Storm Surge - MDL
What is Storm Surge?
Storm Surge is a change in normal tide levels caused by winds and atmospheric pressure associated with a storm. It's primary components are: pressure setup (water level change due to lower atmospheric pressure), geostrophic adjustment (adjustments due to Coriolis forces), and wind setup (water level change due to wind). Additionally, waves riding on top of the surge and tide, increase the water level via wave setup and wave runup.
|Conceptually, winds from a storm create a current in the water. The winds push the surface water forward, it sinks, flows under the storm and comes back up behind the storm only to be pushed forward again.|
|Unfortunately the reverse flow occurs at roughly 200 to 400 feet deep, so the cycle gets disrupted as the storm moves over shallower water. The result is the water piles up in front of the storm and eventually splashes and sloshes overland.|
Storm intensity, forward speed, size, central pressure, shape, and angle of approach to the coast all determine how strong the surge will be. The shape of the bays and estuaries and slope of the ocean bottom also play a large role. For instance, given the same storm, coastal areas adjacent to a steeply sloping ocean bottom will experience less storm surge than areas adjacent to shallow slopes.
Models and Guidance
MDL develops and maintains a suite of models to produce storm surge guidance for both tropical and extra-tropical storms. The key difference is that tropical storms (e.g., hurricanes), are more readily modeled by a parametric wind model, whereas extra-tropical storms (e.g., winter storms), are broader with a more gradual wind gradient so can use a gridded wind field.
Deterministic: For tropical (e.g. hurricane) storm surge, MDL developed the Sea Lake and Overland Surges from Hurricanes (SLOSH) model, which uses an internal parametric wind model to provide surface forcing for the finite difference equations that solve the Navier-Stokes equations of motion. For extra-tropical storm surge, MDL developed the Extra-Tropical Storm Surge (ETSS) model which replaced the SLOSH model's parametric wind model with wind and pressure grids from the Global Forecast System (GFS) for surface forcing.
Real-Time Ensemble: Since the primary uncertainty in storm surge guidance is the uncertainty of the wind, MDL has created two real-time ensemble versions. For the tropical version, MDL created the Probabilistic tropical-cyclone storm Surge (P-Surge) model by using the National Hurricane Center's (NHC) official forecast and its 5-year average errors to derive an ensemble of SLOSH model runs. For the extra-tropical version, MDL created the Probabilistic Extra-Tropical Storm Surge (P-ETSS) model by using the wind and pressure grids from the US's 31 member Global Ensemble Forecast System (GEFS) and Canada's 21 member Global Ensemble Prediction System (GEPS).
Climatological Ensemble: Further, since hurricane evacuations are based on the potential for hurricane storm surge, MDL developed the concept of estimating that potential by (a) establishing an ensemble of all the ways a particular Saffir-Simpson category storm can make landfall, and (b) taking the maximum of each ensemble member's SLOSH result. The maximums are referred to as Maximum Envelopes of Water (MEOWs) when the ensemble members are grouped by common direction, forward speed and category. They are referred to as Maximum of MEOWs (MOMs) when the ensemble members are grouped by just common category. The MOM's form the risk portion of the National Hurricane Program's evacuation plans.