Inland penetration of sea‐breeze fronts

Abstract The advance inland of the sea‐breeze past Lasham, 45 km from the south coast of England, has been measured over a period of twelve years. In this time 76 sea‐breeze fronts passed Lasham, 36 passed Reading and 12 reached Harwell, 85km from the coast. the rate of advance from Lasham to Harwell in calm or light offshore winds was close to 3 ms −1 , and the latter station was passed after sunset at a mean time of 2105 GMT. With onshore winds fronts only appear 30 or 40 km inland, and some fronts may be modified by effects due to the east coast. In some cases detailed measurements were made using pilot balloons, an instrumented motor‐glider and radar. the fronts were shown to have raised heads about twice the depth of the following flow and within the heads dry patches occur. Variable secondary flows are inferred and comparison with water‐tank models shows that several dynamical features in the tank and in the atmosphere are similar. Measurements support the picture of the development of the front during the day as a gravity current, and theoretical rates of advance are deduced. the observed acceleration of the front in the late afternoon is explained in terms of the increased temperature contrast at the front due to the decrease in solar heating of the sea‐air. Shortly before sunset, a sea‐breeze vortex may be detached from the rest of the flow. In a case study, it is shown that friction at the upper surface is at least five times that at the ground, and heat and humidity budgets are given for the detached vortex.