From analyses based on Singular Value Decomposition of rainfall and 500 hPa geopotential height anomalies, we have identified two atmospheric teleconnection pattern linking interannual variability of summertime precipitation over East Asia and the continental United States. The first pattern is associated with enhanced rainfall over the Yangtze River region to above-normal rainfall over the US northern Great Plains and the Midwest, and reduced rainfall over the Atlantic coast. It features zonally elongated 500 hPa height anomalies over the subtropical and extratropical western North Pacific coupled to a regional circulation pattern over North American that regulates moisture transport from the Gulf of Mexico to the Northern Great Plains. The second pattern shows enhanced rainfall anomalies over the Huaihe River, northeastern and southern China and deficient rainfall over the central US. It connects the East Asian and North American continents via a pan-Pacific wavetrain signal, possibly stemming from Rossby wave dispersion from fluctuations of large-scale heat sources, and sinks in the Indo-Pacific region. Examination of associated sea surface temperature variability in the North Pacific suggests that the first pattern may be influenced by El Niño in the preceding spring, but becomes increasing decoupled from tropical SST during the summer and fall. However, the second pattern has no significant relationship with El Niño. Analysis of extreme rainfall statistics between regions in East Asia and North America suggests that occurrence of the aforementioned teleconnection patterns is associated with increased probability for extreme rainfall events over the Yangtze River Valley, coupled to increased probability of anomalies of the same sign over the US Midwest, and of the opposite sign over the US Mid-Atlantic coast. Our results suggest that the summertime teleconnection patterns should be further explored for additional sources of potential predictability of summertime floods or droughts over regions of East Asia and North America.
A numerical prediction experiment is conducted on Super Typhoon Herb, 1996, that made landfall in northern Taiwan and caused severe wind and flood damage. A version of the Naval Research Laboratory's (NRL) Coupled Ocean/Atmospheric Prediction System (COAMPS) with a triple-nested grid, (81, 27, and 9-km horizontal resolutions) and 30 levels in the vertical is used in this experiment. Starting from archived global analysis fields, COAMPS generates equally good track predictions on all three grids. The average location errors of three grids were 58 km at 24 h, and 77 km at 48 h. The prediction of wind fields, especially at the critical time just prior to Herb's landfall on both the medium mesh and the fine mesh agrees very well with subjective analyses by local experts. Quantitative precipitation prediction on the fine mesh is very good both in amount and distribution. Precipitation predictions on the medium and coarse meshes, however, are less skillful. The result from a supplementary numerical experiment with the same three-grid configuration of 9-27-81 km resolution, but with the medium resolution terrain (pertinent for 27 km) on the 9-km grid suggests that high-resolution dynamics, and the terrain afforded on the 9-km mesh, are both important ingredients for quantitative precipitation prediction. Therefore, for accurate quantitative precipitation prediction, it is important to place the highresolution mesh over the complex terrain, so that the terrain effect on the structure of the storm can be represented on the finest possible resolution. This study demonstrates that a well-behaved, generalpurpose, mesoscale numerical model can be a useful tool for tropical cyclone prediction.
The relationship between topography and daytime cloud activity over the Tibetan Plateau and surrounding areas was examined by using Geostationary Meteorological Satellite (GMS) Visible (VIS) and Infrared (IR) images during the premonsoon, and monsoon periods in 1998. Previous studies using IR images have already confirmed the strong diurnal variation of convective activity over the Tibetan Plateau. This study relies primarily on VIS images to analyze daytime cloud distribution because VIS images are more adaptable to low-level clouds and they offer better spatial resolution than IR images. IR images are used to judge whether cloud tops are high or low. High-level clouds are prominent over the large-scale (100-300 km) mountain ranges, but fewer clouds are observed in the major valleys in the plateau during premonsoon and monsoon periods at 15 Local Solar Time (LST), when the amount of cloud cover is at its maximum from 09 LST to 15 LST. However, the relationship between cloud distribution and topography is not always clear when the horizontal scale of the topography is less than 100 km. Less cloud coverage was observed over the plateau in the morning during the premonsoon period. On the contrary, clouds frequently appeared over the southeastern part of the plateau in the morning during the monsoon period. Low-level clouds often cover the southern slope of the Himalayas, and the frequency of cloud coverage exceeds 75% at 15 LST in the monsoon period. While the cloud tops are growing high elevation in the afternoon over the plateau, low-level clouds cover the southern slope of the Himalayas through the daytime (09-15 LST).
The impact of surface hydrological processes in the simulation of tropical climate by the Community Climate Model Version 2 (CCM2) has been studied. Two ten year climate simulations have been analysed to study the effect of surface hydrology feedback. In one of the simulations the surface moisture was determined interactively (VAR_HYD), while in the other simulation the surface moisture was fixed to annual climatological values (FIX_HYD). The simulated values of soil moisture in the VAR_HYD simulation are higher than the annual climatological values, both during northern summer and winter seasons. The impact of surface hydrology on precipitation was larger during northern summer than during the northern winter season. The impact of surface hydrology specification was found to be not entirely local in the model. It appears to have a remote impact on the other parts of the tropics. The precipitation increased over the Indian region, and off-equatorial West Pacific in the interactive hydrology simulation (vis-a-vis FIX_HYD) and reduced over the equatorial West Pacific regions. The simulation of precipitation also improves over the East Pacific region with the incorporation of interactive hydrology. The differences in precipitation between the VAR_HYD and the FIX_HYD simulations are associated with differences in vertical moist static stability and 500 hPa vertical velocity. From harmonic analysis of OLR it was found that the power and frequency in the intraseasonal scales is simulated more realistically with the incorporation of interactive surface hydrology. The interactive hydrology was found to be necessary for the simulation of meridional propagations of convective zones over the Indian region (on intraseasonal time-scales). The meridional migration is not observed in the simulation with fixed surface hydrology, and is on account of non-monotonic variation of vertically integrated moist static energy. The specification of continental surface hydrological processes did not however have a significant impact on the simulation of the equatorially trapped waves, such as the Madden-Julian Oscillation (MJO), though the propagations are discontinuous in the FIX_HYD simulation.
