Cases-97: Late-morning Warming and Moistening of the Convective Boundary Layer over the Walnut River Watershed

Aircraft, radiosonde, surface-flux, and boundary-layer wind profiler data from the Cooperative Atmosphere Surface Exchange Study’s 1997 field project, CASES-97, are combined with synoptic data to study the evolution of the vertically-averaged mixed-layer potential temperature [ ] and mixing-ratio [Q] on three nearly-cloudless days from 1000 CST to 1200 CST (local noon is approximately 1230 CST). This was achieved through examination of the terms in the time-tendency (‘budget’) equations for [ ] and [Q]. We estimate three of the terms – local time rate of change, vertical flux divergence, and horizontal advection. For the [Q]-budget, vertical flux divergence usually dominates, but horizontal advection is significant on one of the three days. The [Q]-budget balances for two of the three days to within the large experimental error. For the -budget, vertical flux divergence accounts for most of the morning warming, with horizontal advection of secondary importance. The residual in the -budget has the same sign for all three days, indicating that not all the heating is accounted for. We can balance the [ ]-budgets to within experimental error on two of the three days by correcting the vertical-flux divergence for apparent low biases in the flux measurements of one of the aircraft and in the surface fluxes, and accounting for direct heating of the mixed layer by radiative flux divergence allowing for the effects of carbonaceous aerosols. The [ ]-budget with these corrections also balances on the third day if horizontal gradients from synoptic maps are used to estimate the horizontal advection. However, the corrected budget for this day does not balance if the horizontal gradient in the advection term is estimated using CASES-97 aircraft and radiosondes; we suggest that persistent mesoscale circulations led to an overestimate of the horizontal gradient and hence horizontal advection.