scholarly journals Tropical Intraseasonal Variability in the MRI-20km60L AGCM*

2009 ◽  
Vol 22 (8) ◽  
pp. 2006-2022 ◽  
Author(s):  
Ping Liu ◽  
Yoshiyuki Kajikawa ◽  
Bin Wang ◽  
Akio Kitoh ◽  
Tetsuzo Yasunari ◽  
...  
Keyword(s):  
Intraseasonal Variability ◽  
Power Spectra ◽  
Cumulus Parameterization ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Loose Coupling ◽  
Low Level ◽  
Mature Phase ◽  
The Mean ◽  
Ceof Analysis

Abstract This study documents the detailed characteristics of the tropical intraseasonal variability (TISV) in the MRI-20km60L AGCM that uses a variant of the Arakawa–Schubert cumulus parameterization. Mean states, power spectra, propagation features, leading EOF modes, horizontal and vertical structures, and seasonality associated with the TISV are analyzed. Results show that the model reproduces the mean states in winds realistically and in convection comparable to that of the observations. However, the simulated TISV is less realistic. It shows low amplitudes in convection and low-level winds in the 30–60-day band. Filtered anomalies have standing structures. Power spectra and lag correlation of the signals do not propagate dominantly either in the eastward direction during boreal winter or in the northward direction during boreal summer. A combined EOF (CEOF) analysis shows that winds and convection have a loose coupling that cannot sustain the simulated TISV as realistically as that observed. In the composited mature phase of the simulated MJO, the low-level convergence does not lead convection clearly so that the moisture anomalies do not tilt westward in the vertical, indicating that the low-level convergence does not favor the eastward propagation. The less realistic TISV suggests that the representation of cumulus convection needs to be improved in this model.

scholarly journals Air, Sea, and Land Interactions of the Continental U.S. Hydroclimate

2009 ◽  
Vol 10 (2) ◽  
pp. 353-373 ◽  
Author(s):  
Vasubandhu Misra ◽  
P. A. Dirmeyer
Keyword(s):  
United States ◽  
General Circulation Model ◽  
General Circulation ◽  
Winter Season ◽  
Circulation Model ◽  
Moisture Flux ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Moisture Flux Convergence ◽  
The Mean

Abstract Multidecadal simulations over the continental United States by an atmospheric general circulation model coupled to an ocean general circulation model is compared with that forced by observed sea surface temperature (SST). The differences in the mean and the variability of precipitation are found to be larger in the boreal summer than in the winter. This is because the mean SST differences in the two simulations are qualitatively comparable between the two seasons. The analysis shows that, in the boreal summer season, differences in moisture flux convergence resulting from changes in the circulation between the two simulations initiate and sustain changes in precipitation between them. This difference in precipitation is, however, further augmented by the contributions from land surface evaporation, resulting in larger differences of precipitation between the two simulations. However, in the boreal winter season, despite differences in the moisture flux convergence between the two model integrations, the precipitation differences over the continental United States are insignificant. It is also shown that land–atmosphere feedback is comparatively much weaker in the boreal winter season.

scholarly journals The Residual-Mean Circulation in the Tropical Tropopause Layer Driven by Tropical Waves

2014 ◽  
Vol 71 (4) ◽  
pp. 1305-1322 ◽  
Author(s):  
David A. Ortland ◽  
M. Joan Alexander
Keyword(s):  
Seasonal Variations ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Minor Effect ◽  
Equatorial Waves ◽  
Latent Heating ◽  
Quasi Biennial Oscillation ◽  
Tropical Tropopause Layer ◽  
Tropical Tropopause ◽  
The Mean

Abstract Latent heating estimates derived from rainfall observations are used to construct model experiments that isolate equatorial waves forced by tropical convection from midlatitude synoptic-scale waves. These experiments are used to demonstrate that quasi-stationary equatorial Rossby waves forced by latent heating drive most of the observed residual-mean upwelling across the tropopause transition layer within 15° of the equator. The seasonal variation of the equatorial waves and the mean meridional upwelling that they cause is examined for two full years from 2006 to 2007. Changes in equatorial Rossby wave propagation through seasonally varying mean winds are the primary mechanism for producing an annual variation in the residual-mean upwelling. In the tropical tropopause layer, averaged within 15° of the equator and between 90 and 190 hPa, the annual cycle varies between a maximum upwelling of 0.4 mm s−1 during boreal winter and spring and a minimum of 0.2 mm s−1 during boreal summer. This variability seems to be due to small changes in the mean wind speed in the tropics. Seasonal variations in latent heating have only a relatively minor effect on seasonal variations in tropical tropopause upwelling. In addition, Kelvin waves drive a small downward component of the total circulation over the equator that may be modulated by the quasi-biennial oscillation.

