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L. Xin
and
G. W. Reuter

Abstract

A nonhydrostatic axisymmetric cloud model is used to quantify the effects of persistent mesoscale convergence on cumulus development and convective rainfall. The model was initialized by environmental conditions adopted from sounding and Doppler radar velocity data sampled on 19 August 1992 in central Alberta. The sounding showed a moist warm air mass with a moderate amount of convective available potential energy and the wind field had boundary layer convergence but almost no vertical shear in the lowest 5 km. The simulated rainfall intensity and accumulation compared well with radar observations.

The dependence of the convective rainfall on the characteristics of the convergence zone is investigated by intercomparing model simulators with different convergence magnitudes, convergence depths, and convergence profiles. Increasing the magnitude or the depth of convergence causes stronger convection and more precipitation. Rainfall increases monotonically (but nonstrictly linearly) with the convergence magnitude. Doubling the convergence magnitude from 1 × 10−4 to 2 × 10−4 s−1 increases the rainfall by a factor of 2.6, while rainfall increases by only 2.3 times when the convergence is doubled from 1.25 × 10−4 to 2.5 × 10−4 s−1. The nonlinear effects become even more apparent when changing the depth of convergent layers. Even when keeping the vertical mass flux constant, the depth of the convergence affects greatly the timing and amount of the surface rainfall. This is related to the fact that humidity tends to decrease with height and therefore the upward moisture flux is weakest for the deepest convergence layer for a fixed upward momentum flux. The model suggests that rainfall is mostly controlled by the amount of vapor converging into the column below cloud base.

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L. Xin
and
G. W. Reuter

Abstract

The volume velocity processing (VVP) technique is used with a simulated wind field to determine the accuracy of kinematic quantities for different numbers of wind parameters and different sizes of analysis volumes. Accurate estimates of divergence, deformation, and vertical shear are obtained if the VVP method contains seven wind parameters and the analysis volumes have a range of about 20 km and an azimuthal extent of about 40°. The seven-parameter VVP method is applied to a convective storm in central Alberta, Canada. The analysis showed that low-level convergence and moderate vertical shear preceded the enhancement of precipitation, while low-level divergence suppressed the convection.

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Xin Hai Gao
and
John L. Stanford

Abstract

Low-frequency analyses are reported for four years of 3-day-mean satellite microwave and infrared data representative of temperatures in the stratosphere. In data representative of 30–150 mb temperatures oscillations with 39–51 day periods are observed as a tropical dipole pattern in the Indonesia/central Pacific. In addition, the first evidence is presented for such oscillations in the southeast Pacific. Furthermore, significant 39–51 day oscillations are observed in the mid- and upper stratosphere, centered near 60°S latitude.

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Xin Tao
,
John E. Walsh
, and
William L. Chapman

Abstract

Simulations of Arctic temperatures by 19 general circulation models are examined as part of a diagnostic subproject of the Atmospheric Model Intercomparison Project (AMIP). The forcing of all the models by observed sea surface temperatures and sea ice from a 10-yr period (1979–1988) permits comparative evaluations of the model biases as well as the models’ simulations of the interannual variations contained in the observational data. The models capture the latitudinal and seasonal variability of surface air temperatures in the Arctic, although a cold bias of −3.3°C (std dev = 3.4°C) is apparent over northern Eurasia during spring, especially in the models that do not include vegetative masking of the high-albedo snow. The 19-model mean bias over northern North America is less than 2°C in all seasons. Over the Arctic Ocean, the spring temperatures generally have a warm bias that averages 3.0 (std dev = 2.9°C), although the bias is smaller in the models in which the prescribed albedo of sea ice is highest. For the summer season, correlations between simulated cloudiness and surface air temperatures are negative and statistically significant, but the corresponding correlations for the winter months are small and statistically insignificant The models without gravity wave drag are generally colder than the other models at the Arctic surface, especially during autumn.

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Xin-Hai Gao
,
Wen-Bi Yu
, and
John L. Stanford

Abstract

Four years of satellite-derived microwave and infrared radiances are analyzed for the three-dimensional and seasonal variation of semiannual oscillations (SAO) in stratospheric temperatures, with particular focus on high latitudes, to investigate the effect of stratospheric warmings on SAO. Separate analyses of individual seasons in each hemisphere reveal that the strongest SAO in temperature occur in the Northern Hemisphere (NH) winter polar upper stratosphere. These results, together with the latitudinal structure of the temperature SAO and the fact that the NH polar SAO is nearly out of phase with the lower latitude, SAO, are consistent with the existence of a global-scale, meridional circulation on the SAO time scale. The results suggest that polar

stratospheric warmings are an important source of SAO in both high and low latitude stratospheric temperature fields. Interannual variations, three-dimensional phase structure, and zonal asymmetry of SAO are also detailed. The SH stratospheric SAO is dominated by a localized feature in the high-latitude, eastern hemisphere which tilts westward with height.

