scholarly journals The Dominant Patterns of Intraseasonal Rainfall Variability in May–October and November–April over the Tropical Western Pacific

2019 ◽  
Vol 147 (8) ◽  
pp. 2941-2960 ◽  
Author(s):  
Sunil Kumar Pariyar ◽  
Noel Keenlyside ◽  
Bhuwan Chandra Bhatt ◽  
Nour-Eddine Omrani
Keyword(s):  
Rainfall Variability ◽  
Surface Wind ◽  
Daily Rainfall ◽  
Moisture Flux ◽  
Feedback Mechanism ◽  
Western Pacific ◽  
Moisture Flux Convergence ◽  
Low Level ◽  
Northward Propagation ◽  
Rainfall Anomalies

Abstract The space–time structure of intraseasonal (10–90 day) rainfall variability in the western tropical Pacific is studied using daily 3B42 TRMM and ERA-Interim reanalysis data for the period 1998–2014. Empirical orthogonal function (EOF) analysis of 10–90-day filtered daily rainfall anomalies identifies two leading modes in both May–October and November–April; together these modes explain about 11%–12% of the total intraseasonal variance over the domain in both seasons and up to 60% over large areas of the western Pacific in both climatological periods. The two leading modes in May–October are linearly related to each other and both are well correlated with the Madden–Julian oscillation (MJO) indices. Although the two leading EOF modes in November–April are linearly independent of each other, both show statistically significant correlations with the MJO. The phase composites of 30–80-day filtered data show that the two leading modes are associated with strong eastward and northward propagation of rainfall anomalies in May–October, and eastward and southward propagation of rainfall anomalies in November–April. The eastward propagation of rainfall anomalies in both seasons and southeastward propagation related with EOF2 in November–April is linked to the development of low-level moisture flux convergence ahead of the active convection. Similarly, the northward propagation in May–October is also connected with low-level moisture flux convergence, but surface wind and evaporation variations are also important. The wind–evaporation–SST feedback mechanism drives the southeastward propagation of rainfall anomalies associated with EOF1 in November–April. The different mechanisms for southeastward propagation associated with two leading modes in November–April suggest dynamically different relations with the MJO.

scholarly journals Variations in the Summertime Atmospheric Hydrologic Cycle Associated with Seasonal Precipitation Anomalies over the Southwestern United States

2006 ◽  
Vol 7 (4) ◽  
pp. 788-807 ◽  
Author(s):  
Bruce T. Anderson ◽  
Hideki Kanamaru ◽  
John O. Roads
Keyword(s):  
New Mexico ◽  
Large Scale ◽  
Moisture Flux ◽  
Hydrologic Cycle ◽  
Seasonal Precipitation ◽  
Model Simulations ◽  
Moisture Flux Convergence ◽  
Rainfall Season ◽  
Rainfall Anomalies ◽  
The Mean

Abstract This paper examines year-to-year variations in the large-scale summertime hydrologic cycle over the southwestern United States using a suite of regional model simulations and surface- and upper-air-based observations. In agreement with previous results, it is found that observed interannual precipitation variations in this region can be subdivided into two spatiotemporal regimes—one associated with rainfall variability over the southwestern portion of the domain centered on Arizona and the other associated with variations over the southeastern portion centered on western Texas and eastern New Mexico. Because of the limited duration of the model simulation data, it is possible to only investigate one positive rainfall season over the Arizona region and one negative rainfall season over the New Mexico region. From these investigations it appears that for the positive rainfall anomalies over Arizona excess seasonal precipitation is balanced by both enhanced evaporation and vertically integrated large-scale moisture flux convergence. Vertical profiles of these terms indicate that the anomalous large-scale moisture flux convergence is actually related to a decrease in the mean large-scale moisture flux divergence aloft; below 800 mb there is a decrease in the mean moisture flux convergence typically found at these levels, which in turn produces anomalous moisture divergence from the region. For the negative rainfall anomalies over New Mexico similar results, but of opposite sign, are found; one exception is that at the lowest levels there is an additional (negative) contribution to the vertically integrated moisture flux convergence anomaly related to a weakening of the mean low-level moisture flux convergence during the low-rainfall year. Further studies using two different model simulations with the same large-scale dynamic forcing but differing initial soil moisture values indicate that similar balances are also found for rainfall anomalies related to surface soil moisture changes within the domain, suggesting that the changes in large-scale moisture flux convergence described above can be attributed to both year-to-year variations in the regional land–atmosphere interactions as well as variations in the large-scale circulation patterns.

