Extreme precipitation trends associated with tropical cyclones in the core of the North American monsoon

In this study the results of two regional fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) simulations forced at their boundaries with low-pass-filtered North American Regional Reanalysis (NARR) composite fields from which synoptic-scale variability was removed are presented. The filtered NARR data are also assimilated into the inner domain through the use of field nudging. The purpose of this research is to investigate wet and dry onset modes in the core region of the North American monsoon (NAM). Key features of the NAM that are present in the NARR fields and assimilated into the regional simulations include the position of the midlevel anticyclone, low-level circulation over the Gulf of California, and moisture flux patterns into the core monsoon region, for which the eastern Pacific is the likely primary source of moisture. The model develops a robust diurnal cycle of deep convection over the peaks of the Sierra Madre Occidental (SMO) that results solely from its radiation scheme and internal dynamics, in spite of the field nudging. The wet onset mode is related to a regional land–sea thermal contrast (LSTC) that is ~2°C higher than in the dry mode, and is further characterized by a northward-displaced midlevel anticyclone, a stronger surface pressure gradient along the Gulf of California, larger mean moisture fluxes into the core region from the eastern Pacific, a stronger diurnal cycle of deep convection, and the more northward distribution of precipitation along the axis of the SMO. A proposed regional LSTC mechanism for NAM onset interannual variability is consistent with the differences between both onset modes.

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Simulations of the 2004 North American Monsoon: NAMAP2

Journal of Climate ◽

10.1175/2009jcli3138.1 ◽

2009 ◽

Vol 22 (24) ◽

pp. 6716-6740 ◽

Cited By ~ 27

Author(s):

D. S. Gutzler ◽

L. N. Long ◽

J. Schemm ◽

S. Baidya Roy ◽

M. Bosilovich ◽

...

Keyword(s):

Boundary Conditions ◽

North American ◽

Warm Season ◽

North American Monsoon ◽

Regional Models ◽

Global Models ◽

Low Level ◽

The Core ◽

The North ◽

Low Level Jet

Abstract The second phase of the North American Monsoon Experiment (NAME) Model Assessment Project (NAMAP2) was carried out to provide a coordinated set of simulations from global and regional models of the 2004 warm season across the North American monsoon domain. This project follows an earlier assessment, called NAMAP, that preceded the 2004 field season of the North American Monsoon Experiment. Six global and four regional models are all forced with prescribed, time-varying ocean surface temperatures. Metrics for model simulation of warm season precipitation processes developed in NAMAP are examined that pertain to the seasonal progression and diurnal cycle of precipitation, monsoon onset, surface turbulent fluxes, and simulation of the low-level jet circulation over the Gulf of California. Assessment of the metrics is shown to be limited by continuing uncertainties in spatially averaged observations, demonstrating that modeling and observational analysis capabilities need to be developed concurrently. Simulations of the core subregion (CORE) of monsoonal precipitation in global models have improved since NAMAP, despite the lack of a proper low-level jet circulation in these simulations. Some regional models run at higher resolution still exhibit the tendency observed in NAMAP to overestimate precipitation in the CORE subregion; this is shown to involve both convective and resolved components of the total precipitation. The variability of precipitation in the Arizona/New Mexico (AZNM) subregion is simulated much better by the regional models compared with the global models, illustrating the importance of transient circulation anomalies (prescribed as lateral boundary conditions) for simulating precipitation in the northern part of the monsoon domain. This suggests that seasonal predictability derivable from lower boundary conditions may be limited in the AZNM subregion.

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The Diurnal Cycle of Precipitation over the North American Monsoon Region during the NAME 2004 Field Campaign

Journal of Climate ◽

10.1175/2007jcli1642.1 ◽

2008 ◽

Vol 21 (4) ◽

pp. 771-787 ◽

Cited By ~ 16

Author(s):

Emily J. Becker ◽

Ernesto Hugo Berbery

Keyword(s):

Diurnal Cycle ◽

North American ◽

Gulf Of California ◽

Warm Season ◽

Moisture Flux ◽

North American Monsoon ◽

The Core ◽

The North ◽

Eta Model ◽

Diurnal Cycle Of Precipitation

Abstract The structure of the diurnal cycle of warm-season precipitation and its associated fields during the North American monsoon are examined for the core monsoon region and for the southwestern United States, using a diverse set of observations, analyses, and forecasts from the North American Monsoon Experiment field campaign of 2004. Included are rain gauge and satellite estimates of precipitation, Eta Model forecasts, and the North American Regional Reanalysis (NARR). Daily rain rates are of about the same magnitude in all datasets with the exception of the Climate Prediction Center (CPC) Morphing (CMORPH) technique, which exhibits markedly higher precipitation values. The diurnal cycle of precipitation within the core region occurs earlier in the day at higher topographic elevations, evolving with a westward shift of the maximum. This shift appears in the observations, reanalysis, and, while less pronounced, in the model forecasts. Examination of some of the fields associated with this cycle, including convective available potential energy (CAPE), convective inhibition (CIN), and moisture flux convergence (MFC), reveals that the westward shift appears in all of them, but more prominently in the latter. In general, warm-season precipitation in southern Arizona and parts of New Mexico shows a strong effect due to northward moisture surges from the Gulf of California. A reported positive bias in the NARR northward winds over the Gulf of California limits their use with confidence for studies of the moist surges along the Gulf; thus, the analysis is complemented with operational analysis and the Eta Model short-term simulations. The nonsurge diurnal cycle of precipitation lags the CAPE maximum by 6 h and is simultaneous with a minimum of CIN, while the moisture flux remains divergent throughout the day. During surges, CAPE and CIN have modifications only to the amplitude of their cycles, but the moisture flux becomes strongly convergent about 6 h before the precipitation maximum, suggesting a stronger role in the development of precipitation.

