Ensemble-Based Error and Predictability Metrics Associated with Tropical Cyclogenesis. Part II: Wave-Relative Framework

Abstract The predictability of selected variables associated with tropical cyclogenesis is examined using 10-day ECMWF ensemble forecasts for 21 events from the 2010 Atlantic hurricane season. Variables are associated with the strength of the pregenesis disturbance, quantified via circulation and thickness anomaly, and the favorability of the immediate environment via moisture and vertical wind shear. For approximately half of the cases, the predicted strength of the genesis signal is directly related to the predicted favorability of the environment. For the remainder of the cases, predictability is more directly associated with the strength and location of the analyzed disturbance. Some commonalities among the majority of the sample are also observed. Forecast joint distributions demonstrate that 700-hPa relative humidity of less than 60% within 300 km of the circulation center is a limiting factor for genesis. Genesis is also predicted and found to occur in the presence of significant wind shear (~15 m s−1), but almost exclusively when the core and environment of the wave are both very moist. The ensemble also demonstrates the potential to predict error standard deviation of variables averaged within 300- and 1000-km radii about individual tropical waves. Forecasts with greater ensemble standard deviation tend to be, on average, associated with greater mean error, especially for forecasts of less than 7 days. However, model biases, particularly a dry core and weak circulation bias, become pronounced at longer lead times. Overall, these results demonstrate that both the environmental conditions favorable to genesis and the genesis events themselves may be predictable to a week or more

On the existence of the logarithmic surface layer in the inner-core of hurricanes

Q. J. R. Meteorol. Soc., 1394128

A View of Tropical Cyclones from Above: The Tropical Cyclone Intensity Experiment

Tropical cyclone (TC) outflow and its relationship to TC intensity change and structure were investigated in the Office of Naval Research Tropical Cyclone Intensity (TCI) field program during 2015 using dropsondes deployed from the innovative new High-Definition Sounding System (HDSS) and remotely sensed observations from the Hurricane Imaging Radiometer (HIRAD), both on board the NASA WB-57 that flew in the lower stratosphere. Three noteworthy hurricanes were intensively observed with unprecedented horizontal resolution: Joaquin in the Atlantic and Marty and Patricia in the eastern North Pacific. Nearly 800 dropsondes were deployed from the WB-57 flight level of ∼60,000 ft (∼18 km), recording atmospheric conditions from the lower stratosphere to the surface, while HIRAD measured the surface winds in a 50-km-wide swath with a horizontal resolution of 2 km. Dropsonde transects with 4–10-km spacing through the inner cores of Hurricanes Patricia, Joaquin, and Marty depict the large horizontal and vertical gradients in winds and thermodynamic properties. An innovative technique utilizing GPS positions of the HDSS reveals the vortex tilt in detail not possible before. In four TCI flights over Joaquin, systematic measurements of a major hurricane’s outflow layer were made at high spatial resolution for the first time. Dropsondes deployed at 4-km intervals as the WB-57 flew over the center of Hurricane Patricia reveal in unprecedented detail the inner-core structure and upper-tropospheric outflow associated with this historic hurricane. Analyses and numerical modeling studies are in progress to understand and predict the complex factors that influenced Joaquin’s and Patricia’s unusual intensity changes

A Review of Targeted Observations

Abstract It has long been conceived that numerical weather forecasts will benefit from the assimilation of supplementary observations that augment the conventional observational network. In particular, the concept of “targeting” observations in selected regions to improve a forecast of a high-impact weather event had been promoted and tested prior to and during the World Meteorological Organization/World Weather Research Programme’s The Observing System Research and Predictability Experiment (THORPEX) era (2005–14), through field campaigns and assimilation experiments. The end of the THORPEX era provided an appropriate opportunity to review the outcomes and, in particular, the evaluations of the influence of assimilating targeted observations on numerical weather predictions. The main outcome in the extratropics was that the influence of the targeted observations was positive though small (typically an average forecast error reduction of less than 10%). In the tropics, the targeted observations usually improved tropical cyclone track forecasts. Significantly, the results from these and other experiments were found to be sensitive to the sample chosen, the method of verification, and the numerical weather prediction system including the data assimilation scheme and the treatment of observations. Recommendations for the future include innovations to optimize the use of the Global Observing System via better exploitation of routinely available resources together with new instrumentation; expanding into the convective scale and mesoscale; investing quantitative evaluations and improving our understanding of how observations affect forecasts; and assessing the socioeconomic value of improved forecasts. A comprehensive bibliography of approximately 200 papers is provided in the online supplement to this paper

