Development Of Mesoscale Convective Systems Research Articles

Impacts of Large‐Scale Urbanization and Irrigation on Summer Precipitation in the Mid‐Atlantic Region of the United States

AbstractThis study investigates how urbanization and irrigation in the United States east of the Rocky Mountains affect summer precipitation in the Mid‐Atlantic region (MAR) using convection‐permitting regional model simulations with/without urbanization or irrigation. A feature tracking algorithm is used to identify precipitation from mesoscale convective systems (MCSs), isolated deep convection (IDC), and non‐convective systems (NC). Overall, urbanization suppresses all three types of precipitation in the MAR by reducing water vapor content and convective available potential energy, while irrigation enhances IDC and NC precipitation but suppresses MCS precipitation. Examination of the MCS and IDC initiation locations indicates that irrigation suppresses MAR precipitation produced by MCSs initiated in the Great Plains and Midwest (GP) but enhances MAR precipitation from MCSs initiated locally within the region. Irrigation induces a mid‐level cyclonic circulation anomaly centering in the Southeast, hindering the eastward propagation and development of MCSs from the GP.

The characteristics of mesoscale convective systems generated over the Yunnan–Guizhou Plateau during the warm seasons

AbstractThe characteristics of mesoscale convective systems (MCSs) over the Yunnan–Guizhou Plateau (YGP) during the warm seasons (April–August) are investigated using an automatic tracking algorithm based on long‐term (2000–2018) hourly geostationary satellites data of temperature of black body. A total of 1,845 MCSs generated over the YGP are identified and further classified into the eastward moving type (EMT; ~13.1%) and noneastward moving/dissipating type (NDT; ~86.9%). The two types of MCSs exhibit varying characteristics. The EMTs are mainly active in the eastern flank of the YGP with a longer mean lifespan (~16.5 hr), while the NDTs occur anywhere over the YGP with a preference for the central YGP with a shorter mean lifespan (~7.6 hr). The MCSs are observed most frequently during June–July, while the ratio of EMTs reaches the highest in April due to the strongest steering flows. The abundant moisture supply plays a vital role in the generation and development of MCSs in June, whereas the dynamic forcing and high CAPE are favourable to MCSs in July. In terms of the diurnal cycle, the NDTs are generally initiated in the afternoon, reach mature in the late afternoon, and dissipate at night. By contrast, the mature stage of EMTs shows double diurnal peaks in the late afternoon and early morning. The MCSs are usually generated in an instable environment accompanied by strong vertical wind shear and intense low‐level water vapour flux over the YGP, and the MCSs tend to vacate the YGP with strong mid‐level westerlies and instable environment in the downstream regions. Compared to MCSs in Tibetan Plateau, the ratio of the EMTs is higher with longer lifespans though fewer MCSs are generated over the YGP.

The Impacts of Hydrometeors and Water Vapor in Stratiform Region on the Evolutions of MCSs and Embedded Convective Cells in a Pre‐Summer Rainfall Event Over South China

AbstractBoth the stratiform and convective regions play important roles in the development of mesoscale convective systems (MCSs). In this study, the impacts of hydrometeors and water vapor in stratiform region on the evolution of MCSs during a heavy rainfall event over South China are investigated by the Weather Research and Forecasting (WRF) model with Thompson microphysics. A series of sensitivity experiments including the reductions of water vapor to 95%, 80% (water vapor sensitivity experiments), and the reductions of hydrometeor mixing ratios to 10% (hydrometeor sensitivity experiments) in stratiform region are conducted, respectively. Results show that the control run reasonably reproduces the spatial distribution and temporal evolution of convective rainbands. The hydrometeor flux convergence partly indicates the changes of convection organization and movement direction. The intensities of precipitation and MCSs are significantly weakened by decreasing moisture advection to convective region in water vapor sensitivity experiments, but are a little weaker in hydrometeor sensitivity experiments. The enhanced water vapor advection and reduced sedimentation and evaporation of rain water in hydrometeor sensitivity experiments are responsible for the restoration of stratiform cloud. The stratiform region with more hydrometeors is generally more conducive to the development of convection than less one when they are at the same evolution stage. However, it is found that stronger convective cell can also exist in the stratiform region with less hydrometeors that is generated by the old convection cloud and at its dissipating stage, owing to the more transfer of water vapor and hydrometeors from stratiform to convective regions.

