Extended Abstract (172K)4B.4
Convective momentum transport observed during the TOGA COARE IOP: Implications for parameterization
Wen-wen Tung, Univ. of California, Los Angeles, CA; and M. Yanai
The effects of convective momentum transport (CMT) on the large-scale motions over the western Pacific warm pool are examined using the momentum budget residual, R=(X,Y), deduced from the objectively analyzed in-situ observations during the TOGA COARE Intensive Observing Period (IOP, November 1992-February 1993). The time series of R exhibit multi-scale temporal behavior, showing modulations by various tropical disturbances, including the Madden-Julian oscillation (MJO). Similarity has been found among the power spectra of X, Y, and ITBB (a satellite-observed index of deep convective activity). Using wavelet transform and multi-scale decomposition, tropical disturbances with periods larger than 1 day are found to evolve in phase generally between the |R| and ITBB time series. During the convective phase of MJO, disturbances with shorter periods in both |R| and ITBB are also enhanced. These results suggest a link between deep convection and the acceleration/deceleration of the large-scale horizontal motion through CMT.
IOP-mean deceleration (i.e., cumulus friction) and downscale kinetic energy (K) transfer occupy a deep tropospheric layer. Also, the average momentum transport is "downgradient" when examining the product between convective momentum flux and large-scale vertical wind shear. However, CMT during the IOP is not only highly variant in time but also case-dependent. Downscale K transfer occurs in about 60-65% of time in the lower troposphere below 500 hPa, while upscale and downscale K transfers occur with nearly equal frequency in the upper troposphere between 350-200 hPa. Distinctly different frequency distributions of K transfers near the surface, middle troposphere, and near the tropopause suggest the existence of different energy cascading regimes associated with different cloud types. Further classification of K transfers by physical parameters such as the vertical shear of the basic flow and CAPE are being attempted.
Furthermore, the dependence of the direction of CMT on mesoscale convective organizations documented in many previous observations is found to be detectable in the 2.5 degree x 2.5 degree analysis (grid-size of GCMs). Couplets of vorticity and vorticity budget residual appear in the upper troposphere over nonlinear mesoscale convective systems (MCSs). Upscale K transfer is found in the line-normal direction of a linear MCS (squall line). During the westerly wind phase of the MJO, convection seems to play dual roles. First, as the westerlies are initiated in the lower troposphere, CMT is upgradient and helps maintain mid-level easterly shear. The upscale K transfer may help trigger the westerly wind burst (WWB). Second, in the later stage with strong lower-to-mid-level westerlies, CMT is mostly downgradient and reduces the mid-level zonal wind shear. The first role appears to be played by shallower convection and the second by very deep convection. Comparisons of the observed CMT with the Wu and Yanai (1994) parameterization are in progress.
Session 4B, Convection III (Parallel with Sessions 4A, 4C, & 4D)
Tuesday, 30 April 2002, 8:30 AM-10:30 AM
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