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C-T. Chen and V. Ramaswamy

Abstract

The sensitivity of the global climate to perturbations in the microphysical properties of low clouds is investigated using a general circulation model coupled to a static mixed layer ocean with fixed cloud distributions and incorporating a new broadband parameterization for cloud radiative properties. A series of GCM experiments involving globally uniform perturbations in cloud liquid water path or effective radius (albedo perturbations), along with one for a doubling of carbon dioxide (greenhouse perturbation), lead to the following results: 1) The model's climate sensitivity (ratio of global-mean surface temperature response to the global-mean radiative forcing) is virtually independent (to ∼10%) of the sign, magnitude, and the spatial pattern of the forcings considered, thus revealing a linear and invariant nature of the model's global-mean response. 2) Although the total climate feedback is very similar in all the experiments, the strengths of the individual feedback mechanisms (e.g., water vapor, albedo) are different for positive and negative forcings. 3) Changes in moisture, tropospheric static stability, and sea ice extent govern the vertical and zonal patterns of the temperature response, with the spatial distribution of the response being quite different from that of the radiative forcing. 4) The zonal surface temperature response pattern, normalized with respect to the global mean, is different for experiments with positive and negative forcings, particularly in the polar regions of both hemispheres, due to differing changes in sea ice. 5) The change in the surface radiative fluxes is different for the carbon dioxide doubling and cloud liquid water path decrease experiments, even though both cases have the same radiative forcing and a similar global-mean surface temperature response; this leads to differences in the vigor of the hydrologic cycle (evaporation and precipitation rates) in these two experiments.

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C-T. Chen and V. Ramaswamy

Abstract

The sensitivity of the global climate to spatially localized (20°–70°N) perturbations in the microphysical properties of low clouds is investigated using a general circulation model coupled to a mixed layer ocean with fixed cloud distributions. By comparing with earlier experiments involving globally uniform perturbations, insights are obtained into the climate responses to spatially inhomogeneous radiative forcings, such as that due to the contrast in the effective drop radius of land and ocean clouds and the anthropogenic sulfate aerosol-induced alteration of cloud albedo. The main findings of this study are as follows: 1) The model's climate sensitivity (ratio of global-mean surface temperature response to the global-mean radiative forcing) is virtually independent of the distribution and magnitude of forcing. 2) Although the total feedback is very similar in the different experiments, the strengths of the individual feedback mechanisms (water vapor, albedo, lapse rate) are dissimilar. 3) For the localized perturbations, the climate response is essentially confined to the hemisphere in which the forcing occurs, owing to a poor interhemispheric energy exchange. 4) In spite of no forcing in the Southern Hemisphere in the localized experiments, there is a weak “remote” temperature response there. 5) For both global and localized perturbations, the temperature response in the tropical upper troposphere is larger than in the lower troposphere due to moist convective processes; in the localized experiments, while there is a strong vertical gradient in the temperature change at the Northern Hemisphere mid and high latitudes, the temperature change throughout the lower and midtroposphere of the Southern Hemisphere is uniform. 6) The localized experiments induce notable changes in the mean meridional circulation and precipitation near the equator, which are not obtained for the global perturbation cases. 7) The pattern of temperature response of the land and ocean areas in the Northern Hemisphere midlatitudes depends on whether the forcing occurs over both types of surfaces or over land only; the results suggest that the well-known contrast in drop radii between continental and maritime clouds exerts a significant influence on the surface temperature distribution within the zone and on the manner in which the surface energy balance is maintained.

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George T. J. Chen and H. C. Chou

Abstract

Six cases of prefrontal squall lines were observed over the Taiwan Strait and western Taiwan during the 1987 Taiwan Area Mesoscale Experiment (TAMEX). The mean propagation speed was 10 m s−1, and the mean life span was 11.4 h for the six squall lines. All the lines occurred ahead of the Mei-Yu front and moved away from the front with time. The mean environmental conditions associated with the squall lines were analyzed by compositing the six cases. The environmental conditions observed during the TAMEX squall lines were found with characteristics between tropical and midlatitude squall lines. The steering level was near 7 km during the mature stage. A low-level jet at 3–4 km was present, with strong vertical shear in the presquall environment below 700 hPa. The squall lines were oriented 45° to the shear in the 1–3-km layer, like midlatitude cases. The CAPE, however, is similar to the tropical squall lines. The inflow ahead of the squall lines was deeper and stronger below 400 hPa, and the CAPE was higher during the mature stage as compared to the intensifying stage. The squall-line collapse is correlated with decreasing CAPE and low-level inflow ahead of the lines.

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C-P. Chang and George T-J. Chen

Abstract

The earliest onset of the Asian summer monsoon occurs in early to middle May over the South China Sea. This onset is signified by the development of low-level westerlies and leads to heavy convective rainfall over southern China (pre-Mei-Yu). In June, low-level westerly surges over the northern South China Sea are associated with the Mei-Yu rainfall system in the Yangtze region and southern Japan. In this work, the ECMWF data for 1981–86 are used to study the tropical circulations associated with the development of low-level westerlies during both events.

