Search Results

You are looking at 1 - 10 of 32 items for

  • Author or Editor: B. Yu x
  • All content x
Clear All Modify Search
P. Philip and B. Yu

Abstract

Rainfall in the southwest of Western Australia (SWWA) has decreased significantly over recent decades. Previous studies documented this decrease in terms of the change in rainfall depth or decrease in the frequency of rainfall events for selected sites. Although rainfall volume is of vital importance to determine water resources availability for a given region, no study has yet been undertaken to examine the change in rainfall volume in SWWA. The aim of this study is to examine the spatiotemporal changes in rainfall volume and to attribute this change to the changes in wet area and rainfall depth. Gridded daily rainfall data at 0.05° resolution for the period from 1911 to 2018 were used for an area of 265 952 km2 in SWWA. For the whole region and most zones, rainfall volume decreased, which was mostly due to a decrease in the wet area, despite an increase in the mean rain depth. In the regions near the coast with mean annual rainfall ≥ 600 mm, 84% of the decrease in rainfall volume could be attributed to a decrease in the wet area, whereas the decrease in rainfall depth only played a minor role. The regions near the coast with a higher number of rain days showed a decreasing trend in wet area, and the regions farther inland with a lower number of rain days showed an increasing trend in wet area. On the coast, the rate of decrease in rainfall has been reduced, and heavy rainfall, in fact, has increased over the past 30 years, although there was no concurrent change in the southern annular mode (SAM). This suggests that the relationship between SAM and rainfall could have changed and that other climate drivers could also be responsible for the recent rainfall trend and variations in the coastal regions of SWWA.

Restricted access
B. Yu, A. Shabbar, and F. W. Zwiers

Abstract

This study provides further evidence of the impacts of tropical Pacific interannual [El Niño–Southern Oscillation (ENSO)] and Northern Pacific decadal–interdecadal [North Pacific index (NPI)] variability on the Pacific–North American (PNA) sector. Both the tropospheric circulation and the North American temperature suggest an enhanced PNA-like climate response and impacts on North America when ENSO and NPI variability are out of phase. In association with this variability, large stationary wave activity fluxes appear in the mid- to high latitudes originating from the North Pacific and flowing downstream toward North America. Atmospheric heating anomalies associated with ENSO variability are confined to the Tropics, and generally have the same sign throughout the troposphere with maximum anomalies at 400 hPa. The heating anomalies that correspond to the NPI variability exhibit a center over the midlatitude North Pacific in which the heating changes sign with height, along with tropical anomalies of comparable magnitudes. Atmospheric heating anomalies of the same sign appear in both the tropical Pacific and the North Pacific with the out-of-phase combination of ENSO and NPI. Both sources of variability provide energy transports toward North America and tend to favor the occurrence of stationary wave anomalies.

Full access
Edmund K. M. Chang and Daniel B. Yu

Abstract

Gridded data produced by the ECMWF reanalysis project have been analyzed to document the properties of wave packets in the Northern Hemisphere winter midlatitude upper troposphere. Based on results from earlier investigations, 300-hPa meridional wind variations were chosen for analysis. Wave packet envelopes were also defined by performing complex demodulation on the wind data. The properties of the wave packets are mainly illustrated using time-lagged one-point correlation maps performed both on υ′ and wave packet envelopes.

The results show that, over most regions in the Northern Hemisphere winter, with the exception of the deep Tropics and near the Aleutian low, medium-scale waves (dominant wavenumber 5–8) exhibit the characteristics of downstream development and occur within wave trains that propagate with eastward group velocities much faster than the phase speeds of individual phases of the waves. Their group velocity is highly correlated with the local time mean 200–400-hPa wind, while the phase speed is well correlated with the 500–700-hPa flow.

A wave coherence index has been defined to show the geographical variations in the downstream development tendency of wave propagation. The results show that wave packets are most coherent along a band that extends from North Africa into southern Asia, toward the Pacific storm track, across North America, then over the central North Atlantic back toward North Africa. The maximum coherence occurs over southern Asia. This band can be regarded as the waveguide for upper-tropospheric wave packets in the Northern Hemisphere winter. Over this band, wave packets generally stay coherent significantly longer than individual troughs and ridges. There is also a secondary waveguide across Russia toward the Pacific, acting as a second source of waves that propagate across the Pacific storm track. Away from the primary waveguide, while wave packet coherence is less, the waves still show the characteristics of downstream development.

