XIAOLEI ZOU and QINGNONG XIAO J. Atmos. Sci., 57,836-860. 報告：黃 小 玲 Studies on the Initialization and Simulation of a Mature Hurricane Using a Variational Bogus Data Assimilation Scheme XIAOLEI ZOU and QINGNONG XIAO J. Atmos. Sci., 57,836-860. 報告：黃　小　玲

Introduction Accurate prediction of hurricane track and intensity change is still a challenging task. The skill of the hurricane intensity forecast varies from case to case and model to model and is probably more difficult than the track forecast. Lack of data over ocean at both the tropical cyclone scale and the large scale in forming the model initial condition. Due to the lack of data, one of the major difficulties in numerical prediction of tropical cyclones is model initialization.

Hurricane Felix (1995) moved north and westward after it reached maximum intensity at 08/12 1800 UTC. Choose a forecast period from 08/16 0000 ~ 08/19 0000 UTC

Experiment design The fine mesh Ci domain moves along the hurricane track. C1(0~18), C2(18~42), C3(42~55), and C4(55~72)

Pc is the central pressure of the hurricane, 963 hPa Fujita’s formula (1952, Geophys. Mag., 23) is used as a basic reference for us to formulate axisymmetric SLP pattern of the bogused surface low. Pc is the central pressure of the hurricane, 963 hPa is the estimation of the SLP at an infinite distance, 1035 hPa is obtained by ship report. r is the radial distance from cyclone center. R0 = 150 km is estimated by NCEP .

ti is carried out at 5-min intervals, R is a circular 2D domain of a 300-km circle centered at the hurricane center at the lowest σ level (σ =0.995), rl is the physical location in the 3D space representing satellite winds available. Hl is a linear interpolation scheme. WP, Wu, and Wv are diagonal weighting matrices and their values are determined empirically. P,u,v, represent SLP, zonal, and meridional wind components. Jb is a simple background term between the model state and the MM5 analysis based on the large-scale NCEP analysis.

CTRL: NCEP 2.5o resolution global analysis. BG: only using observation to MM5 assimilation, without qc and qr BGSAT: same as BG, except observations include WVWVs. BGM: same as BG, more has qc and qr. Initial half-hour rainfall distribution. Satellite-derived water vapor wind vectors (WVWVs)

GOES-8 NCEP 08/16 0000 UTC 08/16 0000 UTC between 150 and 250 hPa below 500 hPa NCEP 08/16 0000 UTC

Initial structures within a mature hurricane generated by BDA 200 hPa_Du 200 hPa_Dv 850 hPa_Du 850 hPa_Dv

200 hPa 200 hPa CTL BG 850 hPa 850 hPa

BG is too high and not realistic 13 oC 36g/kg BG is too high and not realistic

Through forecast model constraint, variational assimilation of a bogused surface low alone is able to recover several structural features that a mature hurricane has: a warm-core vortex with winds swirling in and out of the center in the low and upper troposphere, and a saturated ascent around the eye.

f can be any of the model variables (u, v, T, q, p’,and w) k (=1, 2, …, 10) is number of iterations during the minimization procedure. The first major adjustment in the initial condition during the minimization of JBG comes mainly from the dynamical constraint, and the second major adjustment is associated with the latent heat release due to the heavy precipitation that occurred near the center of the initial vortex.

BG BGSAT: change of divergence field at 200 hPa. BGSAT

Initial half-hour rainfall. BGM Initial half-hour rainfall. BGM Initial half-hour rainfall.

BG (36g/kg) BGM (32g/kg)

Improvements in the prediction of Hurricane Felix

BGSAT CTL 08/16 1850 UTC 08/16 1800 UTC

CTL rainfall during 36 to 42 h BGSAT

Summary The dynamic and thermodynamic structures of the initial vortex obtained by the BDA procedures are examined, and the improvements to the prediction of the hurricane track, the intensity change, and the structural features are demonstrated. BDA scheme can generate the asymmetric structure of the initial vortex . BGSAT is able to generate large amounts of precipitation right from the beginning of model integration,alleviating the spinup problem associated with the traditional hurricane bogusing scheme.