Tropical Cold-Point Tropopause: Climatology, Seasonal Cycle, and Intraseasonal Variability Derived from COSMIC GPS Radio Occultation Measurements

2012 ◽  
Vol 25 (15) ◽  
pp. 5343-5360 ◽  
Author(s):  
Joowan Kim ◽  
Seok-Woo Son
Keyword(s):  
Seasonal Cycle ◽  
Radio Occultation ◽  
Deep Convection ◽  
Intraseasonal Variability ◽  
Kelvin Waves ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Frequency Spectra ◽  
Gps Radio Occultation ◽  
Cold Point

Abstract The finescale structure of the tropical cold-point tropopause (CPT) is examined using high-resolution temperature profiles derived from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) global positioning system (GPS) radio occultation measurements for 4 yr from September 2006 to August 2010. The climatology, seasonal cycle, and intraseasonal variability are analyzed for three CPT properties: temperature (T-CPT), pressure (P-CPT), and sharpness (S-CPT). Their relationships with tropospheric and stratospheric processes are also discussed. The climatological P-CPT is largely homogeneous in the deep tropics, whereas T-CPT and S-CPT exhibit local minima and maxima, respectively, at the equator in the vicinity of deep convection regions. All three CPT properties, however, show coherent seasonal cycle in the tropics; the CPT is colder, higher (lower in pressure), and sharper during boreal winter than during boreal summer. This seasonality is consistent with the seasonal cycle of tropical upwelling, which is largely driven by stratospheric and near-tropopause processes, although the amplitude of the seasonal cycle of T-CPT and S-CPT is modulated by tropospheric circulations. On intraseasonal time scales, P-CPT and T-CPT exhibit homogeneous variability in the deep tropics, whereas S-CPT shows pronounced local variability and seasonality. The wavenumber–frequency spectra reveal that intraseasonal variability of CPT properties is primarily controlled by Kelvin waves, with a nonnegligible contribution by Madden–Julian oscillation convection. The Kelvin waves, which are excited by deep convection but often propagate along the equator freely, explain the homogeneous P-CPT and T-CPT variabilities. On the other hand, the vertically tilted dipole of temperature anomalies, which is associated with convectively coupled equatorial waves, determines the local structure and seasonality of S-CPT variability.

scholarly journals Cloud climatologies from the infrared sounders AIRS and IASI: strengths and applications

2017 ◽  
Vol 17 (22) ◽  
pp. 13625-13644 ◽  
Author(s):  
Claudia J. Stubenrauch ◽  
Artem G. Feofilov ◽  
Sofia E. Protopapadaki ◽  
Raymond Armante
Keyword(s):  
Surface Temperature ◽  
Cloud Amount ◽  
Co2 Concentration ◽  
Boreal Summer ◽  
Ancillary Data ◽  
Low Level ◽  
Geographical Patterns ◽  
Cloud Properties ◽  
The Mean ◽  
High Level