Full access
G. J. Boer, S. Fourest, and B. Yu

Abstract

The mid- and high-latitude variations of mass and momentum, variously termed the Arctic and Antarctic oscillations and/or the Northern and Southern Annular Modes also have a signature in the moisture budget. This is investigated, using associated regression pattern analysis, in the control simulation of the Canadian Centre for Climate Modelling and Analysis (CCCma) coupled climate model and in NCEP reanalysis data. The analyzed behavior is similar in the two hemispheres and is characterized, when pressure is low in polar regions, by anomalous westerlies and a thermodynamically indirect meridional cell poleward of midlatitudes, and by anomalous easterlies and a thermodynamically direct meridional cell equatorward of midlatitudes. Moisture transports are associated primarily with advection by the lower branches of these anomalous meridional circulation cells and secondarily with anomalous eddy transports giving, in this phase of the mode, moisture divergence and decreased precipitation at midlatitudes and moisture convergence and increased precipitation at poleward latitudes. The opposite behavior is seen when pressure is high in polar regions. Patterns tend to be more zonally symmetric in the Southern than the Northern Hemisphere in both model results and reanalyses.

The meridional circulations and eddy transports that affect the moisture budget at lower levels in the atmosphere extend throughout the troposphere and are those associated also with the momentum budget of the annular modes. The annular modes should be considered as interlinked variations of mass, momentum, and moisture.

The amplitudes of the annular modes and associated moisture quantities exhibit an upward trend in both hemispheres in simulations with the CCCma coupled model forced with historical and projected greenhouse gas concentrations and aerosol loadings for the period 1900–2100. It appears, however, that the behavior of the annular modes and their signature in the moisture budget is basically unchanged and is superimposed on a forced climate trend.

Full access
B. Root, T-Y. Yu, M. Yeary, and M. B. Richman

Abstract

Radar measurements are useful for determining rainfall rates because of their ability to cover large areas. Unfortunately, estimating rainfall rates from radar reflectivity data alone is prone to errors resulting from variations in drop size distributions, precipitation types, and other physics that cannot be represented in a simple, one-dimensional ZR relationship. However, improving estimates is possible by utilizing additional inputs, thereby increasing the dimensionality of the model.

The main purpose of this study is to determine the value of surface observations for improving rainfall-rate estimation. This work carefully designed an artificial neural network to fit a model that would relate radar reflectivity, surface temperature, humidity, pressure, and wind to observed rainfall rates. Observations taken over 13 years from the Oklahoma Mesonet and the KTLX WSR-88D radar near Oklahoma City, Oklahoma, were used for the training dataset. While the artificial neural network underestimated rainfall rates for higher reflectivities, it did have an overall better performance than the best-fit ZR relation. Most importantly, it is shown that the surface data contributed significant value to an unaugmented radar-based rainfall-rate estimation model.

Full access
Yi-Leng Chen, Yu-Xia Zhang, and Norman B-F. Hui

Abstract

A case study of a relatively dry front during TAMEX IOP-4 is presented. At 0000 UTC 27 May, the broad cloud band extended from the China plain and southern Japan to east of 150°E, along and north of the surface front. This front possessed appreciable baroclinity.

As the midtropospheric trough moved eastward, the surface front advanced southeastward and eventually separated from the quasi-stationary cloud band. The depth of the cold air decreased as it penetrated to the subtropics. In the vicinity of Taiwan, the frontal surface was shallow with a depth ≈ 1 km. The convection occurred in two different regimes; the large-scale cloud band associated with the eastward-moving trough, and cumuli along the surface front.

As the surface front approached Taiwan, the southwest flow started to increase, and a mesolow formed over the southeastern coast due to the interaction between the southwest flow and the mountainous topography. The air moved over the mountains and descended adiabatically. During the frontal passage, the mesolow deepened as the approaching front from the north also became a flow barrier. After the frontal passage, the mesolow was filled by the advancing cold air.

As the mesolow in southeastern Taiwan filled, a new mesolow formed over the southwestern plain. The surface front and isobars were distorted during the frontal passage as the shallow cold air moved around the topography. The relatively low pressure over the southwestern plain resulted hydrostatically from the cold air behind the front not reaching the southwestern plain, while surrounding areas experienced rising pressure with the arrival of the cold air.

Full access
B. Yu, H. Lin, V. V. Kharin, and X. L. Wang

Abstract

The interannual variability of wintertime North American surface temperature extremes and its generation and maintenance are analyzed in this study. The leading mode of the temperature extreme anomalies, revealed by empirical orthogonal function (EOF) analyses of December–February mean temperature extreme indices over North America, is characterized by an anomalous center of action over western-central Canada. In association with the leading mode of temperature extreme variability, the large-scale atmospheric circulation features an anomalous Pacific–North American (PNA)-like pattern from the preceding fall to winter, which has important implications for seasonal prediction of North American temperature extremes. A positive PNA pattern leads to more warm and fewer cold extremes over western-central Canada. The anomalous circulation over the PNA sector drives thermal advection that contributes to temperature anomalies over North America, as well as a Pacific decadal oscillation (PDO)-like sea surface temperature (SST) anomaly pattern in the midlatitude North Pacific. The PNA-like circulation anomaly tends to be supported by SST warming in the tropical central-eastern Pacific and a positive synoptic-scale eddy vorticity forcing feedback on the large-scale circulation over the PNA sector. The leading extreme mode–associated atmospheric circulation patterns obtained from the observational and reanalysis data, together with the anomalous SST and synoptic eddy activities, are reasonably well simulated in most CMIP5 models and in the multimodel mean. For most models considered, the simulated patterns of atmospheric circulation, SST, and synoptic eddy activities have lower spatial variances than the corresponding observational and reanalysis patterns over the PNA sector, especially over the North Pacific.

