A high-resolution modeling study of convective initiation on 19 June 2002 during IHOP

The 19 June 2002 convective initiation (CI) case is simulated numerically using the Advanced Regional Prediction System (ARPS) with a 1km horizontal resolution grid nested inside a 3km domain. The case involves three distinct CIs accompanying a cold front-dryline interaction. Standard as well as special upper-air and surface observations collected during the International H2O Project (IHOP_2002) field experiment are simulated into the initial condition. The control experiment initiating from an ARPS3DVAR analysis only at 1800 UTC captured the three CIs rather well. The first convection started ~20 min earlier while the other two ~30 min later than the observations, with location errors ranging between 5 and 80 km.

The general deepening of the well-mixed moist boundary layer is clearly essential to the CIs. Mesoscale convergence associated with the confluent flow around the dryline and/or the cold front produces obvious uplift of the moisture layer and provides favorable conditions for the CI there. Localized forcing is essential to the CIs due to the horizontal convective rolls (HCRs) on both sides of the primary dryline/front convergence boundary (PDCB/PFCB) and their interactions with the stronger southeast flow.

The first focused convective cell A-I initiates near the PDCB with a relative dry background of q_v ~ 8 g kg^-1 over southeast Colorado. The CI location settles where the PDCB is driven as a northwest bump by the stronger southeast flow and interacts with an intense misocyclone, the middle level HCR on the southeast side of the PDCB evidently strengthens the strong updraft associated with the CI.

The second focused cell B-I also generates near the PDCB but with a moister condition of q_v ~ 12 g kg^-1 over northwest Kansan. Stronger HCRs are well organized to the west of the PDCB and extend up to ~4 km MSL. Cell B-I initiates over the middle level moisture convergence band (MCB) on the northwest side of the PDCB, which has a phase shift comparing with the low-level ones due to the circulations of the strong HCRs. A middle level HCR is still found on the southeast side of the PDCB within 2-5 km MSL enhancing the updraft flow to trigger the convective cell.

Instead of the PDCB, the third focused cell C-I initiates near PFCB with a further moister condition of q_v ~ 15 g kg^-1 over south-central Nebraska. Strong misocyclones and zonal middle level MCB are found embedding in the front circulation about 1 km above ground. The cell initiates along the middle level MCB where the circulations of two misocyclones around induce local convergence maximum. This location is also just above the strong updraft near the PFCB due to the ahead-front southeast flow interacting with the rear-front HCR.

The trajectory analysis demonstrates the source particles for the CIs are mainly embedded in the stronger south flow east to the PDCB/PFCB. The centers of misocyclones or maximum vorticities in this case usually do not collocate with the maximum localized forcing or the CI locations, but their can enhance the local convergence maximum and modulate the preferred locations for CI.