Abstract. Global cloud climatologies have been built from 13 years of Atmospheric Infrared Sounder (AIRS) and 8 years of Infrared Atmospheric Sounding Interferometer (IASI) observations, using an updated Clouds from Infrared Sounders (CIRS) retrieval. The CIRS software can handle any infrared (IR) sounder data. Compared to the original retrieval, it uses improved radiative transfer modelling, accounts for atmospheric spectral transmissivity changes associated with CO2 concentration and incorporates the latest ancillary data (atmospheric profiles, surface temperature and emissivities). The global cloud amount is estimated to be 0.67–0.70, for clouds with IR optical depth larger than about 0.1. The spread of 0.03 is associated with ancillary data. Cloud amount is partitioned into about 40 % high-level clouds, 40 % low-level clouds and 20 % mid-level clouds. The latter two categories are only detected in the absence of upper clouds. The A-Train active instruments, lidar and radar of the CALIPSO and CloudSat missions, provide a unique opportunity to evaluate the retrieved AIRS cloud properties. CIRS cloud height can be approximated either by the mean layer height (for optically thin clouds) or by the mean between cloud top and the height at which the cloud reaches opacity. This is valid for high-level as well as for low-level clouds identified by CIRS. IR sounders are particularly advantageous to retrieve upper-tropospheric cloud properties, with a reliable cirrus identification, day and night. These clouds are most abundant in the tropics, where high opaque clouds make up 7.5 %, thick cirrus 27.5 % and thin cirrus about 21.5 % of all clouds. The 5 % annual mean excess in high-level cloud amount in the Northern compared to the Southern Hemisphere has a pronounced seasonal cycle with a maximum of 25 % in boreal summer, in accordance with the moving of the ITCZ peak latitude, with annual mean of 4° N, to a maximum of 12° N. This suggests that this excess is mainly determined by the position of the ITCZ. Considering interannual variability, tropical cirrus are more frequent relative to all clouds when the global (or tropical) mean surface gets warmer. Changes in relative amount of tropical high opaque and thin cirrus with respect to mean surface temperature show different geographical patterns, suggesting that their response to climate change might differ.

Resonant Forcing of Mixed Layer Inertial Motions by Atmospheric Easterly Waves in the Northeast Tropical Pacific*

2010 ◽  
Vol 40 (2) ◽  
pp. 401-416 ◽  
Author(s):  
John B. Mickett ◽  
Yolande L. Serra ◽  
Meghan F. Cronin ◽  
Matthew H. Alford
Keyword(s):  
Wind Stress ◽  
Mixed Layer ◽  
Tropical Pacific ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Wind Fields ◽  
Easterly Waves ◽  
Low Level ◽  
Resonant Forcing ◽  
Reversed Rotation

Abstract Westward-propagating atmospheric easterly waves contribute to much of the variability of the low-level wind fields within the northeast tropical Pacific. With the dominant period of these waves (3–5 days) close to the local inertial period (2.4 days at 12°N to 5.7 days at 5°N), there is the expectation that the associated winds may resonantly force mixed layer inertial motions in this region. The authors test this hypothesis using a simple slab model and roughly 4½ yr of wind data from four NOAA Tropical Atmosphere Ocean/Eastern Pacific Investigation of Climate Processes (TAO/EPIC) buoys along 95°W at 12°, 10°, 8°, and 5°N. The degree of resonance is determined by comparing model simulations using observed wind stress with simulations forced with reversed-rotation wind stress. Results strongly indicate that Pacific easterly waves (PEWs) resonantly force inertial motions in the region. This resonance shows both significant seasonality and latitudinal dependence that appears to be related to the meridional position and intensity of the PEWs. North of the zonal axis of the mean track of the PEWs, the low-level winds associated with the waves rotate predominantly clockwise with time and resonantly force mixed layer inertial motions. South of this axis, the winds rotate counterclockwise, resulting in dissonant (antiresonant) forcing. As this axis migrates annually from about 4°N during the boreal winter/spring to a maximum northerly position of about 8°–10°N in the late boreal summer/early fall, the region of strongest resonance follows, consistently remaining to its north. Model output suggests that resonant forcing results in roughly 10%–25% greater net annual flux of kinetic energy from the wind to mixed layer inertial motions than in neutral or nonresonant conditions. This finding has strong implications for mixed layer properties, air–sea coupling, and the generation of near-inertial internal waves.

MJO in the NCAR CAM2 with the Tiedtke Convective Scheme*

2005 ◽  
Vol 18 (15) ◽  
pp. 3007-3020 ◽  
Author(s):  
Ping Liu ◽  
Bin Wang ◽  
Kenneth R. Sperber ◽  
Tim Li ◽  
Gerald A. Meehl
Keyword(s):  
Climate Model ◽  
Convective Available Potential Energy ◽  
Intraseasonal Variability ◽  
Power Spectra ◽  
Boreal Winter ◽  
Central Pacific ◽  
Standard Version ◽  
Central Pacific Ocean ◽  
Model Version ◽  
Convective Scheme

Abstract The boreal winter Madden–Julian oscillation (MJO) remains very weak and irregular in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model version 2 (CAM2) as in its direct predecessor, the Community Climate Model version 3 (CCM3). The standard version of CAM2 uses the deep convective scheme of Zhang and McFarlane, as in CCM3, with the closure dependent on convective available potential energy (CAPE). Here, sensitivity tests using several versions of the Tiedtke convective scheme are conducted. Typically, the Tiedtke convection scheme gives an improved mean state, intraseasonal variability, space–time power spectra, and eastward propagation compared to the standard version of the model. Coherent eastward propagation of MJO-related precipitation is also much improved, particularly over the Indian–western Pacific Oceans. A composite life cycle of the model MJO indicates that over the Indian Ocean wind-induced surface heat exchange (WISHE) functions, while over the western/central Pacific Ocean aspects of frictional moisture convergence are evident in the maintenance and eastward propagation of the oscillation.

scholarly journals Estimate of the Predictability of Boreal Summer and Winter Intraseasonal Oscillations from Observations

2011 ◽  
Vol 139 (8) ◽  
pp. 2421-2438 ◽  
Author(s):  
Ruiqiang Ding ◽  
Jianping Li ◽  
Kyong-Hwan Seo
Keyword(s):  
Spatial Distribution ◽  
Intraseasonal Variability ◽  
Intraseasonal Oscillation ◽  
Longwave Radiation ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Potential Predictability ◽  
The North ◽  
Intraseasonal Oscillations ◽  
The North Pacific

AbstractTropical intraseasonal variability (TISV) shows two dominant modes: the boreal winter Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO). The two modes differ in intensity, frequency, and movement, thereby presumably indicating different predictabilities. This paper investigates differences in the predictability limits of the BSISO and the boreal winter MJO based on observational data. The results show that the potential predictability limit of the BSISO obtained from bandpass-filtered (30–80 days) outgoing longwave radiation (OLR), 850-hPa winds, and 200-hPa velocity potential is close to 5 weeks, comparable to that of the boreal winter MJO. Despite the similarity between the potential predictability limits of the BSISO and MJO, the spatial distribution of the potential predictability limit of the TISV during summer is very different from that during winter. During summer, the limit is relatively low over regions where the TISV is most active, whereas it is relatively high over the North Pacific, North Atlantic, southern Africa, and South America. The spatial distribution of the limit during winter is approximately the opposite of that during summer. For strong phases of ISO convection, the initial error of the BSISO shows a more rapid growth than that of the MJO. The error growth is rapid when the BSISO and MJO enter the decaying phase (when ISO signals are weak), whereas it is slow when convection anomalies of the BSISO and MJO are located in upstream regions (when ISO signals are strong).

Eastern Pacific Intraseasonal Variability: A Predictability Perspective

2014 ◽  
Vol 27 (23) ◽  
pp. 8869-8883 ◽  
Author(s):  
J. M. Neena ◽  
Xianan Jiang ◽  
Duane Waliser ◽  
June-Yi Lee ◽  
Bin Wang
Keyword(s):  
Large Scale ◽  
Climate Models ◽  
Initial Conditions ◽  
Intraseasonal Variability ◽  
Coupled Model ◽  
Eastern Pacific ◽  
Prediction Skill ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Weather And Climate

Abstract The eastern Pacific (EPAC) warm pool is a region of strong intraseasonal variability (ISV) during boreal summer. While the EPAC ISV is known to have large-scale impacts that shape the weather and climate in the region (e.g., tropical cyclones and local monsoon), simulating the EPAC ISV is still a great challenge for present-day global weather and climate models. In the present study, the predictive skill and predictability of the EPAC ISV are explored in eight coupled model hindcasts from the Intraseasonal Variability Hindcast Experiment (ISVHE). Relative to the prediction skill for the boreal winter Madden–Julian oscillation (MJO) in the ISVHE (~15–25 days), the skill for the EPAC ISV is considerably lower in most models, with an average skill around 10 days. On the other hand, while the MJO exhibits a predictability of 35–45 days, the predictability estimate for the EPAC ISV is 20–30 days. The prediction skill was found to be higher when the hindcasts were initialized from the convective phase of the EPAC ISV as opposed to the subsidence phase. Higher prediction skill was also found to be associated with active MJO initial conditions over the western Pacific (evident in four out of eight models), signaling the importance of exploring the dynamic link between the MJO and the EPAC ISV. The results illustrate the possibility and need for improving dynamical prediction systems to facilitate more accurate and longer-lead predictions of the EPAC ISV and associated weather and short-term climate variability.

