Sensitivity to Horizontal Resolution in the AGCM Simulations of Warm Season Diurnal Cycle of Precipitation over the United States and Northern Mexico

2007 ◽  
Vol 20 (9) ◽  
pp. 1862-1881 ◽  
Author(s):  
Myong-In Lee ◽  
Siegfried D. Schubert ◽  
Max J. Suarez ◽  
Isaac M. Held ◽  
Arun Kumar ◽  
...  
Keyword(s):  
Great Plains ◽  
Diurnal Cycle ◽  
North American ◽  
General Circulation ◽  
Warm Season ◽  
The United States ◽  
Horizontal Resolution ◽  
Moist Convection ◽  
The North ◽  
Diurnal Cycle Of Precipitation

Abstract This study examines the sensitivity of the North American warm season diurnal cycle of precipitation to changes in horizontal resolution in three atmospheric general circulation models, with a primary focus on how the parameterized moist processes respond to improved resolution of topography and associated local/regional circulations on the diurnal time scale. It is found that increasing resolution (from approximately 2° to ½° in latitude–longitude) has a mixed impact on the simulated diurnal cycle of precipitation. Higher resolution generally improves the initiation and downslope propagation of moist convection over the Rockies and the adjacent Great Plains. The propagating signals, however, do not extend beyond the slope region, thereby likely contributing to a dry bias in the Great Plains. Similar improvements in the propagating signals are also found in the diurnal cycle over the North American monsoon region as the models begin to resolve the Gulf of California and the surrounding steep terrain. In general, the phase of the diurnal cycle of precipitation improves with increasing resolution, though not always monotonically. Nevertheless, large errors in both the phase and amplitude of the diurnal cycle in precipitation remain even at the highest resolution considered here. These errors tend to be associated with unrealistically strong coupling of the convection to the surface heating and suggest that improved simulations of the diurnal cycle of precipitation require further improvements in the parameterizations of moist convection processes.

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

2008 ◽  
Vol 21 (4) ◽  
pp. 771-787 ◽  
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.

scholarly journals An Analysis of the Warm-Season Diurnal Cycle over the Continental United States and Northern Mexico in General Circulation Models

2007 ◽  
Vol 8 (3) ◽  
pp. 344-366 ◽  
Author(s):  
Myong-In Lee ◽  
Siegfried D. Schubert ◽  
Max J. Suarez ◽  
Isaac M. Held ◽  
Ngar-Cheung Lau ◽  
...  
Keyword(s):  
United States ◽  
Great Plains ◽  
Diurnal Cycle ◽  
Rocky Mountains ◽  
General Circulation ◽  
Warm Season ◽  
General Circulation Models ◽  
Surface Boundary ◽  
Southeast United States ◽  
Northern Mexico

Abstract The diurnal cycle of warm-season rainfall over the continental United States and northern Mexico is analyzed in three global atmospheric general circulation models (AGCMs) from NCEP, GFDL, and the NASA Global Modeling Assimilation Office (GMAO). The results for each model are based on an ensemble of five summer simulations forced with climatological sea surface temperatures. Although the overall patterns of time-mean (summer) rainfall and low-level winds are reasonably well simulated, all three models exhibit substantial regional deficiencies that appear to be related to problems with the diurnal cycle. Especially prominent are the discrepancies in the diurnal cycle of precipitation over the eastern slopes of the Rocky Mountains and adjacent Great Plains, including the failure to adequately capture the observed nocturnal peak. Moreover, the observed late afternoon–early evening eastward propagation of convection from the mountains into the Great Plains is not adequately simulated, contributing to the deficiencies in the diurnal cycle in the Great Plains. In the southeast United States, the models show a general tendency to rain in the early afternoon—several hours earlier than observed. Over the North American monsoon region in the southwest United States and northern Mexico, the phase of the broad-scale diurnal convection appears to be reasonably well simulated, though the coarse resolution of the runs precludes the simulation of key regional phenomena. All three models employ deep convection schemes that assume fundamentally the same buoyancy closure based on simplified versions of the Arakawa–Schubert scheme. Nevertheless, substantial differences between the models in the diurnal cycle of convection highlight the important differences in their implementations and interactions with the boundary layer scheme. An analysis of local diurnal variations of convective available potential energy (CAPE) shows an overall tendency for an afternoon peak—a feature well simulated by the models. The simulated diurnal cycle of rainfall is in phase with the local CAPE variation over the southeast United States and the Rocky Mountains where the local surface boundary forcing is important in regulating the diurnal cycle of convection. On the other hand, the simulated diurnal cycle of rainfall tends to be too strongly tied to CAPE over the Great Plains, where the observed precipitation and CAPE are out of phase, implying that free atmospheric large-scale forcing plays a more important role than surface heat fluxes in initiating or inhibiting convection.

