Synergistic Interactions between an Upper-Level Jet Streak and Diabatic Processes that Influence the Development of a Low-Level Jet and a Secondary Coastal Cyclone

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  • 1 Laboratory for Atmosphere, NASA/Goddard Space Flight Center, Greenbelt, MD 20771
  • | 2 General Sciences Corporation, Laurel, MD 20707
  • | 3 Development Division, National Meteorological Center, Washington, DC 20233
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Abstract

A series of numerical simulations is presented for the February 1979 Presidents' Day cyclone in order to understand more fully the roles played by upper-level jet streaks the oceanic planetary boundary layer (PBL), and latent heat release in the development of a low-level jet (LU) and secondary cyclogenesis along the East Coast of the United States. Mesoscale model simulations with and without sensible and latent heating show that the diabatic processes, along with the jet streak circulation patterns, contribute to the enhancement of the low-level winds and the initial development of the coastal cyclone. However. none of the mechanisms acting alone is sufficient to yield a satisfactory simulation of the LIJ and secondary cyclogenesis. Furthermore, the model-based diagnostic analyses indicate that a synergistic interaction must exist among these processes to account for the substantial increase in the magnitude of the low-level winds and the decrease in the sea level pressure that mark the secondary cyclogenesis for this case.

The following sequence is derived from the model diagnostic study: 1) Temporally increasing divergence along the axis of an upper-tropospheric jet streak located near the crest of an upper-level ridge is associated with the development of an indirect circulation that spans the entire depth of the troposphere and is displaced to the anticyclonic side of the jet. The lower branch of the indirect circulation appears to extend northwestward from the oceanic PBL up sloping isentropic surfaces toward 700 mb over the Appalachian Mountains. 2) Sensible heating and associated moisture flux within the oceanic PBL warm and moisten the lower branch of the indirect circulation, enhancing precipitation rates and latent heat release west of the coastline. 3) The combination of a shallow direct circulation associated with a developing coastal front, sloping lower-tropospheric isentropic surfaces just to the west of the coastline, and latent heat release contributes to a vertical displacement of parcels within the lower branch of the indirect circulation as they cross the coastline. 4) The vertical displacement of the parcels in a baroclinic environment (in which the pressure gradient force changes with height) results in the rapid increase in the magnitude of the ageostrophic wind and associated unbalanced flow. This imbalance contributes to parcel acceleration resulting in the formation of a LLJ in the lower branch of the indirect circulation over a 2 to 4 h period. 5) The increasing wind speed associated with the developing LLJ is, in turn, responsible for an increase in the horizontal mass flux divergence in the entrance region of the LLJ. The increase in the mass flux divergence in the lower troposphere just above the boundary layer makes a significant contribution to the decreasing sea-level pressure that constitutes the initial development phase of the secondary cyclone along the coast.

Abstract

A series of numerical simulations is presented for the February 1979 Presidents' Day cyclone in order to understand more fully the roles played by upper-level jet streaks the oceanic planetary boundary layer (PBL), and latent heat release in the development of a low-level jet (LU) and secondary cyclogenesis along the East Coast of the United States. Mesoscale model simulations with and without sensible and latent heating show that the diabatic processes, along with the jet streak circulation patterns, contribute to the enhancement of the low-level winds and the initial development of the coastal cyclone. However. none of the mechanisms acting alone is sufficient to yield a satisfactory simulation of the LIJ and secondary cyclogenesis. Furthermore, the model-based diagnostic analyses indicate that a synergistic interaction must exist among these processes to account for the substantial increase in the magnitude of the low-level winds and the decrease in the sea level pressure that mark the secondary cyclogenesis for this case.

The following sequence is derived from the model diagnostic study: 1) Temporally increasing divergence along the axis of an upper-tropospheric jet streak located near the crest of an upper-level ridge is associated with the development of an indirect circulation that spans the entire depth of the troposphere and is displaced to the anticyclonic side of the jet. The lower branch of the indirect circulation appears to extend northwestward from the oceanic PBL up sloping isentropic surfaces toward 700 mb over the Appalachian Mountains. 2) Sensible heating and associated moisture flux within the oceanic PBL warm and moisten the lower branch of the indirect circulation, enhancing precipitation rates and latent heat release west of the coastline. 3) The combination of a shallow direct circulation associated with a developing coastal front, sloping lower-tropospheric isentropic surfaces just to the west of the coastline, and latent heat release contributes to a vertical displacement of parcels within the lower branch of the indirect circulation as they cross the coastline. 4) The vertical displacement of the parcels in a baroclinic environment (in which the pressure gradient force changes with height) results in the rapid increase in the magnitude of the ageostrophic wind and associated unbalanced flow. This imbalance contributes to parcel acceleration resulting in the formation of a LLJ in the lower branch of the indirect circulation over a 2 to 4 h period. 5) The increasing wind speed associated with the developing LLJ is, in turn, responsible for an increase in the horizontal mass flux divergence in the entrance region of the LLJ. The increase in the mass flux divergence in the lower troposphere just above the boundary layer makes a significant contribution to the decreasing sea-level pressure that constitutes the initial development phase of the secondary cyclone along the coast.

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