The Synoptically-Influenced Extreme Precipitation Systems over Asian-Australian Monsoon Region observed by TRMM Precipitation Radar

2021 ◽  
Author(s):  
Wei-Ting Chen ◽  
Hong-Wen Jian ◽  
Peng-Jen Chen Chen ◽  
Chien-Ming Wu ◽  
Kristen L. Rasmussen
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Extreme Rainfall ◽  
Vertical Wind ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Monsoon Region ◽  
Precipitation Radar ◽  
Australian Monsoon ◽  
Low Level

<p>This study investigates the synoptic-scale flows associated with extreme rainfall systems over the Asian–Australian monsoon region (90°E–160°E and 12°S–27°N). On the basis of the statistics of the 17-year Precipitation Radar observations from Tropical Rainfall Measurement Mission, a total of 916 extreme systems, with both the horizontal size and maximum rainfall intensity exceeding the 99.9<sup>th</sup> percentiles of the tropical rainfall systems, are identified over this region. The synoptic wind pattern and rainfall distribution surrounding each system are classified into four major types: vortex, coastal, coastal with vortex, and none of above, with each accounting for 44%, 29%, 7%, and 20%, respectively. The vortex type occurs mainly over the off-equatorial areas in boreal summer. The coast-related types show significant seasonal variations in their occurrence, with high frequency in the Bay of Bengal in boreal summer and on the west side of Borneo and Sumatra in boreal winter. The none-of-the-above type occurs mostly over the open ocean, and in boreal winter, these events are mainly associated with the cold surge events. The environment analysis shows that coast-related extremes in the warm season are found within the areas where high total water vapor and low-level vertical wind shear occur frequently. Despite the different synoptic environments, these extremes show a similar internal structure, with broad stratiform and wide convective core (WCC) rain. Furthermore, the maximum rain rate is located mostly over the convective area, near the convective–stratiform boundary in the system. Our results highlight the critical role of the strength and direction of synoptic flows in the generation of extreme rainfall systems near coastal areas. With the enhancement of the low-level vertical wind shear and moisture by the synoptic flow, the coastal convection triggered diurnally has a higher chance to organize into mesoscale convective systems and hence a higher probability to produce extreme rainfall.</p>

scholarly journals Precipitation over Northern South America and Its Seasonal Variability as Simulated by the CMIP5 Models

2015 ◽  
Vol 2015 ◽  
pp. 1-22 ◽  
Author(s):  
Juan P. Sierra ◽  
Paola A. Arias ◽  
Sara C. Vieira
Keyword(s):  
South America ◽  
Cmip5 Models ◽  
Boreal Summer ◽  
Boreal Winter ◽  
Precipitation Pattern ◽  
Socioeconomic Impacts ◽  
Low Level ◽  
Low Level Jets ◽  
Low Level Jet ◽  
Northern South America

Northern South America is identified as one of the most vulnerable regions to be affected by climate change. Furthermore, recent extreme wet seasons over the region have induced socioeconomic impacts of wide proportions. Hence, the evaluation of rainfall simulations at seasonal and interannual time scales by the CMIP5 models is urgently required. Here, we evaluated the ability of seven CMIP5 models (selected based on literature review) to represent the seasonal mean precipitation and its interannual variability over northern South America. Our results suggest that it is easier for models to reproduce rainfall distribution during boreal summer and fall over both oceans and land. This is probably due to the fact that during these seasons, incoming radiation and ocean-atmosphere feedbacks over Atlantic and Pacific oceans locate the ITCZ on the Northern Hemisphere, as suggested by previous studies. Models exhibit the worse simulations during boreal winter and spring, when these processes have opposite effects locating the ITCZ. Our results suggest that the models with a better representation of the oceanic ITCZ and the local low-level jets over northern South America, such as the Choco low-level jet, are able to realistically simulate the main features of seasonal precipitation pattern over northern South America.

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