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Xin Tan
,
Ming Bao
,
Dennis L. Hartmann
, and
Paulo Ceppi

Abstract

Previous studies have demonstrated that the NAO, the leading mode of atmospheric low-frequency variability over the North Atlantic, could be linked to northeast Pacific climate variability via the downstream propagation of synoptic waves. In those studies, the NAO and the northeast Pacific climate variability are considered as two separate modes that explain the variance over the North Atlantic sector and the east Pacific–North American sector, respectively. A newly identified low-frequency atmospheric regime—the Western Hemisphere (WH) circulation pattern—provides a unique example of a mode of variability that accounts for variance over the whole North Atlantic–North American–North Pacific sector. The role of synoptic waves in the formation and maintenance of the WH pattern is investigated using the ECMWF reanalysis datasets. Persistent WH events are characterized by the propagation of quasi-stationary Rossby waves across the North Pacific–North American–North Atlantic regions and by associated storm-track anomalies. The eddy-induced low-frequency height anomalies maintain the anomalous low-frequency ridge over the Gulf of Alaska, which induces more equatorward propagation of synoptic waves on its downstream side. The eddy forcing favors the strengthening of the midlatitude jet and the deepening of the mid-to-high-latitude trough over the North Atlantic, whereas the deepening of the trough over eastern North America mostly arises from the quasi-stationary waves propagating from the North Pacific. A case study for the 2013/14 winter is examined to illustrate the downstream development of synoptic waves. The roles of synoptic waves in the formation and maintenance of the WH pattern and in linking the northeast Pacific ridge anomaly with the NAO are discussed.

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Xin-Zhong Liang
,
Li Li
,
Kenneth E. Kunkel
,
Mingfang Ting
, and
Julian X. L. Wang

Abstract

The fifth-generation PSU–NCAR Mesoscale Model (MM5)-based regional climate model (CMM5) capability in simulating the U.S. precipitation annual cycle is evaluated with a 1982–2002 continuous baseline integration driven by the NCEP–DOE second Atmospheric Model Intercomparison Project (AMIP II) reanalysis. The causes for major model biases (differences from observations) are studied through supplementary seasonal sensitivity experiments with various driving lateral boundary conditions (LBCs) and physics representations. It is demonstrated that the CMM5 has a pronounced rainfall downscaling skill, producing more realistic regional details and overall smaller biases than the driving global reanalysis. The precipitation simulation is most skillful in the Northwest, where orographic forcing dominates throughout the year; in the Midwest, where mesoscale convective complexes prevail in summer; and in the central Great Plains, where nocturnal low-level jet and rainfall peaks occur in summer. The actual model skill, however, is masked by existing large LBC uncertainties over data-poor areas, especially over oceans. For example, winter dry biases in the Gulf States likely result from LBC errors in the south and east buffer zones. On the other hand, several important regional biases are identified with model physics deficiencies. In particular, summer dry biases in the North American monsoon region and along the east coast of the United States can be largely rectified by replacing the Grell with the Kain–Fritsch cumulus scheme. The latter scheme, however, yields excessive rainfall in the Atlantic Ocean but large deficits over the Midwest. The fall dry biases over the lower Mississippi River basin, common to all existing global and regional models, remain unexplained and the search for their responsible physical mechanisms will be challenging. In addition, the representation of cloud–radiation interaction is essential in determining the precipitation distribution and regional water recycling, for which the new scheme implemented in the CMM5 yields significant improvement.

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Xin-Zhong Liang
,
Jinhong Zhu
,
Kenneth E. Kunkel
,
Mingfang Ting
, and
Julian X. L. Wang

Abstract

This study uses the most recent simulations from all available fully coupled atmosphere–ocean general circulation models (CGCMs) to investigate whether the North American monsoon (NAM) precipitation seasonal–interannual variations are simulated and, if so, whether the key underlying physical mechanisms are correctly represented. This is facilitated by first identifying key centers where observed large-scale circulation fields and sea surface temperatures (SSTs) are significantly correlated with the NAM precipitation averages over the core region (central–northwest Mexico) and then examining if the modeled and observed patterns agree.

Two new findings result from the analysis of observed NAM interannual variations. First, precipitation exhibits significantly high positive (negative) correlations with 200-hPa meridional wind centered to the northwest (southeast) of the core region in June and September (July and August). As such, wet conditions are associated with strong anomalous southerly upper-level flow on the northwest flank during the monsoon onset and retreat, but with anomalous northerly flow on the southeast flank, during the peak of the monsoon. They are often identified with a cyclonic circulation anomaly pattern over the central Great Plains for the July–August peak monsoon and, reversely, an anticyclonic anomaly pattern centered over the northern (southern) Great Plains for the June (September) transition. Second, wet NAM conditions in June and July are also connected with a SST pattern of positive anomalies in the eastern Pacific and negative anomalies in the Gulf of Mexico, acting to reduce the climatological mean gradient between the two oceans. This pattern suggests a possible surface gradient forcing that favors a westward extension of the North Atlantic subtropical ridge. These two observed features connected to the NAM serve as the metric for quantitative evaluation of the model performance in simulating the important NAM precipitation mechanisms.

Out of 17 CGCMs, only the Meteorological Research Institute (MRI) model with a medium resolution consistently captures the observed NAM precipitation annual cycle (having a realistic amplitude and no phase shift) as well as interannual covariations with the planetary circulation patterns (having the correct sign and comparably high magnitude of correlation) throughout the summer. For the metric of correlations with 200-hPa meridional wind, there is general agreement among all CGCMs with observations for June and September. This may indicate that large-scale forcings dominate interannual variability for the monsoon onset and retreat, while variability of the peak of the monsoon in July and August may be largely influenced by local processes that are more challenging for CGCMs to resolve. For the metric of correlations with SSTs, good agreement is found only in June. These results suggest that the NAM precipitation interannual variability may likely be determined by large-scale circulation anomalies, while its predictability based on remote signals such as SSTs may not be sufficiently robust to be well captured by current CGCMs, with the exception of the June monsoon onset which is potentially more predictable.

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Xin-Zhong Liang
,
Hyun I. Choi
,
Kenneth E. Kunkel
,
Yongjiu Dai
,
Everette Joseph
,
Julian X. L. Wang
, and
Praveen Kumar

Abstract

This paper utilizes the best available quality data from multiple sources to develop consistent surface boundary conditions (SBCs) for mesoscale regional climate model (RCM) applications. The primary SBCs include 1) fields of soil characteristic (bedrock depth, and sand and clay fraction profiles), which for the first time have been consistently introduced to define 3D soil properties; 2) fields of vegetation characteristic fields (land-cover category, and static fractional vegetation cover and varying leaf-plus-stem-area indices) to represent spatial and temporal variations of vegetation with improved data coherence and physical realism; and 3) daily sea surface temperature variations based on the most appropriate data currently available or other value-added alternatives. For each field, multiple data sources are compared to quantify uncertainties for selecting the best one or merged to create a consistent and complete spatial and temporal coverage. The SBCs so developed can be readily incorporated into any RCM suitable for U.S. climate and hydrology modeling studies, while the data processing and validation procedures can be more generally applied to construct SBCs for any specific domain over the globe.

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Tianjun Zhou
,
Rucong Yu
,
Jie Zhang
,
Helge Drange
,
Christophe Cassou
,
Clara Deser
,
Daniel L. R. Hodson
,
Emilia Sanchez-Gomez
,
Jian Li
,
Noel Keenlyside
,
Xiaoge Xin
, and
Yuko Okumura

Abstract

The western Pacific subtropical high (WPSH) is closely related to Asian climate. Previous examination of changes in the WPSH found a westward extension since the late 1970s, which has contributed to the interdecadal transition of East Asian climate. The reason for the westward extension is unknown, however. The present study suggests that this significant change of WPSH is partly due to the atmosphere’s response to the observed Indian Ocean–western Pacific (IWP) warming. Coordinated by a European Union’s Sixth Framework Programme, Understanding the Dynamics of the Coupled Climate System (DYNAMITE), five AGCMs were forced by identical idealized sea surface temperature patterns representative of the IWP warming and cooling. The results of these numerical experiments suggest that the negative heating in the central and eastern tropical Pacific and increased convective heating in the equatorial Indian Ocean/Maritime Continent associated with IWP warming are in favor of the westward extension of WPSH. The SST changes in IWP influences the Walker circulation, with a subsequent reduction of convections in the tropical central and eastern Pacific, which then forces an ENSO/Gill-type response that modulates the WPSH. The monsoon diabatic heating mechanism proposed by Rodwell and Hoskins plays a secondary reinforcing role in the westward extension of WPSH. The low-level equatorial flank of WPSH is interpreted as a Kelvin response to monsoon condensational heating, while the intensified poleward flow along the western flank of WPSH is in accord with Sverdrup vorticity balance. The IWP warming has led to an expansion of the South Asian high in the upper troposphere, as seen in the reanalysis.

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