Kelvin Waves during GOAmazon and Their Relationship to Deep Convection

AbstractThe 2014–15 Observations and Modeling of the Green Ocean Amazon (GOAmazon) field campaign over the central Amazon near Manaus, Brazil, occurred in coordination with the larger Cloud Processes of the Main Precipitation Systems in Brazil: A Contribution to Cloud-Resolving Modeling and to the Global Precipitation Measurement (CHUVA) project across Brazil. These programs provide observations of convection over the central Amazon on diurnal to annual time scales. In this study, we address the question of how Kelvin waves, observed in satellite observations of deep cloud cover over the GOAmazon region during the 2014–15 time period, modulate the growth, type, and organization of convection over the central Amazon. The answer to this question has implications for improved predictability of organized systems over the region and representation of convection and its growth on local to synoptic scales in global models. Our results demonstrate that Kelvin waves are strong modulators of synoptic-scale low- to midlevel free-tropospheric moisture, integrated moisture convergence, and surface heat fluxes. These regional modifications of the environment impact the local diurnal cycle of convection, favoring the development of mesoscale convective systems. As a result, localized rainfall is also strongly modulated, with the majority of rainfall in the GOAmazon region occurring during the passage of these systems.

Initiation and Organization Mechanisms of Mesoscale Convective Systems in a Warm-Sector Torrential Rainfall Event over Beijing

A torrential rainfall that occurred in Beijing during the period of 21–22 July 2012 is simulated by the Weather Research and Forecasting Model in order to investigate the probable mechanisms for the initiation and organization of warm-sector mesoscale convective systems (MCSs). The simulated results show that the cyclone, which formed in Hetao area, Inner Mongolia and moved eastward slowly, played a key role in the formation and development of warm-sector precipitation, although the favorable atmospheric environment and the configuration of weather systems are also important, which caused the trigger and organization of convective cells along Taihang Mountains. It is the interaction of the local terrain convergence line and the southerly airflows of Hetao cyclone that cause the continuous trigger of convective cells along Taihang Mountains. While, the triggers of convective cells in the plains are caused by the gravity waves, which is related to the development and eastward movement of Hetao cyclone. It must be pointed out that the merging and coupling between the cells that triggered in Taihang Mountains and moved southwesterly and the cells that triggered in plains and moved northeasterly are the key factors for the formation and development of MCSs during the warm-sector precipitation. In addition, the back-building processes and the cold pool forcing are also important for the formation and development of MCSs in this study.

Observational analyses of topographic effects on convective systems in an extreme rainfall event in Northern China

An extreme rainfall event occurred in northern China between 18 and 21 July 2016, with the strongest precipitation of 783.4 mm occurring within the rainfall zone of Taihang Mountain. In this study, mesoscale convective systems (MCSs) resulting in heavy rainfall are studied using high-resolution surface observations, soundings and radar data. The topographic effect in addition to shear zone cyclonic circulation is mainly responsible for the heavy rainfall. The synoptic low-pressure environmental circulation promotes mesocyclone and convection development in northern China, where a strong and moist low-level jet is observed. The interaction between the mesocyclone and mesoscale topography produces two strong, separate precipitation centers, which are clearly the direct results of three MCSs. Orographic blocking plays a crucial role in the development of MCSs. Diabatic cooling due to raindrop evaporation in association with MCS1 forms a cold pool on the plain east of the central Taihang Mountain area. Convective cell initiation and development in MCS2 is due to the impingement of southeasterly flow near the surface of the hills east of Linzhou basin. Convective cells that are successively generated and shifted along a mesoscale shear line comprise the MCS2 linear distribution and a northwestward rain belt. The northern extreme rainfall center is caused by MCS3 along the southern slopes of an eastward-opening valley. Convective cells are triggered along a shear line, which is affected by a combination of topography, a cold pool in the boundary layer and mesocyclone outer flow. Together, the orographic forcing and cold pool enhance the Taihang Mountain precipitation event.