Composites of horizontal wind, geopotential height, moisture, and vertical velocity during six May onsets and nine June surges, respectively, indicate that both events occur with the approach of a midlatitude trough–front system. The possible triggering of the South China Sea summer monsoon onset by the midlatitude system may explain why the South China Sea onset occurs prior to other regions of the Asian monsoon. During boreal spring, this is the only Asian monscon region where midlatitude fronts can move into the Tropics without having to overcome significant terrain barriers.

Following the two events, opposite teleconnection-like patterns develop in the Tropics in both hemispheres. During the May onsets, the arrival of the midlatitude trough/front appears to lead to a southwestward extension of a cyclogenesis zone into the equatorial Indian Ocean. Along this zone, cyclonic vortices develop over.the Andaman Sea, the Bay of Bengal, and perhaps the southern equatorial Indian Ocean, and increased deep convection is indicated by the OLR composites. During the June surges, a pair of anticyclones develop straddling the equator at the longitudes of Indochina. This anticyclonic couplet is associated with decreased deep convection and propagates westward to dominate the flow changes over.the Bay of Bengal and the southern Indian Ocean. The steady 4–5 m s−1 westward speed and near-perfect symmetry with respect to the equator indicate the possibility of an equatorial Rossby wave generation in conjunction with the June westerly surges in the northern South China Sea.

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C-P. Chang, T-C. Yeh, and J. M. Chen

Abstract

The island of Taiwan is situated in the main path of western North Pacific typhoons. Its dominant central mountain range (CMR), with a hoizontal scale comparable to the radius of a typhoon, often produces significant distortions in the typhoon circulation. A 20-year dataset from 22 surface stations is used to describe the effects of the Taiwan terrain on the surface structure of typhoons.

Empirical orthogonal function analysis on the pressure field is used to identify the primary structure modes. The first mode is a uniform-sign anomaly pattern portraying the decrease in pressure as a typhoon is approaching. The second mode represents the strong terrain-induced west-east pressure gradient that is normal to the main axis of the CMR. The third mode results mainly from the west-cast pressure gradient arising from the relative location of the typhoon center to the east or west of Taiwan, but it also contains a weak south-north pressure gradient that can he attributed to the terrain. A regression technique is then used to determine the surface wind, temperature, relative humidity, and hourly rainfall associated with each pressure mode. In all cases, them fields are consistent, showing the effects of the terrain blocking or deflection and their consequent ascending and descending motions.

The relative importance of each mode depends strongly on the location of the typhoon center. No dependence on the direction or speed of motion is discernible when all cases are considered. When different, persistently smooth tracks are identified, the variations due to motion direction can be recognized because the terrain effect is affected by the mean steering flow. Only two types of smooth tracks that represent clearly different steering flows intersect in an area. At the intersection, a subsequent difference in storm structure over Taiwan exists that can be explained by the difference in the steering flows associated with the two track types.

The leeside secondary low that was often observed on the west coast of Taiwan is found to consist of at least two basic modes. It develops only when the typhoon center is in southeastern Taiwan or an ocean area to the east-southeast. The observed scale of this low is significantly smaller than that which can be produced by an interaction of the mean steering flow and the CMR. This smaller scale is due to a local buildup of the surface pressure south of the lee vortex, which results from the against-mountain return flow of the cyclonic circulation.

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C-P. Chang, J. E. Millard, and G. T. J. Chen

Abstract

The surface pressure, temperature, dew point and wind data over the South China Sea and vicinity during Winter MONEX are examined to determine the timing of the passage of cold surges at various reporting stations. It is found that for more than half of the surge cases during Winter MONEX, a surge occurs in two stages separated by a time interval of several hours to approximately one day. The first stage is often characterized by a significant rise in pressure, while the second stage by a sharp decrease in dew point temperature. Freshening of surface winds may accompany either or both stages. The time separation between the two stages is relatively short at the upstream stations but generally long at the downstream stations. The second stage is associated with a frontal passage. On the other hand, the first stage is not clearly associated with any significant synoptic events. From its very fast propagation speed which increases with an inferred depth scale, and an increase in the local cross-isobar angle of the surface wind during passage, the first stage is identified with a synoptic-scale gravity wave type motion.

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C. Dearden, G. Vaughan, T. Tsai, and J.-P. Chen

Abstract

Numerical simulations are performed with the Weather Research and Forecasting Model to elucidate the diabatic effects of ice phase microphysical processes on the dynamics of two slow-moving summer cyclones that affected the United Kingdom during the summer of 2012. The first case is representative of a typical midlatitude storm for the time of year, while the second case is unusually deep. Sensitivity tests are performed with 5-km horizontal grid spacing and at lead times between 1 and 2 days using three different microphysics schemes, one of which is a new scheme whose development was informed by the latest in situ observations of midlatitude weather systems. The effects of latent heating and cooling associated with deposition growth, sublimation, and melting of ice are assessed in terms of the impact on both the synoptic scale and the frontal scale. The results show that, of these diabatic processes, deposition growth was the most important in both cases, affecting the depth and position of each of the low pressure systems and influencing the spatial distribution of the frontal precipitation. Cooling associated with sublimation and melting also played a role in determining the cyclone depth, but mainly in the more intense cyclone case. The effects of ice crystal habit and secondary ice production are also explored in the simulations, based on insight from in situ observations. However in these two cases, the ability to predict changes in crystal habit did not significantly impact the storm evolution, and the authors found no obvious need to parameterize secondary ice crystal production at the model resolutions considered.

