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L. H. LinHo, Xianglei Huang, and Ngar-Cheung Lau

Abstract

Analysis of observations from 1979 to 2002 shows that the seasonal transition from winter to spring in East Asia is marked with a distinctive event—the onset of the south China spring rain (SCSR). In late February, the reduced thermal contrast between ocean and land leads to weakening of the Asian winter monsoon as well as the Siberian high and the Aleutian low. Meanwhile, convection over Australia and the western Pacific Maritime Continent is suppressed on the passage of the dry phase of a Madden–Julian oscillation (MJO). In conjunction with the seasonal march of monsoon circulation in the Indonesian–Australian sector, this MJO passage weakens the local thermally direct cell in the East Asia–Australia sector. This development is further accompanied by a series of adjustments in both the tropics and midlatitudes. These changes include attenuation of the planetary stationary wave, considerable weakening of the westerly jet stream over much of the central Pacific adjacent to Japan, and reduction of baroclinicity near the East Asian trough. The influence of concurrent local processes in midlatitudes on the SCSR onset is also important. The weakened jet stream is associated with confinement of frontal activities to the coastal regions of East Asia as well as with rapid expansion of the subtropical Pacific high from the eastern Pacific to the western Pacific. A parallel analysis using output from an experiment with a GFDL-coupled GCM shows that the above sequence of circulation changes is well simulated in that model.

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A. Kumar, M. Chen, L. Zhang, W. Wang, Y. Xue, C. Wen, L. Marx, and B. Huang

Abstract

For long-range predictions (e.g., seasonal), it is a common practice for retrospective forecasts (also referred to as the hindcasts) to accompany real-time predictions. The necessity for the hindcasts stems from the fact that real-time predictions need to be calibrated in an attempt to remove the influence of model biases on the predicted anomalies. A fundamental assumption behind forecast calibration is the long-term stationarity of forecast bias that is derived based on hindcasts.

Hindcasts require specification of initial conditions for various components of the prediction system (e.g., ocean, atmosphere) that are generally taken from a long reanalysis. Trends and discontinuities in the reanalysis that are either real or spurious can arise due to several reasons, for example, the changing observing system. If changes in initial conditions were to persist during the forecast, there is a potential for forecast bias to depend over the period it is computed, making calibration even more of a challenging task. In this study such a case is discussed for the recently implemented seasonal prediction system at the National Centers for Environmental Prediction (NCEP), the Climate Forecast System version 2 (CFS.v2).

Based on the analysis of the CFS.v2 for 1981–2009, it is demonstrated that the characteristics of the forecast bias for sea surface temperature (SST) in the equatorial Pacific had a dramatic change around 1999. Furthermore, change in the SST forecast bias, and its relationship to changes in the ocean reanalysis from which the ocean initial conditions for hindcasts are taken is described. Implications for seasonal and other long-range predictions are discussed.

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Chenjie Huang, Y-L. Lin, M. L. Kaplan, and J. J. Charney

Abstract

This study has employed both observational data and numerical simulation results to diagnose the synoptic-scale and mesoscale environments conducive to forest fires during the October 2003 extreme fire event in southern California. A three-stage process is proposed to illustrate the coupling of the synoptic-scale forcing that is evident from the observations, specifically the high pressure ridge and the upper-level jet streak, which leads to meso-α-scale subsidence in its exit region, and the mesoscale forcing that is simulated by the numerical model, specifically the wave breaking and turbulence as well as the wave-induced critical level, which leads to severe downslope (Santa Ana) winds. Two surges of dry air were found to reach the surface in southern California as revealed in the numerical simulation. The first dry air surge arrived as a result of moisture divergence and isallobaric adjustments behind a surface cold front. The second dry air surge reached southern California as the meso-α- to meso-β-scale subsidence and the wave-induced critical level over the coastal ranges phased to transport the dry air from the upper-level jet streak exit region toward the surface and mix the dry air down to the planetary boundary layer on the lee side of the coastal ranges in southern California. The wave-breaking region on the lee side acted as an internal boundary to reflect the mountain wave energy back to the ground and created severe downslope winds through partial resonance with the upward-propagating mountain waves.

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Hung-Lung Huang, William L. Smith, Jun Li, Paolo Antonelli, Xiangqian Wu, Robert O. Knuteson, Bormin Huang, and Brian J. Osborne

Abstract

This paper describes the theory and application of the minimum local emissivity variance (MLEV) technique for simultaneous retrieval of cloud pressure level and effective spectral emissivity from high-spectral-resolution radiances, for the case of single-layer clouds. This technique, which has become feasible only with the recent development of high-spectral-resolution satellite and airborne instruments, is shown to provide reliable cloud spectral emissivity and pressure level under a wide range of atmospheric conditions. The MLEV algorithm uses a physical approach in which the local variances of spectral cloud emissivity are calculated for a number of assumed or first-guess cloud pressure levels. The optimal solution for the single-layer cloud emissivity spectrum is that having the “minimum local emissivity variance” among the retrieved emissivity spectra associated with different first-guess cloud pressure levels. This is due to the fact that the absorption, reflection, and scattering processes of clouds exhibit relatively limited localized spectral emissivity structure in the infrared 10–15-μm longwave region. In this simulation study it is shown that the MLEV cloud pressure root-mean-square errors for a single level with effective cloud emissivity greater than 0.1 are ∼30, ∼10, and ∼50 hPa, for high (200– 300 hPa), middle (500 hPa), and low (850 hPa) clouds, respectively. The associated cloud emissivity root-mean-square errors in the 900 cm−1 spectral channel are less than 0.05, 0.04, and 0.25 for high, middle, and low clouds, respectively.

