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This study is the second part of a two-part series investigating a recent decadal modulation of interannual variability over the western Pacific Ocean around the early 2000s. Observational evidence shows that the anomalous Philippine Sea cyclonic circulation retreats eastward, with the western Pacific rainfall anomaly distribution changing from a north--south tripole pattern to an east--west dipole pattern after 2003-04. These changes are attributed to a change in ElNiño-Southern Oscillation (ENSO) properties and the associated Indo-Pacific sea surface temperature (SST) anomaly pattern. Before the early 2000s, slow-decaying ENSO events induce large SST anomalies in the northern Indian Ocean during the following summer. The northern Indian Ocean SST anomalies act together with the opposite-sign SST anomalies in the tropical central Pacific, leading to a zonally extended anomalous lower-level cyclonic (anticyclonic) circulation and an elongated rainfall anomaly band over the western Pacific. After the early 2000s, ENSO events have a shortened period and a weakened amplitude, and the eastern Pacific SST anomalies tend to undergo a phase transition fromwinter to summer.Consequently, the influence of ENSO on the Indian Ocean SST anomalies is weakened and the contribution of the northern Indian Ocean SST anomalies to the western Pacific summer rainfall variability becomes insignificant. In this case, the western North Pacific summer rainfall is mainly dominated by the well-developed tropical Pacific SST forcing following the early decay of ENSO events. The potential physical mechanism for the two types of ENSO in- fluences is validated with regional decoupled Community Earth System Model experiments. [ABSTRACT FROM AUTHOR]

This study compares monthly total precipitable water (TPW) from the National Aeronautics and Space Administration (NASA) Water Vapor Project (NVAP) and reanalyses of the National Centers for Environmental Prediction (NCEP) (R-1), NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) (R-2), and the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) from January 1988 through December 1999. Based on the means, NVAP exhibits systematic wetter land regions relative to the other datasets reflecting differences in their analyses due to paucity in radiosonde observations. ERA-40 is wetter in the atmospheric convergence zones than the U.S. reanalyses and NVAP ranges in between. Differences in the annual cycle between the reanalyses (especially R-2) and NVAP are also noticeable over the tropical oceans. Analyses on the interannual variabilities show that the ENSO-related spatial pattern in ERA-40 follows more coherently that of NVAP than those of the U.S. reanalyses. The 1997/98 El Niño’s effect on TPW is shown to be strongest only in NVAP, R-1, and ERA-40 during the period of study. All the datasets show TPW decreases in the Tropics following the 1991 Mt. Pinatubo eruption. By subtracting SST-estimated TPW from the datasets, only NVAP and ERA-40 can well represent the spatial pattern of convergence and/or moist-air advection zones in the Tropics. Even though all the datasets are viable for water cycle and climate analyses with discrepancies (wetness and dryness) to be aware of, this study has found that NVAP and ERA-40 perform better than the U.S. reanalyses during the 12-yr period. [ABSTRACT FROM AUTHOR]

Substantial evolution of Normalized Difference Vegetation Index (NVDI)-derived vegetation cover (Fg) exists in the southwestern United States and Mexico. The intraseasonal and wet-/dry-year fluctuations of Fg are linked to observed precipitation in the North American monsoon system (NAMS). The manner in which the spatial and temporal variability of Fg influences the land-atmosphere energy and moisture fluxes, and associated likelihood of moist convection in the NAMS regions, is examined. For this, the regional climate model (RCM) is employed, with three different Fg boundary conditions lo examine the influence of intraseasonal and wet-/dry-year vegetation variability. Results show that a strong link exists between evaporative fraction (EF). surface temperature, and relative humidity in the boundary layer (BL). which is consistent with a positive soil moisture feedback. However, contrary to expectations, higher Fg dots not consistently enhance EF across the NAMS region. This is because the low soil moisture values simulated by the land surface model (LSM) yield high canopy resistance values throughout the monsoon season. As a result, the experiment with the lowest Fg yields the greatest EF and precipitation in the NAMS region, and also modulates regional atmospheric circulation that steers the track of tropical cyclones. In conclusion, the simulated influence of vegetation on land-atmosphere exchanges depends strongly on the canopy stress index parameterized in the LSM. Therefore, a reliable dataset, at appropriate scales, is needed to calibrate transpiration schemes and to assess simulated and realistic vegetation-atmosphere interactions in the NAMS region. [ABSTRACT FROM AUTHOR]

The impact of boundary layer (BL) processes on simulated tropical precipitation was studied using the National Centers for Environmental Prediction (NCEP) Medium-Range Forecast (MRF) Model. A new BL scheme, which is a nonlocal mixing concept of Noh et al. after Troen and Mahrt, was successfully incorporated into the MRF Model. In this study, simulations with 10-member ensembles were conducted for boreal summers of normal, El Niño, and La Niña years, respectively. In particular, the authors focused on the impact on tropical rainfall of the new BL scheme when two different convection schemes are utilized respectively in the model. The new BL scheme improves simulated tropical precipitation overall and in particular reduces the simulated rainfall in the central and eastern equatorial Pacific Ocean. This reduction over the eastern Pacific is a direct effect of the new BL scheme resulting in less mixing of heat and moisture and is irrespective of the convection scheme. The effect of BL processes over the western Pacific, however, is indirectly related to the change of the Walker circulation and highly dependent on the convection scheme selected. [ABSTRACT FROM AUTHOR]