NOAA THORPEX SCIENCE WORKSHOP

  The Ens-DA community considers a flow-dependent anisotropic error covariance matrix based on ensembles to be an appealing, algorithmically simple alternative to either the current three-dimensional variational data assimilation (3D-VAR) techniques, which use the quasi-isotropic stationary matrix, or four-dimensional variational data assimilation (4D-VAR) techniques, which can be expensive to maintain. The newly implemented NAEFS offers a unique opportunity to link research and operations by providing an operational outlet, delivery mechanism, and ultimate performance measure of all types of THORPEX research for the North American community, and eventually, as NAEFS contributes to the development of the future THORPEX Global Interactive Forecast System, at the international level

Using TIGGE Data to Diagnose Initial Perturbations and Their Growth for Tropical Cyclone Ensemble Forecasts

Abstract Ensemble initial perturbations around Typhoon Sinlaku (2008) produced by ECMWF, NCEP, and the Japan Meteorological Agency (JMA) ensembles are compared using The Observing System Research and Predictability Experiment (THORPEX) Interactive Grand Global Ensemble (TIGGE) data, and the dynamical mechanisms of perturbation growth associated with the tropical cyclone (TC) motion are investigated for the ECMWF and NCEP ensembles. In the comparison, it is found that the vertical and horizontal distributions of initial perturbations as well as the amplitude are quite different among the three NWP centers before, during, and after the recurvature of Sinlaku. In addition, it turns out that those variations cause a difference in the TC motion not only at the initial time but also during the subsequent forecast period. The ECMWF ensemble exhibits relatively large perturbation growth, which results from 1) the baroclinic energy conversion in a vortex, 2) the baroclinic energy conversion associated with the midlatitude waves, and 3) the barotropic energy conversion in a vortex. Those features are less distinctive in the NCEP ensemble. A statistical verification shows that the ensemble spread of TC track predictions in NCEP (ECMWF) is larger than ECMWF (NCEP) for 1- (3-) day forecasts on average. It can be inferred that while the ECMWF ensemble starts from a relatively small amplitude of initial perturbations, the growth of the perturbations helps to amplify the ensemble spread of tracks. On the other hand, a relatively large amplitude of initial perturbations seems to play a role in producing the ensemble spread of tracks in the NCEP ensemble

An Examination of Model Track Forecast Errors for Hurricane Ike (2008) in the Gulf of Mexico

Abstract Sources of dynamical model track error for Hurricane Ike (2008) in the Gulf of Mexico are examined. Deterministic and ensemble model output are compared against National Centers for Environmental Prediction (NCEP) Global Forecast System (GFS) analyses to identify potential critical features associated with the motion of Ike and its eventual landfall along the upper Texas coast. Several potential critical features were identified, including the subtropical ridge north of Ike and several synoptic-scale short-wave troughs and ridges over central and western North America, and Tropical Storm Lowell in the eastern North Pacific. Using the NCEP Gridpoint Statistical Interpolation (GSI) data assimilation scheme, the operational GSI analysis from the 0000 UTC 9 September 2008 cycle was modified by perturbing each of these features individually, and then integrating the GFS model using the perturbed initial state. The track of Ike from each of the perturbed runs was compared to the operational GFS and it was found that the greatest improvements to the track forecast were associated with weakening the subtropical ridge north of Ike and strengthening a midlevel short-wave trough over California. A GFS run beginning with an analysis where both of these features were perturbed produced a greater track improvement than either did individually. The results suggest that multiple sources of error exist in the initial states of the operational models, and that the correction of these errors in conjunction with reliable ensemble forecasts would lead to improved forecasts of tropical cyclone tracks and their accompanying uncertainty

Convectively Coupled Kelvin Waves and Tropical Cyclogenesis: Connections through Convection and Moisture

Abstract Recent research has demonstrated a relationship between convectively coupled Kelvin waves (CCKWs) and tropical cyclogenesis, likely due to the influence of CCKWs on the large-scale environment. However, it remains unclear which environmental factors are most important and how they connect to TC genesis processes. Using a 39-year database of African easterly waves (AEWs) to create composites of reanalysis and satellite data, it is shown that genesis may be facilitated by CCKW-driven modifications to convection and moisture. First, stand-alone composites of genesis demonstrate the significant role of environmental preconditioning and convective aggregation. A moist static energy variance budget indicates that convective aggregation during genesis is dominated by feedbacks between convection and longwave radiation. These processes begin over two days prior to genesis, supporting previous observational work. Shifting attention to CCKWs, up to 76% of developing AEWs encounter at least one CCKW in their lifetime. An increase in genesis events following convectively active CCKW phases is found, corroborating earlier studies. A decrease in genesis events following convectively suppressed phases is also identified. Using CCKW-centered composites, we show that the convectively active CCKW phases enhances convection and moisture content in the vicinity of AEWs prior to genesis. Furthermore, enhanced convective activity is the main discriminator between AEW-CCKW interactions that result in genesis versus those that do not. This analysis suggests that CCKWs may influence genesis through environmental preconditioning and radiative-convective feedbacks, among other factors. A secondary finding is that AEW attributes as far east as Central Africa may be predictive of downstream genesis