ALBERT

Description: Atmospheric teleconnections in the medium to long (10-90 days) time scales focusing on the interactions between extratropical circulation and tropical convection are studied. In a continuing effort to study short-term climate variability and atmospheric teleconnection as inferred from satellite observed outgoing longwave radiation, the low frequency variability (LFB) of tropical and extratropical cloud fluctuation over the Pacific was studied. It was found that during the Northern winter, the LFV of tropical cloud fluctuation is dominated by a 40-50 day dipole-like oscillation linking convection over Indonesia and the equatorial central Pacific. Eastward propagating signals appearing as outbursts of convective cloud clusters originating from the Indian Ocean appear to periodically feed energy into this dipole oscillation. It was also found that there are cloud features appearing over East Asia and subsequently over the eastern North Pacific which vary coherently with the tropical dipole anomaly. Based on analysis and an a priori phenomenological model, it is believed that the cloud fluctuations are associated with two space/time extended normal modes of tropical-extratropical interactions over the Pacific involving a coupling between the tropical dipole convective heating anomaly with cold surges over East Asia, and blocking over the eastern North Pacific respectively.

Description: The operational global analyses from the two major U.S. numerical weather prediction centers, the Navy's Fleet Numerical Oceanography Center and the National Meteorological Center, are used to describe the synoptic-scale features of the 1 Nov. 1992 to 28 Feb. 1993 TOGA COARE intensive observing period (IOP). TOGA COARE is an international field experiment in which a large number of research scientists from the Goddard Laboratory for Atmospheres (Code 910) and the Laboratory for Hydrospheres (Code 970) participated. Two high-amplitude intraseasonal (30-60 day) oscillations passed through the TOGA COARE observational network located in the equatorial western Pacific. Associated with the oscillations were two 6-10 day periods of persistent westerly surface winds at the equator or 'westerly wind bursts.' These events are depicted through time series and time-longitude cross sections of divergence/velocity potential, surface winds, precipitation, ocean mixed-layer depth, and sea surface temperature. The high and low frequency components of the flow in which the intraseasonal oscillations were embedded are shown using seasonal, monthly, and 5-day averages of the surface, 850 and 200 mb winds, precipitation, and sea-level pressure, and a time-longitude cross section of tropical cyclone activity. Independent verification of precipitation comes from near real-time satellite estimates, and a reference climatology is given based on 9 years of ECMWF analyses. Daily 00 UTC analyses of surface winds and sea-level pressure for the entire western Pacific and Indian Ocean are provided to trace the evolution of individual synoptic events.

Description: Several heavy precipitation episodes occurred over Taiwan from August 10 to 13, 1994. Precipitation patterns and characteristics are quite different between the precipitation events that occurred from August 10 and I I and from August 12 and 13. In Part I (Chen et al. 2001), the environmental situation and precipitation characteristics are analyzed using the EC/TOGA data, ground-based radar data, surface rainfall patterns, surface wind data, and upper air soundings. In this study (Part II), the Penn State/NCAR Mesoscale Model (MM5) is used to study the precipitation characteristics of these heavy precipitation events. Various physical processes (schemes) developed at NASA Goddard Space Flight Center (i.e., cloud microphysics scheme, radiative transfer model, and land-soil-vegetation surface model) have recently implemented into the MM5. These physical packages are described in the paper, Two way interactive nested grids are used with horizontal resolutions of 45, 15 and 5 km. The model results indicated that Cloud physics, land surface and radiation processes generally do not change the location (horizontal distribution) of heavy precipitation. The Goddard 3-class ice scheme produced more rainfall than the 2-class scheme. The Goddard multi-broad-band radiative transfer model reduced precipitation compared to a one-broad band (emissivity) radiation model. The Goddard land-soil-vegetation surface model also reduce the rainfall compared to a simple surface model in which the surface temperature is computed from a Surface energy budget following the "force-re store" method. However, model runs including all Goddard physical processes enhanced precipitation significantly for both cases. The results from these runs are in better agreement with observations. Despite improved simulations using different physical schemes, there are still some deficiencies in the model simulations. Some potential problems are discussed. Sensitivity tests (removing either terrain or radiative processes) are performed to identify the physical processes that determine the precipitation patterns and characteristics for heavy rainfall events. These sensitivity tests indicated that terrain can play a major role in determining the exact location for both precipitation events. The terrain can also play a major role in determining the intensity of precipitation for both events. However, it has a large impact on one event but a smaller one on the other. The radiative processes are also important for determining, the precipitation patterns for one case but. not the other. The radiative processes can also effect the total rainfall for both cases to different extents.