scholarly journals Estimating the Influence of Evaporation and Moisture-Flux Convergence upon Seasonal Precipitation Rates. Part II: An Analysis for North America Based upon the NCEP–DOE Reanalysis II Model

2009 ◽  
Vol 10 (4) ◽  
pp. 893-911 ◽  
Author(s):  
Bruce T. Anderson ◽  
Alex C. Ruane ◽  
John O. Roads ◽  
Masao Kanamitsu
Keyword(s):  
Large Scale ◽  
Moisture Flux ◽  
Early Summer ◽  
Monsoon Circulation ◽  
Atmospheric Moisture ◽  
Storm Activity ◽  
Moisture Flux Convergence ◽  
Low Level ◽  
Low Level Jet ◽  
Northwestern Mexico

Abstract In this paper, a diagnostic metric—termed the local-convergence ratio—is used to analyze the contribution of evaporation and atmospheric moisture-flux convergence to model-based estimates of climatological precipitation over the North American continent. Generally, the fractional evaporative contribution is largest during spring and summer when evaporation is largest and decreases as evaporation decreases. However, there appears to be at least three regions with distinct spatiotemporal seasonal evolutions of this ratio. Over both the northern and western portions of the continent, the fractional evaporative contribution peaks in spring and early summer and decreases during fall and into winter. Over the northern portion, this fall decrease is related to an increase in atmospheric moisture-flux convergence associated with enhanced meridional moisture fluxes into the region; over the western coastal regions, the fall decrease in evaporative contribution is associated with a decrease in evaporation and an increase in total moisture-flux convergence, most likely associated with increased storm activity. In contrast, over the central portions of the continent, the fractional evaporative contribution to precipitation remains relatively low in spring—when enhanced low-level jet activity increases the low-level atmospheric moisture flux convergence into the region—and instead peaks in summer and fall—when the moisture-flux convergence associated with the low-level jet decreases and precipitation is balanced predominantly by local evaporation. Finally, over the southwestern United States and northwestern Mexico, the fractional evaporative contribution to precipitation is found to contain a wintertime minimum as well as a secondary minimum during summer. This latter feature is due to a substantial increase in low-level atmospheric moisture-flux convergence associated with the large-scale monsoon circulation that influences this region during this time.

scholarly journals Mechanisms for the Dipolar Patterns of Rainfall Variability over Large Islands in the Maritime Continent Associated with the Madden–Julian Oscillation

2020 ◽  
Vol 77 (6) ◽  
pp. 2257-2278
Author(s):  
Jian-Hua Qian
Keyword(s):  
Large Scale ◽  
Rainfall Variability ◽  
Reanalysis Data ◽  
Mountain Range ◽  
Madden Julian Oscillation ◽  
Maritime Continent ◽  
Wake Effect ◽  
Low Level ◽  
Rainfall Anomalies ◽  
Normal Rainfall

Abstract Before the eastward-propagating rainy envelope of a Madden–Julian oscillation (MJO) arrives at the Maritime Continent (MC), some islands in the MC experience dipolar patterns of rainfall variability with opposite signs of rainfall anomalies in two neighboring regions within an island. Similar incoherent rainfall anomalies are also observed after the MJO passed the MC. The mechanisms for these dipolar patterns of rainfall anomalies are investigated by using observed and reanalysis data. It is found that the response of rainfall in the MC depends on the direction of wind anomalies and the availability of atmospheric moisture in different phases of the MJO. The low-level wind anomalies over the MC are easterlies in MJO phases 1–3, which cause above-normal rainfall over the mountainous areas in Java, and in western Borneo, western Sumatra, and western Malay Peninsula, respectively. In phases 5–6, the low-level wind anomalies are westerlies and the positive rainfall anomalies are over the eastern part of the islands. Two physical mechanisms are responsible for this phenomenon of the dipolar patterns of rainfall anomalies: 1) the monsoonal damping effect on rainfall over elongated narrow islands—an inverse relationship between the intensity of the diurnal cycle of sea breezes and valley breezes and the large-scale monsoonal wind speed, and 2) the wake effect on rainfall over large and wide islands—above-normal rainfall on the downwind wake side of an island or mountain range with respect to large-scale wind anomalies.