scholarly journals Simulation of the North American Monsoon by the NCAR CCM3 and Its Sensitivity to Convection Parameterization

2006 ◽  
Vol 19 (12) ◽  
pp. 2851-2866 ◽  
Author(s):  
J. Craig Collier ◽  
Guang J. Zhang
Keyword(s):  
Relative Humidity ◽  
Diurnal Cycle ◽  
North American ◽  
Large Scale ◽  
The United States ◽  
Experimental Simulation ◽  
Specific Humidity ◽  
North American Monsoon ◽  
Hourly Precipitation ◽  
The North

Abstract Two 9-yr runs of the NCAR Community Climate Model version 3 (CCM3) are compared in their simulations of the North American summer monsoon. In a control simulation, the Zhang–McFarlane deep convection scheme is used. For an experimental simulation, the following modifications to the scheme are implemented. The closure is based on the large-scale forcing of virtual temperature, and a relative humidity threshold on convective parcels lifted from the boundary layer is applied. The sensitivity to these modifications for simulating the North American monsoon is investigated. Model validation relies on hourly precipitation rates from surface gauges over the United States, hourly precipitation rates derived from the combination of microwave and radar measurements from NASA’s Tropical Rainfall Measuring Mission (TRMM) satellite over Mexico, and CAPE values as calculated from temperature, specific humidity, and pressure fields from the NCEP–NCAR reanalysis. Results show that the experimental run improves the timing of the monsoon onset and peak in the regions of core monsoon influence considered here, though it increases a negative bias in the peak monsoon intensity in one region of northern Mexico. Sensitivity of the diurnal cycle of precipitation to modifications in the convective scheme is highly geographically dependent. Using a combination of gauge-based rainfall rates and reanalysis-based CAPE, it is found that improvements in the simulated diurnal cycle are confined to a convective regime in which the diurnal evolution of precipitation is observed to lag that of CAPE. For another regime, in which CAPE is observed to be approximately in phase with precipitation, model phase biases increase nearly everywhere. Some of the increased phase biases in the latter regime are primarily because of application of the relative humidity threshold.

A Brief Evaluation of Precipitation from the North American Regional Reanalysis

2007 ◽  
Vol 8 (4) ◽  
pp. 837-846 ◽  
Author(s):  
Melissa S. Bukovsky ◽  
David J. Karoly
Keyword(s):  
United States ◽  
Spatial Distribution ◽  
Diurnal Cycle ◽  
Annual Cycle ◽  
North American ◽  
The United States ◽  
The Other ◽  
The North ◽  
North American Regional Reanalysis ◽  
Regional Reanalysis

Abstract Several aspects of the precipitation climatology from the North American Regional Reanalysis (NARR) are analyzed and compared with two other reanalyses and one set of gridded observations over a domain encompassing the United States. The spatial distribution, diurnal cycle, and annual cycle of precipitation are explored to establish the reliability of the reanalyses and to judge their usefulness. While the NARR provides a much improved representation of precipitation over that of the other reanalyses examined, some inaccuracies are found and have been highlighted as a warning to potential users of the data.

scholarly journals The Relationship of Transient Upper-Level Troughs to Variability of the North American Monsoon System

2009 ◽  
Vol 22 (15) ◽  
pp. 4213-4227 ◽  
Author(s):  
Stephen W. Bieda ◽  
Christopher L. Castro ◽  
Steven L. Mullen ◽  
Andrew C. Comrie ◽  
Erik Pytlak
Keyword(s):  
Diurnal Cycle ◽  
North American ◽  
Low Frequency ◽  
Warm Season ◽  
Early Summer ◽  
North American Monsoon ◽  
The North ◽  
Sst Variability ◽  
Upper Level ◽  
Regional Reanalysis

Abstract Relationships between transient upper-tropospheric troughs and warm season convective activity over the southwest United States and northern Mexico are explored. Analysis of geopotential height and vorticity fields from the North American Regional Reanalysis and cloud-to-ground lightning data indicates that the passage of mobile inverted troughs (IVs) significantly enhances convection when it coincides with the peak diurnal cycle (1800–0900 UTC) over the North American monsoon (NAM) region. The preferred tracks of IVs during early summer are related to the dominant modes of Pacific sea surface temperature (SST) variability. When La Niña–like (El Niño–like) conditions prevail in the tropical Pacific and the eastern North Pacific has a horseshoe-shaped negative (positive) SST anomaly, IVs preferentially track farther north (south) and are slightly (typically one IV) more (less) numerous. These results point to the important role that synoptic-scale disturbances play in modulating the diurnal cycle of precipitation over the NAM region and the significant impact that the statistically supported low-frequency Pacific SST anomalies exert on the occurrence and track of these synoptic transients.

scholarly journals Mechanisms of Tropical Atlantic SST Influence on North American Precipitation Variability*

2010 ◽  
Vol 23 (21) ◽  
pp. 5610-5628 ◽  
Author(s):  
Yochanan Kushnir ◽  
Richard Seager ◽  
Mingfang Ting ◽  
Naomi Naik ◽  
Jennifer Nakamura
Keyword(s):  
North Atlantic ◽  
North American ◽  
General Circulation ◽  
Warm Season ◽  
Circulation Model ◽  
Cold Season ◽  
Tropical Atlantic ◽  
The North ◽  
Sst Variability ◽  
The Impact

Abstract The dynamical mechanisms associated with the impact of year-to-year variability in tropical North Atlantic (TNA) sea surface temperatures (SSTs) on North American precipitation, during the cold and warm halves of the hydrological year (October–September) are examined. Observations indicate that during both seasons warmer-than-normal TNA SSTs are associated with a reduction of precipitation over North America, mainly west of ∼90°W, and that the effect can be up to 30% of the year-to-year seasonal precipitation RMS variability. This finding confirms earlier studies with observations and models. During the cold season (October–March) the North American precipitation variability associated with TNA fluctuations is considerably weaker than its association with ENSO. During the warm season (April–September), however, the Atlantic influence, per one standard deviation of SST anomalies, is larger than that of ENSO. The observed association between TNA SST anomalies and global and North American precipitation and sea level pressure variability is compared with that found in the output of an atmospheric general circulation model (AGCM) forced with observed SST variability, both globally and in the tropical Atlantic alone. The similarity between model output and observations suggests that TNA SST variability is causal. The mechanisms of the “upstream” influence of the Atlantic on North American precipitation are seasonally dependent. In the warm season, warmer-than-normal TNA SSTs induce a local increase in atmospheric convection. This leads to a weakening of the North Atlantic subtropical anticyclone and a reduction in precipitation over the United States and northern Mexico, associated with the anomalous southward flow there. In the cold season, a response similar to the warm season over the subtropical Atlantic is identified, but there is also a concomitant suppression of convection over the equatorial Pacific, which leads to a weakening of the Aleutian low and subsidence over western North America, similar to the impact of La Niña although weaker in amplitude. The impact of TNA SST on tropical convection and the extratropical circulation is examined by a set of idealized experiments with a linear general circulation model forced with the tropical heating field derived from the full AGCM.

scholarly journals Effects of Increased Horizontal Resolution on Simulation of the North American Monsoon in the NCAR CAM3: An Evaluation Based on Surface, Satellite, and Reanalysis Data

2007 ◽  
Vol 20 (9) ◽  
pp. 1843-1861 ◽  
Author(s):  
J. Craig Collier ◽  
Guang J. Zhang
Keyword(s):  
North American ◽  
Large Scale ◽  
Monsoon Season ◽  
The United States ◽  
Horizontal Resolution ◽  
North American Monsoon ◽  
Sierra Madre Occidental ◽  
The North ◽  
Sierra Madre ◽  
Monsoon System

Abstract Simulation of the North American monsoon system by the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM3) is evaluated in its sensitivity to increasing horizontal resolution. For two resolutions, T42 and T85, rainfall is compared to Tropical Rainfall Measuring Mission (TRMM) satellite-derived and surface gauge-based rainfall rates over the United States and northern Mexico as well as rainfall accumulations in gauges of the North American Monsoon Experiment (NAME) Enhanced Rain Gauge Network (NERN) in the Sierra Madre Occidental. Simulated upper-tropospheric mass and wind fields are compared to those from NCEP–NCAR reanalyses. The comparison presented herein demonstrates that tropospheric motions associated with the North American monsoon system are sensitive to increasing the horizontal resolution of the model. An increase in resolution from T42 to T85 results in changes to a region of large-scale midtropospheric descent found north and east of the monsoon anticyclone. Relative to its simulation at T42, this region extends farther south and west at T85. Additionally, at T85, the subsidence is stronger. Consistent with the differences in large-scale descent, the T85 simulation of CAM3 is anomalously dry over Texas and northeastern Mexico during the peak monsoon months. Meanwhile, the geographic distribution of rainfall over the Sierra Madre Occidental region of Mexico is more satisfactorily simulated at T85 than at T42 for July and August. Moisture import into this region is greater at T85 than at T42 during these months. A focused study of the Sierra Madre Occidental region in particular shows that, in the regional-average sense, the timing of the peak of the monsoon is relatively insensitive to the horizontal resolution of the model, while a phase bias in the diurnal cycle of monsoon season precipitation is somewhat reduced in the higher-resolution run. At both resolutions, CAM3 poorly simulates the month-to-month evolution of monsoon rainfall over extreme northwestern Mexico and Arizona, though biases are considerably improved at T85.