Open access
Yu-Chieng Liou, Howard B. Bluestein, Michael M. French, and Zachary B. Wienhoff

Abstract

A three-dimensional data assimilation (3DVar) least squares–type single-Doppler velocity retrieval (SDVR) algorithm is utilized to retrieve the wind field of a tornadic supercell using data collected by a mobile, phased-array, Doppler radar [Mobile Weather Radar (MWR) 05XP] with very high temporal resolution (6 s). It is found that the cyclonic circulation in the hook-echo region can be successfully recovered by the SDVR algorithm. The quality of the SDVR analyses is evaluated by dual-Doppler syntheses using data collected by two mobile Doppler radars [Doppler on Wheels 6 and 7 (DOW6 and DOW7, respectively)]. A comparison between the SDVR analyses and dual-Doppler syntheses confirms the conclusion reached by an earlier theoretical analysis that because of the temporally discrete nature of the radar data, the wind speed retrieved by single-Doppler radar is always underestimated, and this underestimate occurs more significantly for the azimuthal (crossbeam) wind component than for the radial (along beam) component. However, the underestimate can be mitigated by increasing the radar data temporal resolution. When the radar data are collected at a sufficiently high rate, the azimuthal wind component may be overestimated. Even with data from a rapid scan, phased-array, Doppler radar, our study indicates that it is still necessary to calculate the SDVR in an optimal moving frame of reference. Finally, the SDVR algorithm’s robustness is demonstrated. Even with a temporal resolution (2 min) much lower than that of the phased-array radar, the cyclonic flow structure in the hook-echo region can still be retrieved through SDVR using data observed by DOW6 or DOW7, although a difference in the retrieved fields does exist. A further analysis indicates that this difference is caused by the location of the radars.

Full access
Ming Yu Zhou, D. H. Lenschow, B. B. Stankov, J. C. Kaimal, and J. E. Gaynor

Abstract

Data from the Boulder Atmospheric Observatory (BAO) are used to investigate the wave and turbulence structure of the convective atmospheric mixed layer and the overlying inversion. Three cases are discussed, one in considerable detail, in which the depth of the mixed layer is below the top of the 300 m tower at the BAO and is nearly steady state for several hours. Velocity and temperature variances and spectra, coherences between vertical velocity and temperature, and vertical velocities at different levels on the tower are used to show that although the mixed-layer behavior is for the most part similar to that found in previous studies, there are some significant differences due mainly to the relatively large shear term in the turbulence energy equation compared with buoyancy, both within the mixed layer and in the capping inversion. For example, the wavelength of the spectral maximum for vertical velocity in the upper half of the mixed layer is about three times the boundary-layer height, which is about twice that estimated in a previous experiment. The wavelength is up to 5.5 times the mixed-layer height above the top of the mixed layer. Within the mixed layer, terms in the turbulence kinetic energy equation are similar to previous studies. Above the mixed layer, shear production becomes large, and is approximately balanced by the sum of the buoyancy, dissipation and transport terms. The temperature variance and flux budgets also have large terms and significant residuals in the overlying inversion.

Full access
B. Yu, X. L. Wang, X. B. Zhang, J. Cole, and Y. Feng

Abstract

The decadal covariability of northern wintertime land surface temperature and 500-hPa geopotential anomalies is examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research and the Twentieth-Century Reanalyses over the twentieth century and a 996-yr preindustrial climate simulation from the Canadian Earth System Model. Based on the reanalysis data, the covariability is dominated by two leading maximum covariance analysis (MCA) modes. MCA1 is characterized by temperature anomalies over most of Canada, the eastern United States, Mexico, and Eurasian mid- to high latitudes, accompanied by anomalies of opposite sign elsewhere over northern landmasses. MCA2 features temperature anomalies over most of North America, Eurasia, and Greenland with opposite anomalies elsewhere. In the upper troposphere the synoptic vorticity fluxes reinforce the anomalous circulation, while in the lower troposphere advection by the anomalous mean flow offsets the eddy forcing and maintains the decadal temperature perturbation. The MCA1-associated variability has a broad spectrum over decadal–interdecadal time scales, while the MCA2-related variability has a significant power peak around 20 yr. The change of temperature and geopotential trends around 1990 tends to be a decadal-scale shift in winter and has significant features of the leading mode of the decadal covariability. The climate model has broadly similar decadal covariability, including the leading MCA patterns as well as the temporal evolution of the patterns. The decadal temperature and geopotential anomalies primarily covary with the North Atlantic Oscillation but also with the variability of the North Pacific index, while the Southern Oscillation index variability tends to be the least important predictor for the northern decadal temperature and geopotential anomalies.

Full access