The midocean trough in the North Pacific may form a favorable environment for the genesis of some synoptic disturbances. In contrast, the North Pacific anticyclone may hinder the downward penetration of these disturbances into the lower troposphere and prevent the moisture supply to these disturbances from the lower troposphere. Because no thick clouds, rainfall, and destructive surface winds are associated with these disturbances to attract attention, they have not been analyzed or documented. Actually, the upper-level wind speed within these disturbances is sometimes as strong as tropical cyclones and has the possibility of causing air traffic hazards in the western subtropic Pacific. With infrared images of the Japanese Geostationary Meteorological Satellite and the NCEP–NCAR reanalysis data, 25 North Pacific disturbances were identified over six summers (1993–98). Two aspects of these disturbances were explored: spatial structure and basic dynamics. For their structure, the disturbances possess a well-organized vortex in the middle to upper troposphere with a descending dry/cold core encircled by the moist ascending air around the vortex periphery; the secondary circulation of the vortex is opposite to other types of synoptic disturbances. Since vorticity reaches maximum values along the midocean trough line, barotrophic instability is suggested as a likely genesis mechanism of the vortex. After the vortex is formed, the horizontal advection of total vorticity results in its westward propagation, while the secondary circulation hinders this movement. Along its westward moving course, close to East Asia, there is a reduction in vortex size and a tangential speed increase inversely proportional to the vortex size. Diminishing its horizontal convergence/descending motion by the upper-tropospheric East Asian high and the lower-tropospheric monsoon low, the vortex eventually dissipates along the East Asian coast.

Abstract

Finescale parameterizations are of great importance to explore the turbulent mixing in the open ocean due to the difficulty of microstructure measurements. Studies based on finescale parameterizations have greatly aided our knowledge of the turbulent mixing in the open ocean. In this study, we introduce a modified finescale parameterization (MMG) based on shear/strain variance ratio R _ω and compare it with three existing parameterizations, namely, the MacKinnon–Gregg (MG) parameterization, the Gregg–Henyey–Polzin (GHP) parameterization based on shear and strain variances, and the GHP parameterization based on strain variance. The result indicates that the prediction of MG parameterization is the best, followed by the MMG parameterization, then the shear-and-strain-based GHP parameterization, and finally the strain-based GHP parameterization. The strain-based GHP parameterization is less effective than the shear-and-strain-based GHP parameterization, which is mainly due to its excessive dependence on stratification. The predictions of the strain-based MMG parameterization can be comparable to that of the MG parameterization and better than that of the shear-and-strain-based GHP parameterization. Most importantly, MMG parameterization is even effective over rough topography where the GHP parameterization fails. This modified MMG parameterization with prescribed R _ω can be applied to extensive CTD data. It would be a useful tool for researchers to explore the turbulent mixing in the open ocean.

Abstract

The wind–pressure relationship (WPR) for tropical cyclones (TCs) in the western North Pacific is reexamined based on aircraft data, TC best track data, and daily reanalysis data during 1957–87. Minimum sea level pressure (MSLP) was estimated from aircraft reconnaissance, and maximum surface wind speeds (MSWs) were adjusted from the maximum wind speed at flight level. The mean MSLP was found to be higher during 1957–64 than during 1965–87, presumably due to the change in reconnaissance instrumentation and technology, which results in a systematic MSW bias (too high) before 1965 in the China Meteorological Administration (CMA) dataset. Further analyses found that the WPR used in the CMA dataset is more accurate for strong TCs, while the WPR in the Tokyo Regional Specialized Meteorological Center (RSMC) dataset is better for weak TCs after the MSW-RSMC converted by the Dvorak conversion table (1984) and when using the aircraft datasets as a baseline. Several prevailing operational WPRs used in the western North Pacific are reexamined. Results show that the WPR of Knaff and Zehr explains 71% of the variance with a MAE of 9.22 hPa, which represents a significant improvement over other WPRs. Utilizing data after 1965 (a total of 1874 samples), the effects of TC center latitude, size, translation speed, intensification trend, and environmental pressure on the WPRs were examined. Results show that faster-traveling TCs, smaller in size, and located in a higher environmental pressure at lower latitudes, exhibited a higher MSLP for a given MSW. Meanwhile, the latitude, translational speed, and the environmental pressure produces additional improvement, but the TC size and intensity change added only a little skill to the WPR equation.