The radiative effects of several cloud types as classified by the split window (11 and 12 μm) technique were studied using coincident and collocated Earth Radiation Budget Experiment (ERBE) S-8 data and Advanced Very High Resolution Radiometer (AVHRR) data from NOAA-9. The parameter investigated was cloud radiative forcing (CRF), the difference between clear and cloudy shortwave flux (SW) and longwave flux (OLR) at the top of the atmosphere. In computing the CRF, the accuracy of clear SW and OLR is essential. Clear scene IDs in the ERBE dataset were evaluated using coincident and collocated AVHRR image data. The mean visible reflectance and SW for clear footprints defined by the ERBE are reasonably small and are 3.2% and 89.0 Wm-2, respectively. However, the values computed using our technique are smaller, 2.7% and 83.9 Wm-2, respectively. The use of collocated AVHRR image data improves clear footprint definition and implies that care should be taken when computing CRF from ERBE data alone. The CRF from several cloud types classified by the split window were compared. Cumulonimbus clouds show the largest impact on top of the atmosphere radiation for both SW and OLR. Cirrus and lowlevel cumulus clouds have similar effects on OLR, but large differences between them are seen for SW. The impact of low-level cumulus clouds on SW is much larger than that of cirrus clouds. Some optically thin cirrus clouds show positive cloud radiative forcing (warming effect). The relationships between OLR and cloud types (including cloud-free) as classified by the split window technique were investigated. By using brightness temperature differences between the split window channels, OLR estimation is improved for cloud-free and low-level cumulus clouds when compared with OLR estimated by the National Oceanic and Atmospheric Administration (NOAA) operational algorithm.
A two-dimensional cloud-resolving model, including a supply of sensible and latent heat fluxes from the surface, is used to study the development of deep convective clouds over a southern region far from the Meiyu front (wet region) of eastern China. Some deep convective clouds were observed during the latter half of the GAME/HUBEX IOP (GEWEX Asian Monsoon Experiment/Huaihe River Basin Experiment Intensive Observation Periods, GEWEX: Global Energy and Water Cycle Experiment) 1998, although there is less large-scale convergence over this region. Numerical simulations reproduce the development of deep convective clouds, and their generating/decaying time. The development process of a convective mixing layer and generation of shallow convective clouds around the top of this layer are also simulated. The results of sensitivity tests on the surface land-use (i.e., the supply of the sensible and latent heat fluxes) and the relative humidity in the middle troposphere, indicate that there are two effective factors in the development of deep convective clouds over this region. One is the large amount of latent heat flux from the surface, and the other is the moist environment in the middle troposphere. The latent heat flux from the surface supplies water vapor to generate convective clouds. Paddy fields can supply a large amount of latent heat flux into the lower atmosphere and are widely distributed over this region. On the other hand, the moist environment in the middle troposphere can cause shallow convective clouds to become deep because the positive buoyancy in the shallow convective clouds is not lost by the evaporation cooling of the entrained air mass. Additionally, this moist environment in the middle troposphere is formed by the development of shallow convective clouds, which transport water vapor from the convective mixing layer.
In this paper model-generated data sets are examined to address the question of seasonal precipitation forecast skill of the Asian and the North American monsoon systems. In this context the seasonal climate forecast data from a set of coupled atmosphere-ocean models were used. The main question we ask is if there is any useful skill in predicting seasonal anomalies beyond those of climatology. The methodology for prediction is the ‘FSU Superensemble’ which is applied here to the anomalies of the predicted multimodel data sets and the observed (analysis) fields. The skills of seasonal forecasts are evaluated using two different types of parameters: anomaly correlations and root mean square errors. Comparison of skill of the coupled model forecasts and the AMIP hindcasts yields encouraging results. It is noted that the superensemble based anomaly forecasts have somewhat higher skill compared to the bias-removed ensemble mean of member models, individually bias removed ensemble mean of the member models and the climatology. This skill comes partly from the forecast performance of multimodels and partly from the training component built into this system that is based on past collective performance of these multimodels. These components are separated to assess the improvements of the superensemble. Though skill of the forecasts from the superensemble is found to be higher than that of the bias-removed ensemble mean and has shown some usefulness over the climatology, the issue of forecasting a season in advance in quantitative terms still remains a challenge and demands further advancement in climate modeling studies.
This study investigates the regional responses of a climate model to the gradual increase of atmospheric carbon dioxide at 1% per year compounded for quadrupling. Use of NCAR fully coupled Climate System Model (CSM1.2) is attempted, with special emphasis on the simulated sea-level changes in the neighbouring seas of the northwestern Pacific Ocean with an enhanced resolution. Regional warming and sea level change are higher than global changes. At the time of CO2 quadrupling, the model predicts a rise in sea level of 19 cm and 25 cm for the globe and the northwestern Pacific Ocean, respectively, while surface air temperature rises are 2.9°C and 3.0°C for the globe and the northwestern Pacific Ocean, respectively. Based on simulations, climate changes in the Northwestern Pacific Ocean will be more distinctive compared with the global average, mainly due to exceptionally large warming and sea level change near the entrance of the Kuroshio extension.