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Characteristics, Origins, and Impacts of Summertime Extreme Precipitation in the Lake Mead Watershed

Journal of Climate ◽

10.1175/jcli-d-19-0387.1 ◽

2020 ◽

Vol 33 (7) ◽

pp. 2663-2680 ◽

Cited By ~ 1

Author(s):

Michael D. Sierks ◽

Julie Kalansky ◽

Forest Cannon ◽

F. M. Ralph

Keyword(s):

North American ◽

Extreme Precipitation ◽

Extreme Rainfall ◽

North American Monsoon ◽

Lake Mead ◽

Extreme Rainfall Events ◽

Rainfall Events ◽

The North ◽

The U.S ◽

Tier I

AbstractThe North American monsoon (NAM) is the main driver of summertime climate variability in the U.S. Southwest. Previous studies of the NAM have primarily focused on the Tier I region of the North American Monsoon Experiment (NAME), spanning central-western Mexico, southern Arizona, and New Mexico. This study, however, presents a climatological characterization of summertime precipitation, defined as July–September (JAS), in the Lake Mead watershed, located in the NAME Tier II region. Spatiotemporal variability of JAS rainfall is examined from 1981 to 2016 using gridded precipitation data and the meteorological mechanisms that account for this variability are investigated using reanalyses. The importance of the number of wet days (24-h rainfall ≥1 mm) and extreme rainfall events (95th percentile of wet days) to the total JAS precipitation is examined and shows extreme events playing a larger role in the west and central basin. An investigation into the dynamical drivers of extreme rainfall events indicates that anticyclonic Rossby wave breaking (RWB) in the midlatitude westerlies over the U.S. West Coast is associated with 89% of precipitation events >10 mm (98th percentile of wet days) over the Lake Mead basin. This is in contrast to the NAME Tier I region where easterly upper-level disturbances such as inverted troughs are the dominant driver of extreme precipitation. Due to the synoptic nature of RWB events, corresponding impacts and hazards extend beyond the Lake Mead watershed are relevant for the greater U.S. Southwest.

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Dynamics of a Simulated North American Monsoon Gulf Surge Event

Monthly Weather Review ◽

10.1175/mwr-d-12-00294.1 ◽

2013 ◽

Vol 141 (9) ◽

pp. 3238-3253 ◽

Cited By ~ 6

Author(s):

Andrew J. Newman ◽

Richard H. Johnson

Keyword(s):

United States ◽

North American ◽

Gulf Of California ◽

Dynamical Evolution ◽

North American Monsoon ◽

The Core ◽

Southwest United States ◽

Northwest Mexico ◽

The North ◽

Internal Bores

Abstract Gulf surges are transient disturbances that propagate along the Gulf of California (GoC) from south to north, transporting cool moist air toward the deserts of northwest Mexico and the southwest United States during the North American monsoon. They have been shown to modulate precipitation and have been linked to severe weather and flooding in northern Mexico and the southwest United States. The general features and progression of surge events are well documented but their detailed dynamical evolution is still unclear. In this study, a convection-permitting simulation is performed over the core monsoon region for the 12–14 July 2004 gulf surge event and the dynamics of the simulated surge are examined. Initially, convection associated with the tropical easterly wave precursor to Tropical Cyclone Blas creates a disturbance in the southern GoC on early 12 July. This disturbance is a precursor to the gulf surge on 13 July and is a Kelvin shock (internal bore under the influence of rotation) that dissipates in the central GoC. The surge initiates from inflow from the mouth of the GoC along with convective outflow impinging on the southern GoC. Continued convective outflow along the GoC generates multiple gravity currents and internal bores while intensifying the simulated surge as it propagates up the GoC. As the core of the surge reaches the northern GoC, a Kelvin shock is again the best dynamical fit to the phenomenon. Substantial low-level cooling and moistening are associated with the modeled surge along the northern GoC as is observed.

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Mechanical forcing of the North American monsoon by orography

10.21203/rs.3.rs-514304/v1 ◽

2021 ◽

Author(s):

William Boos ◽

Salvatore Pascale

Keyword(s):

North American ◽

Land Surface ◽

Global Climate ◽

Global Climate Model ◽

Stationary Wave ◽

Heat Fluxes ◽

North American Monsoon ◽

The Core ◽

The North ◽

Orographic Rainfall

Abstract The core of the North American monsoon consists of a band of intense rainfall along the west coast of Mexico[1, 2] and is commonly thought to be caused by thermal forcing from both land and the elevated terrain of that region[3-5]. Here we use observations, a global climate model, and stationary wave solutions to show that this rainfall maximum is instead generated when Mexico's Sierra Madre mountains mechanically force an adiabatic stationary wave by diverting extratropical eastward winds toward the equator; eastward, upslope flow in that wave lifts warm and moist air to produce convective rainfall. Land surface heat fluxes do precondition the atmosphere for convection, particularly in summer afternoons, but even if amplified are insufficient for producing the observed rainfall maximum. These results, together with dynamical structures in observations and models, indicate that the core monsoon should be understood as convectively enhanced orographic rainfall in a mechanically forced stationary wave, not as a classic, thermally forced tropical monsoon. This has implications for the response of the North American monsoon to past and future global climate change, making trends in jet stream interactions with orography of central importance.

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