Wind over water

In plain words

Wind slows near the ground because the surface drags on it. Trees, buildings and fields are rough; open water is smooth. So at the standard measuring height of 10 m (33 ft) the wind over the open lake is usually stronger than at an airport on shore. The difference also depends on the air and water temperatures: when the lake is warmer than the air, as it often is in fall, the air over the water is heated from below, mixes, and brings faster wind from higher up down to the surface.

Waves, the crossing rule and water clarity all need the wind over the water, not the wind at the shore. The app therefore uses an over-water wind wherever one exists and scales a land wind up where it has to use one.

Where the over-water wind comes from

1. The National Weather Service gridded forecast for a cell over open water (42.465 N, 82.754 W, mid-lake). It is already an over-water wind and is used as it is.
2. If that cell has no forecast, the forecast for the shore point nearest your launch, multiplied by the land-to-water factor $f = 1.2$.
3. For hours already past (water clarity looks back 48 hours): buoy 45147 in mid-lake when it reports, used as it is; otherwise Selfridge Air National Guard Base (KMTC), multiplied by $f$.

NWS marine products give wind in knots: $1\ \text{kt} = 1.15078\ \text{mph}$. Directions are where the wind comes from, in degrees true; the app groups them into 8 sectors of 45 degrees, N being 337.5-22.5 degrees.

The factor 1.2 is a Starting value: placeholder set in SPEC 5.4 until it is measured. The Shore Protection Manual (1984, chapter 3) gives the ratio of over-water to over-land wind speed for the Great Lakes as a curve that falls as the land wind rises (after Resio and Vincent, 1977), so one constant factor is a simplification.

Source: SPEC 5.4 and 8; lake record (data/seed/lakes.json); API: apps/api/src/lakestate.ts; engine: plan.ts (overWater).

Shore wind in October

Selfridge (KMTC) on the lake's west shore, Oct 3-15 of 2014-2025 (155 days with data): the daily mean wind was 8.2 mph, and on 50% of days it came from the SE, S or SW. Computed now from the seed table with the engine's sectors:

Days past the 7-day forecast use a daily wind of 8 mph for the water temperature projection, Starting value: close to this October mean (Water temperature).

Source: Iowa Environmental Mesonet daily summaries for KMTC (data/seed/weather_daily_selfridge.csv); trip analysis section 2.2.

How the factor will be fitted

Task 1.8, Planned: not done yet: buoy 45147 measures the wind in mid-lake and Selfridge measures it on land. Pairing the two for the same hours over the buoy's seasons gives the ratio directly. The fit will check whether the ratio changes with wind speed, with direction (off the land or along the lake) and with the difference between air and water temperature. The fitted value then replaces 1.2 in the lake record, and the wave disclaimer is revised with the measured error.

Why it matters

Wave height grows faster than wind speed. A 15 mph land wind is 18.0 mph over water with $f = 1.2$ and 19.5 mph with $f = 1.3$; over 30 km of fetch at 4 m depth the modeled $H_s$ rises from 1.90 ft to 2.05 ft, 8% higher. This is one of the error sources in the wave disclaimer.

Constants

Sources

• National Weather Service API, gridded forecasts; NWS marine forecast for zone LCZ460.
• NOAA National Data Buoy Center, station 45147 (Lake St. Clair).
• Iowa Environmental Mesonet, daily and hourly airport observations (KMTC, KDTW).
• U.S. Army Corps of Engineers (1984), Shore Protection Manual, chapter 3; Resio and Vincent (1977), Estimation of winds over the Great Lakes.
• Links: References.