Abstract

Over the last two decades, the Central Weather Bureau of Taiwan and the U.S. National Severe Storms Laboratory have been involved in a research and development collaboration to improve the monitoring and prediction of river flooding, flash floods, debris flows, and severe storms for Taiwan. The collaboration resulted in the Quantitative Precipitation Estimation and Segregation Using Multiple Sensors (QPESUMS) system. The QPESUMS system integrates observations from multiple mixed-band weather radars, rain gauges, and numerical weather prediction model fields to produce high-resolution (1 km) and rapid-update (10 min) rainfall and severe storm monitoring and prediction products. The rainfall products are widely used by government agencies and emergency managers in Taiwan for flood and mudslide warnings as well as for water resource management. The 3D reflectivity mosaic and QPE products are also used in high-resolution radar data assimilation and for the verification of numerical weather prediction model forecasts. The system facilitated collaborations with academic communities for research and development of radar applications, including quantitative precipitation estimation and nowcasting. This paper provides an overview of the operational QPE capabilities in the Taiwan QPESUMS system.

Abstract

Tropical deforestation can result in substantial changes in local surface energy and water budgets, and thus in atmospheric stability. These effects may in turn yield changes in precipitation. The Maritime Continent (MC) has undergone severe deforestation during the past few decades but it has received less attention than the deforestation in the Amazon and Congo rain forests. In this study, numerical deforestation experiments are conducted with global (i.e., Community Earth System Model) and regional climate models (i.e., Regional Climate Model version 4.6) to investigate precipitation responses to MC deforestation. The results show that the deforestation in the MC region leads to increases in both surface temperature and local precipitation. Atmospheric moisture budget analysis reveals that the enhanced precipitation is associated more with the dynamic component than with the thermodynamic component of the vertical moisture advection term. Further analyses on the vertical profile of moist static energy indicate that the atmospheric instability over the deforested areas is increased as a result of anomalous moistening at approximately 800–850 hPa and anomalous warming extending from the surface to 750 hPa. This instability favors ascending air motions, which enhance low-level moisture convergence. Moreover, the vertical motion increases associated with the MC deforestation are comparable to those generated by La Niña events. These findings offer not only mechanisms to explain the local climatic responses to MC deforestation but also insights into the possible reasons for disagreements among climate models in simulating the precipitation responses.

Abstract

Hydroclimate in the montane cloud forest (MCF) regions is unique for its frequent fog occurrence and abundant water interception by tree canopies. Latent heat (LH) flux, the energy flux associated with evapotranspiration (ET), plays an essential role in modulating energy and hydrological cycles. However, how LH flux is partitioned between transpiration (stomatal evaporation) and evaporation (nonstomatal evaporation) and how it impacts local hydroclimate remain unclear. In this study, we investigated how fog modulates the energy and hydrological cycles of MCF by using a combination of in situ observations and model simulations. We compared LH flux and associated micrometeorological conditions at two eddy-covariance sites—Chi-Lan (CL), an MCF, and Lien-Hua-Chih (LHC), a noncloud forest in Taiwan. The comparison between the two sites reveals an asymmetric LH flux with an early peak at 0900 local time in CL as opposed to LHC, where LH flux peaks at noon. The early peak of LH flux and its evaporative cooling dampen the increase in near-surface temperature during the morning hours in CL. The relatively small diurnal temperature range, abundant moisture brought by the valley wind, and local ET result in frequent afternoon fog formation. Fog water is then intercepted by the canopy, sustaining moist conditions throughout the night. To further illustrate this hydrological feedback, we used a land surface model to simulate how varying canopy water interception can affect surface energy and moisture budgets. Our study highlights the unique hydroclimatological cycle in the MCF and, specifically, the inseparable relationship between the canopy and near-surface meteorology during the diurnal cycle.

Abstract

This is the second part of a three-part paper on North American climate in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that evaluates the twentieth-century simulations of intraseasonal to multidecadal variability and teleconnections with North American climate. Overall, the multimodel ensemble does reasonably well at reproducing observed variability in several aspects, but it does less well at capturing observed teleconnections, with implications for future projections examined in part three of this paper. In terms of intraseasonal variability, almost half of the models examined can reproduce observed variability in the eastern Pacific and most models capture the midsummer drought over Central America. The multimodel mean replicates the density of traveling tropical synoptic-scale disturbances but with large spread among the models. On the other hand, the coarse resolution of the models means that tropical cyclone frequencies are underpredicted in the Atlantic and eastern North Pacific. The frequency and mean amplitude of ENSO are generally well reproduced, although teleconnections with North American climate are widely varying among models and only a few models can reproduce the east and central Pacific types of ENSO and connections with U.S. winter temperatures. The models capture the spatial pattern of Pacific decadal oscillation (PDO) variability and its influence on continental temperature and West Coast precipitation but less well for the wintertime precipitation. The spatial representation of the Atlantic multidecadal oscillation (AMO) is reasonable, but the magnitude of SST anomalies and teleconnections are poorly reproduced. Multidecadal trends such as the warming hole over the central–southeastern United States and precipitation increases are not replicated by the models, suggesting that observed changes are linked to natural variability.