The climatic impacts of a large-scale desertification in northeast Brazil (NEB) are assessed by using the Center for Weather Forecasting and Climate Studies–Center for Ocean–Land–Atmosphere Studies (CPTEC–COLA) AGCM. Two numerical runs are performed. In the control run, NEB is covered by its natural vegetation (most of NEB is covered by a xeromorphic vegetation known as caatinga); in the desertification run, NEB vegetation is changed to desert (bare soil). Each run consists of five 1-yr numerical integrations. The results for NEB wet season (March–May) are analyzed. Desertification results in hydrological cycle weakening: precipitation, evapotranspiration, moisture convergence, and runoff decrease. Surface net radiation decreases and this reduction is almost evenly divided between sensible and latent heat flux. Atmospheric diabatic heating decreases and subsidence anomalies confined at lower atmospheric levels are found. The climatic impacts result from the cooperative action of feedback processes related to albedo increase, plant transpiration suppression, and roughness length decrease. On a larger scale, desertification leads to precipitation increase in the oceanic belt close to the northernmost part of NEB (NNEB). In the NEB–NNEB dipole, the anomalies of vertical motion and atmospheric circulation are confined to lower atmospheric levels, that is, 850–700 hPa. At these levels, circulation anomalies resemble the linear baroclinic response of a shallow atmospheric layer (850–700 hPa) to a tropical heat sink placed over NEB at the middle-layer level. Therefore, NEB climate does show sensitivity to a vegetation change to desert. The present work shows the possibility of significant and pronounced climate impacts, on both regional and large scales, if the environmental degradation in NEB continues unchecked. [ABSTRACT FROM AUTHOR]

Four different Atmospheric Radiation Measurement Program Cloud and Radiation Test Bed (ARM–CART) Single-Column Model (SCM) datasets were used to force an SCM in a number of simulations performed to study the influence of land surface fluxes on precipitation. The SCM employed Goddard Earth Observing System (GEOS-2) GCM physics, which includes a recent version of prognostic cloud scheme (Microphysics of Clouds with Relaxed Arakawa–Schubert), and a land model (Simplified Simple Biosphere Model) coupled to a highly resolved soil hydrological description in the vertical. The four ARM–CART datasets employed in these studies are referred as case 1, case 3, case 4, and case 8. The SCM simulation results broadly confirm the previous findings that an increase in the solar absorption and surface evaporation helps to increase the local rainfall, but they also reveal that the magnitude of the rainfall increase is strongly affected by the ability of the background circulation to promote moist convection. The simulated precipitation increase was as large as 50% of the evapotranspiration increase for case 1 that covered a relatively wet period. It was substantially reduced for cases 3 and 4 covering a normal rainfall period and became negligible for case 8, a dry case. A part of evaporation increase became horizontal divergence of water vapor; this would have the potential of increasing the precipitation downstream of the test region. For a particular background circulation, it was found that the evaporation–precipitation relationship, often defined as recycling ratio, is remarkably robust even for a large range of vegetation covers, soil types, and initial soil moistures. Notwithstanding the limitations of only one-way interaction (i.e., the large scale influencing the regional physics and not vice versa), the current SCM simulations show that recycling ratio is a function of the background circulation and not a regional and/or seasonal featur... [ABSTRACT FROM AUTHOR]

An analysis is provided to explain the sensitivity of regional climate to tropical deforestation based on an intermediate-level mechanistic model for land--atmosphere interaction. The analytical framework is made possible by the fact that feedback to local thermodynamics from large-scale dynamics is negligible, so the determining processes take place in situ. The analytical method accurately reproduces the intermediate-level numerical model results for an albedo change scenario and further provides insight into the mechanisms. A three-way balance among large-scale adiabatic cooling, moist convective heating, and radiative heating allows two positive feedback mechanisms, moisture convergence feedback and evaporation feedback, that give rise to the high sensitivity. The analysis also highlights a deficiency in column energy balance commonly used in tropical simple models, which results in a sensitivity that is likely too high. In light of these findings, some immediate needs for further advancing understanding of the problem are discussed. [ABSTRACT FROM AUTHOR]

Parameterizations for cloud single-scattering properties and the scaling of optical thickness in a partial cloud-iness condition have been developed for use in atmospheric models. Cloud optical properties are parameterized for four broad bands in the solar (or shortwave) spectrum; one in the ultraviolet and visible region and three in the infrared region. The extinction coefficient, single-scattering albedo, and asymmetry factor are parameterized separately for ice and water clouds. Based on high spectral-resolution calculations, the effective single-scattering coalbedo of a broad band is determined such that errors in the fluxes at the top of the atmosphere and at the surface are minimized. This parameterization introduces errors of a few percent in the absorption of shortwave radiation in the atmosphere and at the surface. Scaling of the optical thickness is based on the maximum-random cloud-overlapping approximation. The atmosphere is divided into three height groups separated approximately by the 400- and 700-mb levels. Clouds are assumed maximally overlapped within each height group and randomly overlapped among different groups. The scaling is applied only to the maximally overlapped cloud layers in individual height groups. The scaling as a function of the optical thickness, cloud amount, and the solar zenith angle is derived from detailed calculations and empirically adjusted to minimize errors in the fluxes at the top of the atmosphere and at the surface. Different scaling is used for direct and diffuse radiation. Except for a large solar zenith angle, the error in fluxes introduced by the scaling is only a few percent. In terms of absolute error, it is within a few watts per square meter. [ABSTRACT FROM AUTHOR]