the warming hemisphere, but negligible meridional shifts. The dynamic feedbacks of the coupled system weaken the gradient of the atmospheric energy transport (AET) near the equator. This suggests that over a wide range of climates, the ITCZ position is proportional to the cubic root of the cross-equatorial AET, as opposed to the commonly used linear relation.

Summer precipitation characteristics on the southern Tibetan plateau

AbstractPrecipitation is an important source of streamflow and a critical parameter in hydrological studies. However, spatiotemporal variation of precipitation in the Tibetan Plateau (TP) has been poorly characterized due to the limited distribution of meteorological stations mostly in the lowlands. In this study, we set‐up rain gauges to study summer precipitation on mountain slopes in three catchments of the southern TP from 2013 to 2016. We studied the precipitation gradients (PGs) averaged during the Indian monsoon season (PGms), as well as daily PGs (PGd) and unit PGs (PGu) under various precipitation intensity grades (PIGs) and subdiurnal precipitation distributions. We also analysed the correlations between the PGs and precipitation amounts under various PIGs. Results showed that the mean PGms, PGd and PGu in the study catchments were 0.0071–0.0082% for monsoon season, 0.11–0.13 mm·100 m−1 d−1 and 2.2–3.1%·100 m−1 for the total (nonintensity‐graded) precipitation, but varied widely under different PIGs. PGms, PGd and PGu were all strongly correlated to precipitation amounts, which were relatively consistent between years but varied between different catchments. In addition, the correlations showed a steep positive slope under fine precipitation but became flat or even negative with increasing PIG. Generally, two distinct precipitation peaks were noted in each of the three catchments: a dispersed peak in the morning, and a stronger but sharper peak at around 18:00 LST; overall, this subdiurnal pattern was temporally stable. The mean precipitation durations of the three catchments were approximately 1.0–1.4 hr (nonintensity‐graded) and increased from <0.5 hr, to <1 hr, 2–5 hr and 6–11 hr for trace, light, moderate and heavy precipitation intensities, respectively. Among the various PG units, both PGd and PGu better represented precipitation at different altitudes than that of PGms. Summer precipitation in the study catchments was influenced more by valley‐scale convection than by large‐scale monsoon circulation.