Sahel Rainfall–Tropical Easterly Jet Relationship on Synoptic to Intraseasonal Time Scales

Abstract The tropical easterly jet (TEJ) is a characteristic upper-level feature of the West African monsoon (WAM) circulation. Moreover, the TEJ over West Africa is significantly correlated with summer Sahel rainfall on interannual and decadal time scales. In contrast, the relationship between Sahel rainfall and the regional TEJ on synoptic to intraseasonal time scales is unclear. Therefore, this relationship is investigated by means of multiple statistical analyses using temporally highly resolved measurement and reanalysis data. It is shown that average correlations between convective activity and regional TEJ intensity remain below 0.3 for all synoptic to intraseasonal time scales. Especially on the synoptic time scale, the TEJ significantly lags anomalies in convective activity by one or two days, which indicates that convection anomalies are more likely to drive changes in the regional TEJ than vice versa. To further shed light on the role of the TEJ for rainfall over West Africa, a previously proposed effect of TEJ-induced upper-level divergence on the development of mesoscale convective systems (MCSs) is examined more closely. An analysis of nearly 300 Sahelian MCSs shows that their initiation is generally not associated with significant TEJ anomalies or jet-induced upper-level divergence. Furthermore, no statistically significant evidence is found that preexisting TEJ-related upper-level divergence anomalies affect intensity, size, and lifetime of MCSs. A limiting factor of this study is the focus on TEJ-induced upper-level divergence. Therefore, a possible effect of the TEJ on Sahel rainfall via other mechanisms cannot be ruled out and should be subject to future studies.

Spatiotemporal Distribution Characteristics of Mesoscale Convective Systems Producing Short-Duration Heavy Rainfall over the Tianshan Mountain Area

Based on hourly precipitation data and FY-2 satellite infrared (IR) digital satellite imagery collected during the warm season from 2005 to 2015 in the Tianshan Mountains and the adjacent areas in Xinjiang, China, the definition of mesoscale convective systems (MCSs) was revised based on short-duration heavy precipitation processes. The spatiotemporal development of MCSs in terms of the initiation, maturation, dissipation, duration, and movement was statistically analyzed. Most mesoscale systems in the area were dominated by meso-β convective systems (MβCSs), which was in line with the annual heavy precipitation frequency. In meso-α convective systems (MαCSs), persistent elongated convective systems (PECSs) occurred more commonly than mesoscale convective complexes (MCCs). MCSs were common in summer, with the peak frequency of MαCS occurrence in June and the peak frequency of MβCS occurrence mainly in July and August. From the perspective of diurnal variations, MCSs initiated in the late afternoon, developed during the evening, and dissipated before midnight. MCSs in general lasted 6∼9 h between June and July and 9∼11 h in August. The MαCSs lasted approximately 14 h, and the MβCSs lasted from 7 h to 12 h. The development and termination stages of MCSs lasted 3∼6 h and 2∼7 h, respectively. In low-elevation areas and on the windward slope of the mountains, MCSs initiated more easily and more frequently over the northern and western slopes than that over the southern and eastern slopes. The central area of the Junggar basin hosted the development of MCSs, but the distribution of the convective systems at different scales varied. During the termination stage, these mesoscale systems were mainly located at the basin edges. In terms of tracks, most MCSs moved eastward under the influence of the westerlies and the MαCSs moved faster but in a more scattered manner than the MβCSs. Additionally, some unusual tracks appeared because of the effects of the underlying surface topography and environmental wind.

Characteristics of a mesoscale convective system in a warm zone that created an extreme, short rainstorm in Southern China in May 2016

