Convective initiation via outflow boundary interaction with quasi-stationary thermal circulations

Quasi-stationary thermal circulations occur over inhomogeneous surfaces. Large mesoscale temperature gradients are often associated with urban heat islands (UHI), discontinuities between land and water areas, including some inland lakes and rivers, and gradients in soil moisture or solar insolation. The temperature gradients may produce vigorous thermal circulations, and convergence within the warm air. The upward branch of the thermal circulation may initiate convection if it carries parcels to their lifted condensation level (LCL) or their level of free convection (LFC). Sometimes, the thermal circulation alone does not initiate convection, especially when convective inhibition is present or the LCL is high.

In low-shear environments, outflow boundaries may propagate long distances away from their parent storm as a density current. Convergence occurs at the leading edge of the boundary, but this convergence alone may also not be sufficient to initiate convection. However, the convergence and vertical motion which occurs when an outflow boundary interacts with a quasistationary thermal circulation may be sufficient to initiate convection.

In this paper, such an interaction will be examined. On 22 June 2006, a hot air mass with very light winds was in place over the southeastern United States. Scattered convection developed over parts of northern and eastern Alabama during the afternoon, but at Birmingham, Alabama, solar insolation allowed temperatures to rise above 35 C, and a pronounced urban heat island (UHI) developed. Surface observations indicate that the temperature perturbation over the Birmingham area was as large as 4 K, setting up temperature gradients on the order of 0.1 K/km. However, with the hot temperatures, the LCL height was about 2000 m AGL, and no significant convection developed during the afternoon hours around Birmingham.

However, radar observations indicate an outflow boundary, presumably from the convection to the northeast, approached Birmingham around 00 UTC. Once the boundary reached the UHI on the northeast side of Birmingham, convective initiation occurred. The storm developed rapidly, with radar reflectivities reaching 55 dBZ, and the NASA North Alabama Lightning Array indicated vigorous lightning with the storm.

Other such storms may occur when outflow boundaries interact with quasi-stationary thermal circulations associated with inland lakes and rivers, gradients in soil moisture, and gradients in solar insolation. Case studies of these types will be sought.