Surface and Atmospheric Boundary Layer Responses to Diurnal Variations of Sea Surface Temperature in an NWP Model

An atmospheric model is coupled to a sea surface temperature algorithm that calculates the diurnal variability in order to understand the responses to near surface winds and boundary layer temperature, pressure, and moisture in the north Central Atlantic. 7-day case study simulations with diurnally varying sea surface temperatures and daily-constant sea surface temperatures are compared. With the inclusion of diurnal heating during the day, the local heat fluxes are increased and the surface pressure decreases. The extent of the surface-induced heating to the atmosphere is not necessarily restricted to the boundary layer depending on the atmospheric stability and the interaction with synoptic scale weather. The diurnal variations in sea surface temperature also induce positive and negative mean wind speed anomalies on the order of 0.5 m/s over the 7-day period. Hourly changes to the wind field, however, can exceed 3 m/s particularly where modifications to the regional weather occur. A comparison to geostationary data show that the sea surface temperature algorithm overestimates the warming on average, but this overestimation could be exaggerated from several factors including smoothing of the geostationary data. We examine the spatial variability and data distribution of the wind field anomalies in response to the diurnal sea surface temperature gradients at hourly and daily time scales. The changes to the wind field on the first day of diurnal warming exhibit a linear, but temporally lagged response to the direction in which flow crosses the diurnal warming gradient. This trend was also observed on day 2, but does not exist for subsequent days after. It is thought that the larger amplitude responses of interaction with the synoptic scale (secondary feedbacks) dominate the distribution for areas in which we expect wind-sea surface temperature coupling. The surface pressure gradient and Coriolis are deemed the dominant forcing processes in the model that generate the initial wind-diurnal sea surface temperature coupling response on the first day of the simulation. In order to understand the importance of interactive feedbacks of the wind and the diurnal cycle of sea surface temperatures, we compared the duration and amplitude of the diurnal warming produced in a one-way coupled simulation. The one-way coupled simulation allows the diurnal variations in sea surface temperature to influence the surface fluxes at the concurrent time step, but uses winds from a non-diurnally modified SST to calculate the diurnal warming of the sea surface temperature. As compared to two-way coupled simulations, the mean amplitude of the diurnal warming is larger and the duration is longer. This is one way to demonstrate that the integrated hourly feedbacks to the diurnal variability of sea surface temperature are important in producing an accurate duration and amplitude of diurnal heating over the day.