An extremely intense rainstorm occurred on 20 May 2016. It had been the heaviest rainfall event recorded in Xinyi, Guangdong Province, China, since 1960. The black body temperature (TBB) output of FY-2B satellite images, weather radar, and NCEP/NCAR reanalysis data were used to investigate the characteristics of a mesoscale convective system (MCS) associated with the rainstorm. Heavy rainstorms formed under the influence of an eastward migration of a low trough at 500 hPa, typical of warm area heavy rain. In front of the 500 hPa trough, there was strong coupling of high and low jet streams, high convective effective bit energy, strong middle and low water vapor convergence and high ground dew point temperature, creating a favorable environment for the formation and development of MCSs. The convective region of the linear convective cloud developed into an organized MCS, and Xinyi’s heavy rainfall occurred during the MCS lifecycle from formation to maturity to extinction. Five MCSs, with radar echo intensities of 40-55 dBz, created a ‘train effect’, which was the main driver of the heavy rainstorm. The strong echoes observed above 40 dBz were below 0°C, and the strongest echo center was near 2 km. The convective precipitation system had a low mass center and high precipitation efficiency. The echo top (with intensity ≥ 10 dBz) was as high as 19 km, and there was a deep ice phase growth belt above -10°C, which contributed to the heavy rainfall, which peaked at 132.8 mm in one hour.

Convective-Permitting Hindcast Simulations during the North American Monsoon GPS Transect Experiment 2013: Establishing Baseline Model Performance without Data Assimilation

AbstractDuring the North American monsoon global positioning system (GPS) Transect Experiment 2013, daily convective-permitting WRF simulations are performed in northwestern Mexico and the southern Arizona border region using the operational Global Forecast System (GFS) and North American Mesoscale Forecast System (NAM) models as lateral boundary forcing and initial conditions. Compared to GPS precipitable water vapor (PWV), the WRF simulations display a consistent moist bias in the initial specification of PWV leading to convection beginning 3–6 h early. Given appreciable observed rainfall, days are classified as strongly and weakly forced based only on the presence of an inverted trough (IV); gulf surges did not noticeably impact the development of mesoscale convective systems (MCSs) and related convection in northwestern Mexico. Strongly forced days display higher modeled precipitation forecast skill than weakly forced days in the slopes of the northern Sierra Madre Occidental (SMO) away from the crest, especially toward the west where MCSs account for the greatest proportion of all monsoon-related precipitation. A case study spanning 8–10 July 2013 illustrates two consecutive days when nearly identical MCSs evolved over northern Sonora. Although a salient MCS is simulated on the strongly forced day (9–10 July 2013) when an IV is approaching the core monsoon region, a simulated MCS is basically nonexistent on the weakly forced day (8–9 July 2013) when the IV is farther away. The greater sensitivity to the initial specification of PWV in the weakly forced day suggests that assimilation of GPS-derived PWV for these types of days may be of greatest value in improving model precipitation forecasts.

Landfall tropical cyclone rainstorms on the north slope of the Dabie Mountains

The formation and development mechanism of landfall cyclone rainstorms that occur on the north slope of the Dabie Mountains were investigated by the determination of typical occurrences. Interaction between the tropical cyclone and the westerly trough was characterized by the favorable circulation backgrounds of landfall tropical cyclone rainstorms on the north slope of the Dabie Mountains. A conveyor belt was created between the easterly jet flow of the tropical cyclone and the subtropical high pressure of the western equatorial Pacific Ocean and the southerly jet flow of the westerly trough front, creating a huge amount of energy and vapor from the landfall tropical cyclone in the rainstorm area and destabilizing the stratification. These conditions were advantageous to the frontogenesis of a warm front and the development of Mesoscale convective systems (MCS) in the westerly cold air that met the inverted trough located at the northern portion of the tropical cyclone. The existence and development of the mesoscale front area in the ground provide a trigger mechanism for the rainstorm. The MCS occurred and developed in the equivalent potential temperature theta se (θse) frontal zone, which is located between the low pressure area of the typhoon and the cold air, which is located at the rear of the westerly trough. The terrain block slowed or stopped the motion of the low pressure system formed by the landfall tropical cyclone, which was conducive to the enhancement of the rainstorm.

Numerical Simulations of the South American Low Level Jet in Two Episodes of MCSs: Sensitivity to PBL and Convective Parameterization Schemes

The sensitivity of numerical simulations of the low level jet stream (LLJS) in South America to the choice of parameterization schemes for the planetary boundary layer (PBL) and for cumulus convection using the Advanced Research core of the Weather Research and Forecast (WRF) model was assessed for two cases in which the development of Mesoscale Convective Systems (MCSs) was observed in the La Plata Basin at the exit of the LLJS. The MCSs developed under distinct synoptic forcing. Overall, the general area over the La Plata Basin where the wind profiles met LLJS criteria was larger in the situation with stronger frontal forcing. Regarding the impact of the choice of the PBL parameterization scheme upon the simulated LLJS, the nonlocal Yonsei University (YSU) scheme displayed slightly better results for most simulations regardless of the cumulus parameterization scheme utilized. In fact, the characterization of the LLJS in the simulations exhibited no significant sensitivity to the choice of the cumulus parameterization. In situations under stronger [weaker] frontal forcing, less [more] dispersion among the simulations was found regarding the identification of the LLJS.