scholarly journals Observations of the Nocturnal Boundary Layer Associated with the West African Monsoon

2010 ◽  
Vol 138 (8) ◽  
pp. 3142-3156 ◽  
Author(s):  
Caroline L. Bain ◽  
Douglas J. Parker ◽  
Christopher M. Taylor ◽  
Laurent Kergoat ◽  
Françoise Guichard
Keyword(s):  
Boundary Layer ◽  
Case Studies ◽  
Large Scale ◽  
Surface Wind ◽  
West African Monsoon ◽  
Moisture Flux ◽  
Nocturnal Boundary Layer ◽  
Near Surface ◽  
Low Level ◽  
Strong Inversion

Abstract A set of nighttime tethered balloon and kite measurements from the central Sahel (15.2°N, 1.3°W) in August 2005 were acquired and analyzed. A composite of all the nights’ data was produced using boundary layer height to normalize measured altitudes. The observations showed some typical characteristics of nocturnal boundary layer development, notably a strong inversion after sunset and the formation of a low-level nocturnal jet later in the night. On most nights, the sampled jet did not change direction significantly during the night. The boundary layer thermodynamic structure displayed some variations from one night to the next. This was investigated using two contrasting case studies from the period. In one of these case studies (18 August 2005), the low-level wind direction changed significantly during the night. This change was captured well by two large-scale models, suggesting that the large-scale dynamics had a significant impact on boundary layer winds on this night. For both case studies, the models tended to underestimate near-surface wind speeds during the night, which is a feature that may lead to an underestimation of moisture flux northward by models.

scholarly journals Opposite Effects of ENSO on the Rainfall over the Northern and Equatorial Great Horn of Africa and Possible Causes

2020 ◽  
Vol 2020 ◽  
pp. 1-16
Author(s):  
K. Abebe Kiflie ◽  
Li Tao
Keyword(s):  
Rainfall Variability ◽  
Summer Rainfall ◽  
Western Indian Ocean ◽  
Annual Rainfall ◽  
Horn Of Africa ◽  
Tropical Africa ◽  
Low Level ◽  
Tropical Easterly Jet ◽  
Rainfall Anomalies ◽  
Upper Level

In this study, we explore the possible mechanism of opposite ENSO effects on summer rainfall in the JJAS region (northern GHA) and autumn rainfall in the OND region (equatorial GHA). The two regions are identified based on the spatial distribution of high seasonal fractions of annual rainfall for the period 1979–2016. The summer rainfall over the JJAS region is negatively correlated with ENSO. It is because the warm Niño3.4 SST triggers zonal wave one pattern in tropics and forces upper-level westerly anomaly and the low-level easterly anomaly over tropical Africa. Thus, the weakened upper-level Tropical Easterly Jet (TEJ) and the low-level westerly over the JJAS region result in deficient rainfall during JJAS over the northern GHA. For the autumn rainfall variability over the equatorial GHA, IOD is a pivotal factor. But, autumn rainfall anomalies are far greater in ENSO and IOD coexisting years than those in IOD alone years. In other words, ENSO has a significant impact on the autumn rainfall over the equatorial GHA by means of IOD. It is because the warming SST, which is fully developed over western Indian Ocean (IO) in autumn of ENSO developing year, causes low-level convergence over the equatorial GHA and enhances upper-level easterly over tropical Africa. Those conditions are favorable for abundant rainfall over the equatorial GHA in autumn.

scholarly journals The Role of Moisture in Summertime Low-Level Jet Formation and Associated Rainfall over the East Asian Monsoon Region

2015 ◽  
Vol 72 (10) ◽  
pp. 3871-3890 ◽  
Author(s):  
Ruidan Chen ◽  
Lorenzo Tomassini
Keyword(s):  
General Circulation ◽  
North China ◽  
Circulation Model ◽  
Moisture Flux ◽  
Mountain Range ◽  
The Tibetan Plateau ◽  
Moisture Flux Convergence ◽  
Low Level ◽  
Low Level Jet