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C-T. Chen, Erich Roeckner, and Brian J. Soden

Abstract

Water vapor distributions obtained from the fourth generation ECHAM general circulation model are compared with satellite observations of total precipitable water (TPW) from the Special Sensor Microwave/Imager (SSM/1) and upper-tropospheric relative humidity (UTH) from TIROS-N Operational Vertical Sounder (TOVS). In general, the model simulators agree well with satellite observations of the climatological mean, seasonal variation, and interannual variation of moisture. There are, however, biases in the details. Underestimates in TPW and UTH are found off the west coast of continents, especially in the boreal summer over the eastern subtropical Pacific. These biases are related to both enhanced dry advection due to an excessively strong subtropical high and greater large-scale subsidence. A more intense tropical circulation in ECHAM4 is evidenced by the broadening of the high TPW and UTH zone that coincides with the equatorial convective regions. Additionally, interannual anomalies in equatorial UTH and TPW simulated by the model are found to be more sensitive to tropical SST anomalies than are the satellite data. The impact of changes in physical parameterizations upon the moisture distribution is also examined by comparing the simulations from the previous ECHAM3 and the current ECHAM4 models. The dry bias at the equator in ECHAM3 is related to the closure assumption used for deep convection, while the dry bias in UTH over the high-latitude winter hemisphere in ECHAM3 is a result of negative specific humidifies produced by the spectral vapor advection scheme. With the new semi-Lagrangian advection scheme in ECHAM4, the simulated UTH over the same region becomes moister than TOVS observations suggest. The impact of discrepancies in the simulated water vapor distributions upon the radiation budget and cloud distribution in the model are also described.

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G. T. J. Chen, Y. J. Wang, and C-P. Chang

Abstract

This study compares the systematic errors of 36-h surface cyclone and anticyclone forecasts for two operational numerical weather prediction models over East Asia and the western North Pacific Ocean: the U.S. Navy's Operational Global Atmospheric Prediction System (NOGAPS), and Japan Meteorological Agency's Fine-mesh Limited Area Model (JFLM). The study is carried out for the 1983 Mei-Yu season (May–July), which is the wettest season over East Asia based on nontyphoon-produced rainfall. All available 0000 and 1200 GMT forecast runs are evaluated against an independent dataset of subjective analysis produced operationally by the Central Weather Bureau, Taipei. The mean position errors, mean central pressure errors and forecast skill indices for both cyclones and anticyclones in the NOGAPS and JFLM models are examined.

Both NOGAPS and JFLM models are more likely to underforecast than to overforecast the existence and/or genesis of both cyclones and anticyclones. However, over the Tibetan Plateau and its vicinity, both models tend to overforecast the existence and/or genesis of cyclones. They also forecast both cyclones and anticyclones too slow and too far to the north.

Diurnal variations in central pressure errors suggest that the error source is the lack of radiation processes in the JFLM and too strong a diurnal cycle of radiation processes in NOGAPS. Also, the failure to treat adequately the bulk effects of cumulus convection seems to be primarily responsible for the poor forecasts of oceanic cyclone development.

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C-P. Chang, S. C. Hou, H. C. Kuo, and G. T. J. Chen

Abstract

The East Asian summer monsoon (Mei-yu) disturbance of 17–25 June 1992 was the most intense 850-hPa low center of such systems during a 7-yr period. Due to the moisture fluxes associated with the southwesterlies from the warm tropical oceans, diabatic heating has generally been considered the main energy source of these heavy-precipitation disturbances as they propagate eastward from the eastern flank of the Tibetan Plateau across southeastern China and move into the East China Sea. In this study piecewise potential vorticity inversion is used to analyze the physical mechanisms of this intense case, particularly the possible roles of midlatitude baroclinic processes in its development and evolution.

The development of the low-level vortex involved the coupling with two upper-level disturbances, one at 500 hPa that also originated from the eastern flank of the Tibetan Plateau, and another at 300 hPa. Both disturbances appeared later than and upstream of the low-level vortex. Faster eastward movements allowed them to catch up with the low-level vortex and led to a strong vertical coupling and deep tropopause folding. Initially, diabatic heating was the dominant mechanism for the low-level vortex while the tropopause process opposed it. Both mechanisms supported the 500-hPa disturbance, and tropopause folding was the dominant mechanism for the 300-hPa disturbance. As the vertical coupling developed, the tropopause process reversed its earlier role in the low-level disturbance and contributed to its development. Boundary layer and adiabatic effects also became contributive as the disturbance moved out of eastern China to the oceanic region.

The vertical coupling of the three disturbances was a major factor in the development. The timing and position of the middle-tropospheric disturbance was critical in bridging the upper- and lower-level disturbances and a deep tropopause folding. This midlatitude-originated process compounded the diabatic heating effect that was sustained by tropical moist air, leading to the strong intensification.

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