scholarly journals A Teleconnection between Forced Great Plains Snow Cover and European Winter Climate

2008 ◽  
Vol 21 (11) ◽  
pp. 2466-2483 ◽  
Author(s):  
Nicholas P. Klingaman ◽  
Brian Hanson ◽  
Daniel J. Leathers
Keyword(s):  
Snow Cover ◽  
Great Plains ◽  
North American ◽  
The United States ◽  
Northern Great Plains ◽  
Late Winter ◽  
Winter Climate ◽  
Early Season ◽  
Late Season ◽  
The North

Abstract Anomalies in Siberian snow cover have been shown to affect Eurasian winter climate through the North Atlantic Oscillation (NAO). The existence of a teleconnection between North American snow cover and the NAO is far less certain, particularly for limited, regional snow cover anomalies. Using three ensembles of the Community Atmosphere Model, version 2 (CAM2), the authors examined teleconnections between persistent, forced snow cover in the northern Great Plains of the United States and western Eurasian winters. One ensemble allowed the model to freely determine global snow cover, while the other two forced a 72-cm snowpack centered over Nebraska. Of the forced ensembles, the “early-season” (“late season”) simulations initiated the snowpack on 1 November (1 January). The additional snow cover generated lower (higher) sea level pressures and geopotential heights over Iceland (the Azores) and warmer (cooler) temperatures over northern and western (eastern and southeastern) Europe, which suggests the positive NAO phase. Differences between the free-snow-cover and early-season ensembles were never significant until January, which implied either that the atmospheric response required a long lag or that the late-winter atmosphere was particularly sensitive to Great Plains snow. The authors rejected the former hypothesis and supported the latter by noting similarities between the early- and late-season ensembles in late winter for European 2-m temperatures, transatlantic circulation, and an NAO index. Therefore, a regional North American snow cover anomaly in an area of high inter- and intra-annual snow cover variability can show a stronger teleconnection to European winter climate than previously reported for broader snow cover anomalies. In particular, anomalous late-season snow in the Great Plains may shift the NAO toward the positive phase.

Modulation of the Diurnal Cycle of Warm-Season Precipitation by Short-Wave Troughs

2013 ◽  
Vol 70 (6) ◽  
pp. 1710-1726 ◽  
Author(s):  
John D. Tuttle ◽  
Chris A. Davis
Keyword(s):  
Diurnal Cycle ◽  
North American ◽  
Phase Speed ◽  
Warm Season ◽  
Short Wave ◽  
Short Waves ◽  
The North ◽  
The Mean ◽  
Regional Reanalysis ◽  
Wave Passage

Abstract Traveling deep tropospheric disturbances of wavelengths ~1500 km (short waves) have long been known to play an important role in the initiation and maintenance of warm-season convection. To date, relatively few studies have formally documented the climatology of short waves and their relationship to the diurnal heating cycle, the topography, and the diurnal cycle of precipitation. Those that did had to rely on low-resolution global analyses and often could not track short waves across mountain barriers. In this study, 10 yr of the (32 km) NCEP North American Regional Reanalysis (NARR) are used to objectively identify and track short waves in the North American domain. Statistics of short-wave span, duration, phase speed, latitudinal extent, and locations of preferred intensification/decay are presented. Some of the key findings from the climatology include that the lee (windward) of mountain barriers are preferred regions of intensification (decay) and short waves show little evidence of a diurnal cycle and can pass a given point at any time of the day. The second part of the study focuses on the role that short waves play in modulating the diurnal cycle of propagating convection east of the Rocky Mountains. Depending on the timing of short-wave passage, short waves may either significantly enhance the precipitation above the mean or completely disrupt the normal diurnal cycle, causing precipitation to develop at times and locations where it normally does not. While short waves play an important role in modulating the mean precipitation patterns their role is considered to be secondary in nature. The diurnal precipitation signature is prominent even when short waves are not present.

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