Description: Two parallel sets of 10-year long: January 1, 1982 to December 31, 1991, simulations were made with the finite volume General Circulation Model (fvGCM) in which the model integrations were forced with prescribed sea-surface temperature fields (SSTs) available as two separate SST-datasets. One dataset contained naturally varying monthly SSTs for the chosen period, and the oth& had the 12-monthly mean SSTs for the same period. Plots of evaporation, precipitation, and atmosphere-column moisture convergence, binned by l C SST intervals show that except for the tropics, the precipitation is more strongly constrained by large-scale dynamics as opposed to local SST. Binning data by SST naturally provided an ensemble average of data contributed from disparate locations with same SST; such averages could be expected to mitigate all location related influences. However, the plots revealed: i) evaporation, vertical velocity, and precipitation are very robust and remarkably similar for each of the two simulations and even for the data from 1987-ENSO-year simulation; ii) while the evaporation increased monotonically with SST up to about 27 C, the precipitation did not; iii) precipitation correlated much better with the column vertical velocity as opposed to SST suggesting that the influence of dynamical circulation including non-local SSTs is stronger than local-SSTs. The precipitation fields were doubly binned with respect to SST and boundary-layer mass and/or moisture convergence. The analysis discerned the rate of change of precipitation with local SST as a sum of partial derivative of precipitation with local SST plus partial derivative of precipitation with boundary layer moisture convergence multiplied by the rate of change of boundary-layer moisture convergence with SST (see Eqn. 3 of Section 4.5). This analysis is mathematically rigorous as well as provides a quantitative measure of the influence of local SST on the local precipitation. The results were recast to examine the dependence of local rainfall on local SSTs; it was discernible only in the tropics. Our methodology can be used for computing relationship between any forcing function and its effect(s) on a chosen field.

Description: The South China Sea Monsoon Experiment (SCSMEX) was conducted in May-June 1998. One of its major objectives is to better understand the key physical processes for the onset and evolution of the summer monsoon over Southeast Asia and southern China (Lau et al. 2000). Multiple observation platforms (e.g., soundings, Doppler radar, ships, wind seafarers, radiometers, etc.) during SCSMEX provided a first attempt at investigating the detailed characteristics of convection and circulation changes, associated with monsoons over the South China Sea region. SCSMEX also provided precipitation derived from atmospheric budgets (Johnson and Ciesielski 2002) and comparison to those obtained from the Tropical Rainfall Measuring Mission (TRMM). In this paper, a regional climate model and a cloud-resolving model are used to perform multi-day integrations to understand the precipitation processes associated with the summer monsoon over Southeast Asia and southern China. The regional climate model is used to understand the soil - precipitation interaction and feedback associated with a flood event that occurred in and around China's Atlantic River during SCSMEX. Sensitivity tests on various land surface models, cumulus parameterization schemes (CASE), sea surface temperature (SST) variations and midlatitude influences are also performed to understand the processes associated with the onset of the monsoon over the S. China Sea during SCSMEX. Cloud-resolving models (CRMs) use more sophisticated and physically realistic parameterizations of cloud microphysical processes with very fine spatial and temporal resolution. One of the major characteristics of CRMs is an explicit interaction between clouds, radiation and the land/ocean surface. It is for this reason that GEWEX (Global Energy and Water Cycle Experiment) has formed the GCSS (GEWEX Cloud System Study) expressly for the purpose of improving the representation of the moist processes in large-scale models using CRMs. The Goddard Cumulus Ensemble (GCE) model is a CRM and is used to simulate convective systems associated with the onset of the South China Sea monsoon in 1998. The BRUCE model includes the same land surface model, cloud physics, and radiation scheme used in the regional climate model. A comparison between the results from the GCE model and regional climate model is performed.