Impact of wind bogus and cloud‐ and rain‐affected SSM/I data on tropical cyclone analyses and forecasts

Tellus A: Dynamic Meteorology and Oceanography

2016

A B S T R A C T Microwave radiances from the humidity sounder SAPHIR at 183 GHz onboard the MEGHA-TROPIQUES satellite are simulated in cloudy and rainy conditions (all-sky radiances) using short-range forecasts from the operational limited area model ALADIN-Re´union over the Indian Ocean. The simulation of SAPHIR radiances uses cloud and precipitation profiles from the moist physical parameterizations of ALADINRe´union describing deep convection and large-scale precipitation, that are coupled to the fast radiative transfer model RTTOV-SCATT accounting for scattering by hydrometeors. Sensitivity studies show that within cloud systems the SAPHIR sounder is particularly sensitive to solid precipitating hydrometeors. An optimal choice of scattering properties for snow particles is undertaken by a systematic comparison of simulated and observed SAPHIR radiances over a 1-month period in February 2012. This choice has required defining a criterion for rain occurrence from SAPHIR radiances that has been calibrated with the TMPA 3B40RT surface precipitation rate product. Finally, a retrieval technique based on Bayesian inversion is proposed to derive atmospheric profiles within clouds to be assimilated in the ALADIN 3D-Var system in a forthcoming study. The retrieved profiles are evaluated through the simulation of radiances from the microwave imager MADRAS that also flew on MEGHA-TROPIQUES.

Section: Physical Parameterizationsmentioning

confidence: 99%

Towards the assimilation of all-sky microwave radiances from the SAPHIR humidity sounder in a limited area NWP model over tropical regions

Mahfouf¹,

Guerbette²,

Tellus A: Dynamic Meteorology and Oceanography

2016

“…It provided reliable information on the track and the vortex response to the external forcing as it captured many observed characteristics of the storm and key aspects of the interaction, such as the tongue of dry air that gradually encircled the western side of TC Dora in association with the stratospheric intrusion. This 60-h forecast, initialized at t 0 5 0600 UTC 31 January, was carried out using the limited-area atmospheric model AladinRéunion (Montroty et al 2008) in its 2011 operational configuration (hydrostatic, 70 vertical levels, 8-km horizontal resolution, and a fixed domain shown in Fig. 3).…”

Section: B the Reference Experimentsmentioning

confidence: 99%

“…To obtain a realistic vortex structure and position at the base time of the forecast, the Aladin's threedimensional variational data assimilation (3D-Var) of cyclone wind bogus (Montroty et al 2008) was employed. Synthetic wind observations deduced from the Holland (1980) analytical model, plus the mean sea level pressure at the TC center, were produced from the surface to 500 hPa.…”

Section: B the Reference Experimentsmentioning

confidence: 99%

“…These pseudoobservations were assimilated at 0600 UTC 31 January 2007 in an Aladin's first-guess 6-h forecast initialized with an interpolation of the 25-km resolution IFS analysis at 0000 UTC 31 January 2007. One of the advantages of the Aladin-Réunion cyclone initialization is that the 3D-Var introduces realistic balance between the thermodynamic and dynamic fields (wind, temperature, humidity) and thus helps to equilibrate the vortex in the analyses and in the very first forecast hours (Montroty et al 2008). …”

Section: B the Reference Experimentsmentioning

confidence: 99%

See 1 more Smart Citation

On the Sensitivity of Tropical Cyclone Intensification under Upper-Level Trough Forcing

Leroux

Roux

2016

Monthly Weather Review

This study is part of the efforts undertaken to resolve the ''bad trough/good trough'' issue for tropical cyclone (TC) intensity changes and to improve the prediction of these challenging events. Sensitivity experiments are run at 8-km resolution with vortex bogusing to extend the previous analysis of a real case of TC-trough interaction (Dora in 2007). The initial position and intensity of the TC are modified, leaving the trough unchanged to describe a realistic environment. Simulations are designed to analyze the sensitivity of TC prediction to both the variety of TC-trough configurations and the current uncertainty in model analysis of TC intensity and position.Results show that TC intensification under upper-level forcing is greater for stronger vortices. The timing and geometry of the interaction between the two cyclonic potential vorticity anomalies associated with the cutoff low and the TC also play a major role in storm intensification. The intensification rate increases when the TC (initially located 128 northwest of the trough) is displaced 18 closer. By allowing a gradual deformation and equatorward tilting of the trough, both scenarios foster an extended ''inflow channel'' of cyclonic vorticity at midlevels toward the vortex inner core. Conversely, unfavorable interaction is found for vortices displaced 38 or 48 east or northeast. Variations in environmental forcing relative to the reference simulation illustrate that the relationship between intensity change and the 850-200-hPa wind shear is not systematic and that the 200-hPa divergence, 335-350-K mean potential vorticity, or 200-hPa relative eddy momentum fluxes may be better predictors of TC intensification during TC-trough interactions.

“…Bogussing is such a technique: pseudo-observations deduced from an observed coherent structure are assimilated as conventional observations (wind, humidity, etc.). The assumptions that link the image to the pseudo-observation are often simplified (Heming et al, 1995;Michel, 2011;Montroty et al, 2008), and such methods are not fully objective. Michel (2011) concluded about the severe limitations of bogussing.…”

Section: Introductionmentioning

confidence: 99%

A variational formulation for translation and assimilation of coherent structures

Nonlin. Processes Geophys.

2013

Abstract. The assimilation of observations from teledetected images in geophysical models requires one to develop algorithms that would account for the existence of coherent structures. In the context of variational data assimilation, a method is proposed to allow the background to be translated so as to fit structure positions deduced from images. Translation occurs as a first step before assimilating all the observations using a classical assimilation procedure with specific covariances for the translated background. A simple validation is proposed using a dynamical system based on the onedimensional complex Ginzburg-Landau equation in a regime prone to phase and amplitude errors. Assimilation of observations after background translation leads to better scores and a better representation of extremas than the method without translation.