Extreme Rainfall of the South American Monsoon System: A Dataset Comparison Using Complex Networks

Abstract In this study, the authors compare six different rainfall datasets for South America with a focus on their representation of extreme rainfall during the monsoon season (December–February): the gauge-calibrated TRMM 3B42 V7 satellite product; the near-real-time TRMM 3B42 V7 RT, the GPCP 1° daily (1DD) V1.2 satellite–gauge combination product, the Interim ECMWF Re-Analysis (ERA-Interim) product; output of a high-spatial-resolution run of the ECHAM6 global circulation model; and output of the regional climate model Eta. For the latter three, this study can be understood as a model evaluation. In addition to statistical values of local rainfall distributions, the authors focus on the spatial characteristics of extreme rainfall covariability. Since traditional approaches based on principal component analysis are not applicable in the context of extreme events, they apply and further develop methods based on complex network theory. This way, the authors uncover substantial differences in extreme rainfall patterns between the different datasets: (i) The three model-derived datasets yield very different results than the satellite–gauge combinations regarding the main climatological propagation pathways of extreme events as well as the main convergence zones of the monsoon system. (ii) Large discrepancies are found for the development of mesoscale convective systems in southeastern South America. (iii) Both TRMM datasets and ECHAM6 indicate a linkage of extreme rainfall events between the central Amazon basin and the eastern slopes of the central Andes, but this pattern is not reproduced by the remaining datasets. The authors’ study suggests that none of the three model-derived datasets adequately captures extreme rainfall patterns in South America.

Initiation, maintenance, and properties of convection in an extreme rainfall event during SCMREX: Observational analysis

AbstractA long‐lived mesoscale convective system (MCS) with extreme rainfall over the western coastal region of Guangdong on 10 May 2013 during the Southern China Monsoon Rainfall Experiment (SCMREX) is studied. The environmental conditions are characterized by little convective inhibition, low‐lifting condensation level, moderate convective available potential energy and precipitable water, and lack of low‐level jets from the tropical ocean. Repeated convective back building and subsequent northeastward “echo training” of convective cells are found during the MCS's development stages. However, the initiation/maintenance factors and organization of convection differ significantly during the earlier and later stages. From midnight to early morning, convection is continuously initiated as southeasterly flows near the surface impinge on the east side of mesoscale mountains near the coastal lines and then moves northeastward, leading to formation of two quasi‐stationary rainbands. From early morning to early afternoon, new convection is repeatedly triggered along a mesoscale boundary between precipitation‐induced cold outflows and warm air from South China Sea and Gulf of Tokin, resulting in the formation of “band training” of several parallel rainbands that move eastward in a later time, i.e., two scales of “training” of convective elements are found. As the MCS dissipates, a stronger squall line moves into the region from the west and passes over within about 3.5 h, contributing about 10%–15% to the total rainfall amount. It is concluded that terrain, near‐surface winds, warm advection from the upstream ocean in the boundary layer, and precipitation‐generated cold outflows play important roles in initiating and maintaining the extreme rain‐producing MCS.

