Topic Chair: Prof. William M. Frank (USA)
Department of Meteorology
Penn State University
University Park, PA 16802

Email: frank@ems.psu.edu
Fax: 814-865-3663

2.0.1 Introduction

Topic 2 focuses on particular problems related to the landfall of tropical cyclones. While many of the theoretical and forecasting problems discussed in Topic 1 are equally applicable to landfalling storms, the emphasis here is primarily on problems associated uniquely with landfall. These problems are reflected by the names of Working Groups (WG): 2.1 Observing and Forecasting Rainfall, 2.3 Hydrological Models, 2.4 Coastal and Inland Wind Structure Changes, and 2.5 Storm Surge. WG 2.2 (Data Assimilation and Numerical Prediction of Precipitation) is closely related to WG 1.7 as part of the general problem of numerical prediction of tropical cyclones.

This overview report is intended to introduce some of the main issues raised in the following rapporteur reports. It also summarizes briefly the recommendations made by some of the working groups for consideration by the full membership of the workshop. The detailed recommendations and their supporting arguments are presented in more detail in the rapporteur reports that follow.

2.0.2 Issues and Recommendations Observing and Forecasting Rainfall

Loss of life and damage caused by tropical cyclone rainfall has been a growing area of research during the past few years. As discussed in the report of WG 2.1, this has been an area of major concern for many years in China. Other regions are increasingly focusing on this problem as well.

The report of WG 2.1 discusses how tropical cyclone rainfall is affected by different types of synoptic flow patterns, interactions with topography, and changes in the storm structure and intensity. It includes discussion of the physical processes by which these phenomena affect the rainfall.

Forecasting tropical cyclone rainfall can be broken down conceptually into two problems. One problem is forecasting the pattern and rate of rainfall produced by the storm, while the second is forecasting the storm’s track. For a given rainfall pattern, the total amount of rainfall experienced at any one point will depend on the path followed by the storm and the speed with which it follows that path. The pattern and track problems are not fully independent, since the storm’s track influences the rainfall pattern by determining just how it will interact with topography and environmental flow features.

The WG 2.1 report discusses the use of mesoscale numerical models in forecasting tropical cyclone rainfall. It also surveys the use of statistical and empirical forecasting techniques. The report summarizes the use of satellite, radar, rain gages, and automatic weather stations to measure rainfall and the roles of these measurement systems in short-range rainfall forecasting. Data Assimilation and Numerical Prediction of Precipitation

WG 2.2 discusses a range of topics related to the numerical prediction of rainfall in landfalling tropical cyclones. These include reviews of the current status of data assimilation and model physics. They place particular emphasis on the assimilation of data that define the moisture and motion fields. The group also summarizes recent progress in satellite and radar observations and the assimilation of these types of data. A major theme of the report is the need to take steps to deal with the explosive growth of remotely sensed observations. This will involve both developing methods of handling the data and finding better ways to assimilate the observations into models. This group has prepared a group of recommendations that are summarized here as follows:

  1. Hydrological Models of Precipitation

    WG 2.3 examines the operational models used to forecast flooding caused by tropical cyclones. This problem includes both estimating the rainfall that will occur during the cyclone landfall and using hydrological models to estimate the resulting flooding. The group reviews the types of models used in different regions and summarizes their characteristics. They also discuss verification procedures and problems.

    The group concludes that there is still much uncertainty in forecasting flooding from tropical cyclones, and they group the problems into three major areas. The first area concerns the adequacy of the flood model itself. Tropical cyclone rainfall is intense and is often very localized, and these factors tend to make the forecasts difficult. There is usually little information available in real time to assess how well the model is performing. The second problem area is inadequacy of model calibration. This is particularly difficult in smaller basins with complex terrain and strong local gradients of rainfall. The third major problem area is comprised of the constraints on real-time operations. These constraints include problems in receiving adequate data in a timely fashion, changes in the river characteristics during the event, inadequate documentation of the initial catchment state, and errors in predictions of the ocean flooding.

    The WG 2.3 report contains several recommendations, which are summarized briefly below:

    Recommendations related to meteorological inputs:

    • Improve real-time forecasts of the flood-producing potential of approaching tropical cyclones.
    • Improve forecasts of the landfall location.
    • Develop improved quantitative precipitation forecasts (QPFs) for tropical cyclones.
    • Improve the quality and real-time availability of radar rainfall estimates.

    Recommendations concerning hydrology:

    • Incorporate continuous soil moisture accounting models.
    • Develop digital representation of hydrological model sub-areas.
    • Incorporate radar rainfall estimates into short-range flood forecasts.
    • Develop improved physics-based distributed models.
    • Develop strategies to improve and standardize model calibration.
    • Integrate advanced hydrological models with the use of GIS.
    • Develop models capable of forecasting landslide and debris flow conditions. Coastal and Inland Wind Structure Changes

WG 2.4 analyzes the dramatic changes that occur in tropical cyclones as their core regions begin to move over land. They note that the surface generally becomes rougher and more heterogeneous, the surface fluxes of sensible heat and moisture are dramatically reduced, the boundary layer tends to cool and stabilize, a variety of mesoscale circulation features are often created, and orographic effects may greatly increase local rainfall and produce locally intense winds.

The group also discusses a range of issues concerned with observations of landfalling tropical cyclones. These include a review of the newer remote sensing techniques for monitoring the storms, problems with in-situ wind observations of extremely high winds over land, and the problems that beset the performance of boundary layer models when applied under these extreme conditions.

The WG 2.4 report makes several recommendations for future research to improve understanding and our ability to forecast winds as tropical cyclones move inland. These are summarized below: Storm Surge

Storm surge is the net elevation of sea level that occurs along the coast as a tropical cyclone makes landfall or moves along a track that is close to the shore. Although the manner in which a tropical cyclone causes death and destruction varies greatly from one storm to the next, the storm surge has been responsible for the greatest losses of life. Storm surge theory is relatively well advanced compared to many aspects of tropical cyclone forecasting. When the storm track and wind distributions are well forecast and the bathymetry is well known, surge models can produce impressively accurate forecasts of the pattern of flooding. However, there are still a great many problems related to the forecasting of storm surge and the measures that need to be taken to minimize its effects.

The report of WG 2.5 focuses primarily on recent developments in storm surge modeling and methods of using them more effectively. It reviews the basic techniques used in the models and gives an overview of the operational models that are used in different regions. It also discusses recent work on evaluating surge models using data from sensors mounted on aircraft and on satellites.

WG 2.5 make several recommendations for future work on the subject of storm surge, and these are summarized briefly below: