The observed frequency shifts of the high-degree solar fundamental mode are explained using a simple geometrical optics approximation .
The predicted fractional frequency shift is -0.4375 < V ^ { 2 } > / v _ { g } ^ { 2 } , where < V ^ { 2 } > is the mean squared velocity of the convection flow at the surface of the Sun , v _ { g } is the group velocity of the mode .
The data converge to the predicted behavior at very high degrees , with < V ^ { 2 } > ^ { 1 / 2 } = 1.3 ~ { } { km / s } .