Abstract The southwesterly low-level jet (LLJ) located to the east of the Tibetan Plateau in southern China plays an important role in summertime convective initiation over north China. This study adopts a novel perspective and uses hindcast experiments in order to investigate the role of moisture in LLJ and associated heavy rainfall formation, employing a global atmospheric general circulation model (AGCM). In the sensitivity experiments, an increase of humidity in the inflow region leads to a weaker LLJ but stronger diurnal wind oscillations. The weaker LLJ is due to a decreased lower-tropospheric east–west pressure gradient resulting from a low pressure anomaly over southeastern China induced by deep convection and related condensational heating. On the other hand, the stronger diurnal variation of the LLJ originates from stronger day-and-night thermal differences over the sloping terrain, which is related to drier conditions over the mountain range. Moreover, the increased humidity and decreased LLJ counteract one another to impact precipitation in the outflow region. The change of precipitation is mainly determined by the altered moisture flux divergence. If the increase in humidity dominates, then the moisture flux convergence is enhanced and favors more precipitation over north China. Otherwise, if the decreased LLJ dominates, then the moisture flux convergence is reduced, which constrains precipitation. It is highlighted that the moist diabatic and dynamic processes are intimately coupled, and that a correct simulation of moisture flux convergence is vital for AGCMs to reproduce the LLJ-related precipitation, particularly the nocturnal precipitation peak, which is a deficiency in many current models.

scholarly journals The Impact of Forcing Datasets on the High-Resolution Simulation of Tropical Storm Ivan (2004) in the Southern Appalachians

2012 ◽  
Vol 140 (10) ◽  
pp. 3300-3326 ◽  
Author(s):  
Xiaoming Sun ◽  
Ana P. Barros
Keyword(s):  
High Resolution ◽  
Global Analysis ◽  
Surface Wind ◽  
Tropical Storm ◽  
Southern Appalachians ◽  
Low Level ◽  
Simulated Precipitation ◽  
Forced Simulation ◽  
The Impact ◽  
Resolution Simulation

Abstract The influence of large-scale forcing on the high-resolution simulation of Tropical Storm Ivan (2004) in the southern Appalachians was investigated using the Weather Research and Forecasting model (WRF). Two forcing datasets were employed: the North American Regional Reanalysis (NARR; 32 km × 32 km) and the NCEP Final Operational Global Analysis (NCEP FNL; 1° × 1°). Simulated fields were evaluated against rain gauge, radar, and satellite data; sounding observations; and the best track from the National Hurricane Center (NHC). Overall, the NCEP FNL forced simulation (WRF_FNL) captures storm structure and evolution more accurately than the NARR forced simulation (WRF_NARR), benefiting from the hurricane initialization scheme in the NCEP FNL. Further, the performance of WRF_NARR is also negatively affected by a previously documented low-level warm bias in NARR. These factors lead to excessive precipitation in the Piedmont region, delayed rainfall in Alabama, as well as spatially displaced and unrealistically extreme rainbands during its passage over the southern Appalachians. Spatial filtering of the simulated precipitation fields confirms that the storm characteristics inherited from the forcing are critical to capture the storm’s impact at local places. Compared with the NHC observations, the storm is weaker in both NARR and NCEP FNL (up to Δp ~ 5 hPa), yet it is persistently deeper in all WRF simulations forced by either dataset. The surface wind fields are largely overestimated. This is attributed to the underestimation of surface roughness length over land, leading to underestimation of surface drag, reducing low-level convergence, and weakening the dissipation of the simulated cyclone.

scholarly journals Assessing Future Changes in the East Asian Summer Monsoon Using CMIP5 Coupled Models

2013 ◽  
Vol 26 (19) ◽  
pp. 7662-7675 ◽  
Author(s):  
Kyong-Hwan Seo ◽  
Jung Ok ◽  
Jun-Hyeok Son ◽  
Dong-Hyun Cha
Keyword(s):  
Summer Monsoon ◽  
East Asian Summer Monsoon ◽  
Asian Summer Monsoon ◽  
East Asian ◽  
Moisture Flux ◽  
Moisture Convergence ◽  
Cmip5 Models ◽  
Moisture Flux Convergence ◽  
The North ◽  
Asian Summer

Abstract Future changes in the East Asian summer monsoon (EASM) are estimated from historical and Representative Concentration Pathway 6.0 (RCP6) experiments of the fifth phase of the Coupled Model Intercomparison Project (CMIP5). The historical runs show that, like the CMIP3 models, the CMIP5 models produce slightly smaller precipitation. A moisture budget analysis illustrates that this precipitation deficit is due to an underestimation in evaporation and ensuing moisture flux convergence. Of the two components of the moisture flux convergence (i.e., moisture convergence and horizontal moist advection), moisture convergence associated with mass convergence is underestimated to a greater degree. Precipitation is anticipated to increase by 10%–15% toward the end of the twenty-first century over the major monsoonal front region. A statistically significant increase is predicted to occur mostly over the Baiu region and to the north and northeast of the Korean Peninsula. This increase is attributed to an increase in evaporation and moist flux convergence (with enhanced moisture convergence contributing the most) induced by the northwestward strengthening of the North Pacific subtropical high (NPSH), a characteristic feature of the future EASM that occurred in CMIP5 simulations. Along the northern and northwestern flank of the strengthened NPSH, intensified southerly or southwesterly winds lead to the increase in moist convergence, enhancing precipitation over these areas. However, future precipitation over the East China Sea is projected to decrease. In the EASM domain, a local mechanism prevails, with increased moisture and moisture convergence leading to a greater increase in moist static energy in the lower troposphere than in the upper troposphere, reducing tropospheric stability.

scholarly journals Modeling and forecasting of rainfall reoccurrence changes using Markov Switching in Iran

2021 ◽  
Vol 3 (8) ◽  
Author(s):  
Majid Javari
Keyword(s):  
Environmental Planning ◽  
Rainfall Variability ◽  
Markov Switching ◽  
Daily Rainfall ◽  
First Order ◽  
Markov Switching Models ◽  
Markov Transition Matrix ◽  
Switching Models ◽  
Markov Transition ◽  
Modeling And Forecasting

AbstractThis paper represents the recurrence (reoccurrence) changes in the rainfall series using Markov Switching models (MSM). The switching employs a dynamic pattern that allows a linear model to be combined with nonlinearity models a discrete structure. The result is the Markov Switching models (MSM) reoccurrence predicting technique. Markov Switching models (MSM) were employed to analyze rainfall reoccurrence with spatiotemporal regime probabilities. In this study, Markov Switching models (MSM) were used based on the simple exogenous probability frame by identifying a first-order Markov process for the regime probabilities. The Markov transition matrix and regime probabilities were used to analyze the rainfall reoccurrence in 167 synoptic and climatology stations. The analysis results show a low distribution from 0.0 to 0.2 (0–20%) per day spatially from selecting stations, probability mean of daily rainfall recurrence is 0.84, and a different distribution based on the second regime was found to be more remarkable to the rainfall variability. The rainfall reoccurrence in daily rainfall was estimated with relatively low variability and strong reoccurrence daily with ranged from 0.851 to 0.995 (85.1–99.5%) per day based on the spatial distribution. The variability analysis of rainfall in the intermediate and long variability and irregular variability patterns would be helpful for the rainfall variability for environmental planning.

scholarly journals TEMPORAL RAINFALL DISAGGREGATION BY A SIMPLE RANDOM CASCADE MODEL

2018 ◽  
Vol 80 (6) ◽  
Author(s):  
Siti Mariam Saad ◽  
Abdul Aziz Jemain ◽  
Noriszura Ismail
Keyword(s):  
Rainfall Variability ◽  
Daily Rainfall ◽  
Poisson Model ◽  
Rain Gauge ◽  
Peninsular Malaysia ◽  
Cascade Model ◽  
Statistical Measures ◽  
Hourly Rainfall ◽  
Cascade Models ◽  
Log Normal

This study evaluates the utility and suitability of a simple discrete multiplicative random cascade model for temporal rainfall disaggregation. Two of a simple random cascade model, namely log-Poisson and log-Normal  models are applied to simulate hourly rainfall from daily rainfall at seven rain gauge stations in Peninsular Malaysia. The cascade models are evaluated based on the capability to simulate data that preserve three important properties of observed rainfall: rainfall variability, intermittency and extreme events. The results show that both cascade models are able to simulate reasonably well the commonly used statistical measures for rainfall variability (e.g. mean and standard deviation) of hourly rainfall. With respect to rainfall intermittency, even though both models are underestimated, the observed dry proportion, log-Normal  model is likely to simulate number of dry spells better than log-Poisson model. In terms of rainfall extremes, it is demonstrated that log-Poisson and log-Normal  models gave a satisfactory performance for most of the studied stations herein, except for Dungun and Kuala Krai stations, which both located in the east part of Peninsula.

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