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E. Mollo-Christensen, N. E. Huang, L. F. Bliven, and S. R. Long

Abstract

A wave sensor, consisting of parallel, evenly spaced capacitance wires, whose output is the sum of the water surface deflections at the wires, has been built and tested in a wave tank. The probe output simulates Bragg scattering of electromagnetic waves from a water surface with waves; it can be used to simulate electromagnetic probing of the sea surface by radar. Our study establishes that the wave probe, called the “Harp” for short, will simulate Bragg scattering, and that it can also be used to study nonlinear wave processes.

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Qifen Yuan, Thordis L. Thorarinsdottir, Stein Beldring, Wai Kwok Wong, Shaochun Huang, and Chong-Yu Xu

Abstract

In applications of climate information, coarse-resolution climate projections commonly need to be downscaled to a finer grid. One challenge of this requirement is the modeling of subgrid variability and the spatial and temporal dependence at the finer scale. Here, a postprocessing procedure for temperature projections is proposed that addresses this challenge. The procedure employs statistical bias correction and stochastic downscaling in two steps. In the first step, errors that are related to spatial and temporal features of the first two moments of the temperature distribution at model scale are identified and corrected. Second, residual space–time dependence at the finer scale is analyzed using a statistical model, from which realizations are generated and then combined with an appropriate climate change signal to form the downscaled projection fields. Using a high-resolution observational gridded data product, the proposed approach is applied in a case study in which projections of two regional climate models from the Coordinated Downscaling Experiment–European Domain (EURO-CORDEX) ensemble are bias corrected and downscaled to a 1 km × 1 km grid in the Trøndelag area of Norway. A cross-validation study shows that the proposed procedure generates results that better reflect the marginal distributional properties of the data product and have better consistency in space and time when compared with empirical quantile mapping.

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Yuanhong Guan, Jieshun Zhu, Bohua Huang, Zeng-Zhen Hu, and James L. Kinter III

Abstract

Evaluating the climate hindcasts for 1982–2009 from the NCEP CFS Reanalysis and Reforecast (CFSRR) project using the Climate Forecast System, version 2 (CFSv2), this study identifies substantial areas of high prediction skill of the sea surface temperature (SST) in the South Pacific. The skill is the highest in the extratropical oceans on seasonal-to-interannual time scales, and it is only slightly lower than that for the El Niño–Southern Oscillation (ENSO). Two regions with the highest prediction skills in the South Pacific in both the CFSv2 and persistence hindcasts coincide with the active centers of opposite signs in the South Pacific Ocean dipole (SPOD) mode, a seesaw between the subtropical and extratropical SST in the South Pacific with a strong phase locking to austral summer. Interestingly, the CFSv2 prediction exhibits skillful predictions made three seasons ahead, more superior to the persistence forecast, suggesting significant dynamical predictability of the SPOD. An austral “spring predictability barrier” is noted in both the dynamical and persistence hindcasts. An analysis of the observational and model data suggests that the SPOD mode is significantly associated with ENSO, as an oceanic response to the atmospheric planetary wave trains forced by the anomalous atmospheric heating in the western Pacific. Although previous studies have demonstrated that the pattern of subtropical SST dipole is ubiquitous in the Southern Ocean, the SPOD has been least known and studied, compared with its counterparts in the south Indian and Atlantic Oceans. Since the SPOD is the most predictable oceanic mode in the whole Southern Hemisphere, its climate effects for local and remote regions should be further studied.

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M. Thurai, G. J. Huang, V. N. Bringi, W. L. Randeu, and M. Schönhuber

Abstract

Drop shapes derived from a previously conducted artificial rain experiment using a two-dimensional video disdrometer (2DVD) are presented. The experiment involved drops falling over a distance of 80 m to achieve their terminal velocities as well as steady-state oscillations. The previous study analyzed the measured axis ratios (i.e., ratio of maximum vertical to maximum horizontal chord) as a function of equivolumetric spherical drop diameter (D eq) for over 115 000 drops ranging from 1.5 to 9 mm. In this paper, the actual contoured shapes of the drops are reported, taking into account the finite quantization limits of the instrument. The shapes were derived from the fast line-scanning cameras of the 2DVD. The drops were categorized into D eq intervals of 0.25-mm width and the smoothed contours for each drop category were superimposed on each other to obtain their most probable shapes and their variations due to drop oscillations. The most probable shapes show deviation from oblate spheroids for D eq > 4 mm, the larger drops having a more flattened base, in good agreement with the equilibrium (nonoblate) shape model of Beard and Chuang. Deviations were noted from the Beard and Chuang model shapes for diameters larger than 6 mm. However, the 2DVD measurements of the most probable contour shapes are the first to validate the Beard and Chuang model shapes for large drops, and further to demonstrate the differences from the equivalent oblate shapes. The purpose of this paper is to document the differences in radar polarization parameters and the range of error incurred when using the equivalent oblate shapes versus the most probable contoured shapes measured with the 2DVD especially for drop size distributions (DSDs) with large median volume diameters (>2 mm).

