What I've Always Wanted to Know, but Have Been Afraid to
Ask, About Tropical Cyclone Intensity
Stephen J. Lord
NCEP, Camp Springs, MD
Current thought is that the primary controlling factors on hurricane intensity are 1) the underlying
sea surface temperature (SST), 2) planetary boundary layer (PBL) structure and fluxes, 3)
interaction with mid-latitude, synoptic-scale phenomena such as troughs and vertical shear, and
4) internal structural processes such as concentric eye wall cycles. Detailed discussion of all four
of these factors is given by the wide variety of papers presented as part of this Symposium.
Lacking, however, is a unified discussion of their relative importance, of their ability to account
completely (individually or combined) for intensity changes in the wide variety of synoptic
scenarios Nature presents to us each hurricane season, and most of all, the demonstrated ability to
predict intensity with sufficient skill (with respect to climatology and persistence) in an
operational setting. Such is the fledgling state of our discipline at this time.
It is, therefore, appropriate to ask some questions to, hopefully, guide our progress toward a
significantly improved ability to forecast hurricane intensity. I am somewhat afraid to ask them
(hence the title of this talk) as the answers may be discouraging, but nevertheless they should be
raised so that they can be seen perhaps as needless worrying by those who progress past them in
solving the intensity forecast problem. Generic examples to illustrate the following will be
shown in the Conference presentation.
- Given the influence of SST and mid-latitude troughs, are
track forecasts sufficiently accurate
to enable skillful intensity forecasts? How much better can track forecasts become?
- Are current estimates of analyzed SST in the storm
environment sufficiently accurate and of sufficient resolution?
- While vastly improved, initialization of the hurricane
vortex for numerical forecast models
remains a difficult problem. If a coupled atmosphere-ocean forecast model is used, initializing
one without the other may introduce significant biases. What level of detail is necessary to
provide skillful and unbiased forecasts? Will a major problem be the ability to obtain the correct
balance in air-sea and convective fluxes as the forecast proceeds? This problem is quite
analogous to the coupled forecasting of El Nino: the atmospheric model cannot respond if the
SST forecast is poor; the SST forecast may be poor because the atmospheric forecast model does
not provide sufficiently accurate wind stress.
- For numerical models, increased horizonal and
vertical resolution may lead to decreased bias, particularly for more intense storms. Given the
inferred influence of internal structural processes on intensity, how high must resolution be to
capture them? What additional physics must be parameterized at that resolution? Can
operational NWP centers afford such a resolution?
- How well is the PBL represented in numerical forecast
models? The state of the PBL is
determined by ocean surface fluxes, convective updraft and downdraft fluxes, and turbulent
entrainment in regions that are not dominated by convective effects. Most parameterizations of
these physical processes have been evaluated under weaker wind regimes than in a hurricane and
few observations exist for validation over the ocean. Are there large compensating errors in
models that currently exist?
- What future role will ensemble forecasting play in intensity
forecasting? What uncertainties in
track forecast are generated by uncertainties in forecasting the storm environment? For weak
storms, intensification may be a probabilistic process defined by the internal structure. In
general, what role do "triggering" processes, such as strong convective events, play in intensity
forecasting? Can this role be demonstrated reliably with forecast models? If demonstrated to be
skillful, can we afford ensemble forecasting for intensity?