Initiation and Organizational Modes of an Extreme-Rain-Producing Mesoscale Convective System along a Mei-Yu Front in East China

Abstract The initiation and organization of a quasi-linear extreme-rain-producing mesoscale convective system (MCS) along a mei-yu front in east China during the midnight-to-morning hours of 8 July 2007 are studied using high-resolution surface observations and radar reflectivity, and a 24-h convection-permitting simulation with the nested grid spacing of 1.11 km. Both the observations and the simulation reveal that the quasi-linear MCS forms through continuous convective initiation and organization into west–east-oriented rainbands with life spans of about 4–10 h, and their subsequent southeastward propagation. Results show that the early convective initiation at the western end of the MCS results from moist southwesterly monsoonal flows ascending cold domes left behind by convective activity that develops during the previous afternoon-to-evening hours, suggesting a possible linkage between the early morning and late afternoon peaks of the mei-yu rainfall. Two scales of convective organization are found during the MCS's development: one is the east- to northeastward “echo training” of convective cells along individual rainbands, and the other is the southeastward “band training” of the rainbands along the quasi-linear MCS. The two organizational modes are similar within the context of “training” of convective elements, but they differ in their spatial scales and movement directions. It is concluded that the repeated convective backbuilding and the subsequent echo training along the same path account for the extreme rainfall production in the present case, whereas the band training is responsible for the longevity of the rainbands and the formation of the quasi-linear MCS.

Complex networks identify spatial patterns of extreme rainfall events of the South American Monsoon System

We investigate the spatial characteristics of extreme rainfall synchronicity of the South American Monsoon System (SAMS) by means of Complex Networks (CN). By introducing a new combination of CN measures and interpreting it in a climatic context, we investigate climatic linkages and classify the spatial characteristics of extreme rainfall synchronicity. Although our approach is based on only one variable (rainfall), it reveals the most important features of the SAMS, such as the main moisture pathways, areas with frequent development of Mesoscale Convective Systems (MCS), and the major convergence zones. In addition, our results reveal substantial differences between the spatial structures of rainfall synchronicity above the 90th and above the 95th percentiles. Most notably, events above the 95th percentile contribute stronger to MCS in the La Plata Basin.

A Conceptual Model for the Identification of Active Red Sea Trough Synoptic Events over the Southeastern Mediterranean

AbstractA phenomenon characterized by a tongue of low pressure extending northward from the southern Red Sea [Red Sea Trough (RST)] toward the eastern Mediterranean Sea (EM) is analyzed. In general, the RST is associated with hot and dry weather, resulting from east-southeasterly flows in the lower troposphere. In some cases, the RST is found to be accompanied by an upper-tropospheric trough extending from the north over the EM. Such conditions are associated with unstable stratification, favoring the development of mesoscale convective systems. This kind of RST has been defined as an “active” RST (ARST). The ARST phenomenon represents a serious threat to human society in the northeastern Africa–southeastern Mediterranean region, being in some cases associated with devastating floods. In this study, a conceptual model of the ARST phenomenon is discussed, and then an algorithm for the identification of ARST events is presented. The identification algorithm has been applied to a multiyear NCEP–NCAR reanalysis data archive for both RST and ARST events. From the results of a composite analysis of several different atmospheric flow parameters associated with ARST events, the key features associated with ARST events are identified. The results from the analysis of the composite patterns support the suggestion that high amounts of moisture transported from tropical Africa in the form of an atmospheric river to the Red Sea–EM play a key role in determining the intensity of the ARST events.

Characteristics of Precipitation Systems and Their Environment Observed during the Onset of the Western North Pacific Summer Monsoon in 2008

This paper has investigated the structure, propagation, orientation, and rainfall of precipitation systems and environmental wind and thermodynamic structures observed by the Doppler radar and the radiosonde system on board the R/V Mirai around a fixed site (12°N, 135°E) during the onset of the western North Pacific summer monsoon in 2008. The monsoon onset occurred in mid-June and was accompanied by a remarkable change in large-scale circulation over the observational area.Statistical analyses indicate that precipitation systems evolved deeply and widely upon the monsoon onset. The development of mesoscale convective systems contributed to the formation of much more rainfall. Precipitation systems tended to move along with low-level winds during the pre-onset and the early-onset periods and with mid-level winds during the latter-onset period. The increase in the height and the area of precipitation systems possessed much stronger correlation with the evolution of the low-level shear and the mid-level humidity than with other environmental variables.The internal structure of two precipitation systems that occurred in the environment with the distinct low-level shear and mid-level humidity has also been analyzed. The shear-parallel precipitation system during the early-onset period occurred in the environment with the dry middle troposphere and the weak low-level shear and was characterized by the descending rear inflow and weaker updrafts. On the other hand, the shear-normal precipitation system during the latter-onset period occurred in the environment with the moist middle troposphere and the strong low-level shear and was featured with the elevated rear inflow and stronger updrafts.According to the observational results, possible interaction processes between precipitation systems and their environment during the monsoon onset are discussed.

Numerical Investigations on the Formation of Tropical Storm Debby during NAMMA-06

Abstract Mesoscale model forecasts were carried out beginning at 0000 UTC 19 August for simulating Tropical Disturbance 4, which was named Tropical Storm Debby on 22 August 2006. The Weather Research and Forecasting model, with 25-km grid spacing and an inner nested domain of 5-km grid spacing, was used. The development of a small closed vortex at approximately 0600 UTC 20 August 2006 at 850 hPa was found off the coast of Guinea in agreement with satellite images in the 5-km simulation. Intense convection offshore and over the Guinea Highlands during the morning of 20 August 2006 led to the production of a vortex formation by 1400 UTC at 700 hPa. Sensitivity tests show that the Guinea Highlands play an important role in modulating the impinging westerly flow, in which low-level flow deflections (i.e., northward turning) enhance the cyclonic circulation of the vortex formation. Yet, the moist air can be transported by the northward deflection flow from lower latitudes to support the development of mesoscale convective systems (MCSs). Although the model forecast is not perfect, it demonstrates the predictability of the formation and development of the tropical disturbance associated with the Guinea Highlands.

The Role of Upstream Midtropospheric Circulations in the Sierra Nevada Enabling Leeside (Spillover) Precipitation. Part II: A Secondary Atmospheric River Accompanying a Midlevel Jet

Abstract The synoptic structure of two case studies of heavy “spillover” or leeside precipitation—1–2 January 1997 and 30–31 December 2005—that resulted in Truckee River flooding are analyzed over the North Pacific beginning approximately 7 days prior to the events. Several sequential cyclone-scale systems are tracked across the North Pacific, culminating in the strengthening and elongation of a polar jet stream’s deep exit region over northern California and Nevada. These extratropical cyclones separate extremely cold air from Siberia from an active intertropical convergence zone with broad mesoscale convective systems and tropical cyclones. The development of moisture surges resulting in leeside flooding precipitation over the Sierra Nevada is coupled to adjustments within the last wave in the sequence of cyclone waves. Stage I of the process occurs as the final wave moves across the Pacific and its polar jet streak becomes very long, thus traversing much of the eastern Pacific. Stage II involves the development of a low-level return branch circulation [low-level jet (LLJ)] within the exit region of the final cyclone scale wave. Stage III is associated with the low-level jet’s convergence under the upper-level divergence within the left exit region, which results in upward vertical motions, dynamic destabilization, and the development of mesoscale convective systems (MCSs). Stage IV is forced by the latent heating and subsynoptic-scale ridging caused by each MCS, which results in a region of diabatic isallobaric accelerations downstream from the MCS-induced mesoridge. During stage IV the convectively induced accelerating flow, well to the southeast of the upper-level jet core, organizes a midlevel jet and plume of moisture or midlevel atmospheric river, which is above and frequently out of phase with (e.g., southeast of) the low-level atmospheric river described in Ralph et al. ahead of the surface cold front. Stage V occurs as the final sequential midlevel river arrives over the Sierra Nevada. It phases with the low-level river, allowing upslope and midlevel moisture advection, thus creating a highly concentrated moist plume extending from near 700 to nearly 500 hPa, which subsequently advects moisture over the terrain. When simulations are performed without upstream convective heating, the horizontal moisture fluxes over the Sierra Nevada are reduced by ∼30%, indicating the importance of convection in organizing the midlevel atmospheric rivers. The convective heating acts to accelerate the midlevel jet flow and create the secondary atmospheric river between ∼500 and 700 hPa near the 305-K isentropic surface. This midlevel moisture surge slopes forward with height and transports warm moist air over the Sierra Nevada to typically rain shadowed regions on the lee side of the range. Both observationally generated and model-generated back trajectories confirm the importance of this convectively forced rapid lifting process over the North Pacific west of the California coast ∼12 h and ∼1200 km upstream prior to heavy leeside spillover precipitation over the Sierra Nevada.