The measured contours for D eq > 1.5 mm were fitted to a modified conical equation, and scattering calculations were performed to derive the complex scattering amplitudes for forward and backscatter for H and V polarizations primarily at 5.34 GHz (C band) but also at 3 GHz (S band) and 9 GHz (X band). Calculations were also made to derive the relevant dual-polarization radar parameters for measured as well as model-based drop size distributions. When comparing calculations using the contoured shapes against the equivalent oblate spheroid shapes, good agreement was obtained for cases with median volume diameter (D 0) less than around 2 mm. Small systematic differences in the differential reflectivity (Z dr) values of up to 0.3 dB were seen for the larger D 0 values when using the oblate shapes, which can be primarily attributed to the shape differences in the resonance region, which occurs in the 5.5–7-mm-diameter range at C band. Lesser systematic differences were present in the resonance region at X band (3–4 mm). At S band, the impact of shape differences in the polarimetric parameters were relatively minor for D 0 up to 2.5 mm. Unusual DSDs with very large D 0 values (>3 mm) (e.g., as can occur along the leading edge of severe convective storms or aloft due localized “big drop” zones) can accentuate the Z dr difference between the contoured shape and the oblate spheroid equivalent, especially at C band. For attenuation-correction schemes based on differential propagation phase, it appears that the equivalent oblate shape approximation is sufficient using a fit to the axis ratios from the 80-m fall experiment given in this paper. For high accuracy in developing algorithms for predicting D 0 from Z dr, it is recommended that the fit to the most probable contoured shapes as given in this paper be used especially at C band.

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Guoyong Leng, Maoyi Huang, Qiuhong Tang, Huilin Gao, and L. Ruby Leung

Abstract

Human alteration of the land surface hydrologic cycle is substantial. Recent studies suggest that local water management practices including groundwater pumping and irrigation could significantly alter the quantity and distribution of water in the terrestrial system, with potential impacts on weather and climate through land–atmosphere feedbacks. In this study, the authors incorporated a groundwater withdrawal scheme into the Community Land Model, version 4 (CLM4). To simulate the impact of irrigation realistically, they calibrated the CLM4 simulated irrigation amount against observations from agriculture censuses at the county scale over the conterminous United States. The water used for irrigation was then removed from the surface runoff and groundwater aquifer according to a ratio determined from the county-level agricultural census data. On the basis of the simulations, the impact of groundwater withdrawals for irrigation on land surface and subsurface fluxes were investigated. The results suggest that the impacts of irrigation on latent heat flux and potential recharge when water is withdrawn from surface water alone or from both surface and groundwater are comparable and local to the irrigation areas. However, when water is withdrawn from groundwater for irrigation, greater effects on the subsurface water balance are found, leading to significant depletion of groundwater storage in regions with low recharge rate and high groundwater exploitation rate. The results underscore the importance of local hydrologic feedbacks in governing hydrologic response to anthropogenic change in CLM4 and the need to more realistically simulate the two-way interactions among surface water, groundwater, and atmosphere to better understand the impacts of groundwater pumping on irrigation efficiency and climate.

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Richard C. Y. Li, Wen Zhou, Johnny C. L. Chan, and P. Huang

Abstract

The present study investigates the modulation by the Madden–Julian oscillation (MJO) and the impact of the El Niño–Southern Oscillation (ENSO) on tropical cyclone (TC) genesis in the western North Pacific (WNP) during the period 1975–2010. Results reveal a stronger modulation of cyclogenesis by the MJO during El Niño years, while the modulations in neutral and La Niña years are comparable to each other.

The asymmetric background modification by ENSO is found to greatly affect the extent of MJO modulation under different ENSO conditions. First, MJO activity is intensified and extends farther eastward during El Niño years, instead of being confined west of 150°E as in neutral and La Niña periods. Thus, the influence of MJO is stronger and more zonally widespread in El Niño years, causing significant differences in cyclogenesis parameters in most parts of the WNP. In El Niño years, cyclogenesis is further enhanced in the active phase due to synchronization of MJO signals with favorable background ENSO conditions. While in the inactive phase, the dominance of the strong MJO signals leads to further suppression in TC formation. This leads to overall enhancement of the MJO–TC relationship during El Niño years. On the other hand, the MJO signals confined to the western region west of 150°E in neutral and La Niña years lead to changes in TC-related parameters mainly in the western region, which contribute to the comparatively weaker TC modulations. It can thus be concluded that the MJO has an asymmetric modulation on cyclogenesis in the WNP under different ENSO conditions.

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