Numerical Simulation of an Explosively Deepening Cyclone in the Eastern Pacific

Ying-Hwa Kuo National Center for Atmospheric Research, Boulder, Colorado

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Richard J. Reed Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

A series of nine experiments were conducted using a version of the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. Objectives were: 1) to test the ability of a high resolution limited-area model to simulate an extraordinary cyclogenesis that occurred in the eastern Pacific in November 1982; 2) to examine the effects of various physical processes on the storm development and 3) to determine the reasons for the failure of the operational Limited-area Fine-mesh Model (LFM) to forecast the event. Six of the experiments employed a 40 km grid and three employed an 80 km grid. Initial data for seven of the experiments consist of fields interpolated from the National Meteorological Center (NMC) operational analysis supplemented by subjective soundings created by Reed and Albright. The supplementary data were withheld in two of the experiments. Principal findings are:

1) The control experiment, which utilized the supplementary dataset, a 40 km grid and an explicit moisture scheme, simulated a mejor cyclone with a central pressure of 969 mb and a deepening rate of 31 mb per 24 h (observed values were 950 mb and 48 mb per 24 h). The path of the cyclone was well predicted, as were several features of the storm that could be verified by satellite and aircraft observations.

2) A vertical cross section taken immediately ahead of the storm center at the time of rapid deepening revealed a symmetrically neutral or slightly unstable state in and near the warm frontal zone and a narrow, sloping sheet of rapidly ascending air (w > 50 cm s−1) at the frontal boundary. Low-level convergence exceeded 1.0×10−4 s−1 as the air approached the zone. Vorticity grew from near zero to 6–7 f in only a few hours.

3) Moist processes were essential to the rapid development. Dry simulations produced deepenings of only 13–15 mb in the 24 hour period, implying that roughly half the intensification in the control experiment can be ascribed to dry baroclinicity and half to latent beat release and its interactions with baroclinicity.

4) Surface energy fluxes had no significant impact on the development during the 24 hour period of rapid deepening.

5) An experiment with parameterized convective and nonconvective precipitation yielded essentially the same final pressure as the control experiment. However, the time of most rapid deepening was delayed in the simulation with parameterized convection. The delay was related to differences in the vertical heating profile in the two experiments.

6) Reduction of the grid size from 80 km to 40 km had only a minor effect on the central pressure, suggesting that further reduction would not eliminate the 19 mb error in the predicted central pressure.

7) A considerably weaker cyclone (982 mb vs 971 mb central pressure) resulted when the supplementary data were withheld in an experiment conducted on the 80 km grid.

8) An experiment designed to match most closely the conditions of the LFM forecast yielded the weakest development of all. It is speculated that the absence of development in the LFM forecast stemmed from short-comings of the initial analysis.

9) A possible cause of the failure of the present experiments to fully capture the storm intensity is the deficiency of middle and upper-level observations, and the attendant uncertainties in the upper-level analyses, in the prestorm period.

Abstract

A series of nine experiments were conducted using a version of the Pennsylvania State University/National Center for Atmospheric Research (PSU/NCAR) mesoscale model. Objectives were: 1) to test the ability of a high resolution limited-area model to simulate an extraordinary cyclogenesis that occurred in the eastern Pacific in November 1982; 2) to examine the effects of various physical processes on the storm development and 3) to determine the reasons for the failure of the operational Limited-area Fine-mesh Model (LFM) to forecast the event. Six of the experiments employed a 40 km grid and three employed an 80 km grid. Initial data for seven of the experiments consist of fields interpolated from the National Meteorological Center (NMC) operational analysis supplemented by subjective soundings created by Reed and Albright. The supplementary data were withheld in two of the experiments. Principal findings are:

1) The control experiment, which utilized the supplementary dataset, a 40 km grid and an explicit moisture scheme, simulated a mejor cyclone with a central pressure of 969 mb and a deepening rate of 31 mb per 24 h (observed values were 950 mb and 48 mb per 24 h). The path of the cyclone was well predicted, as were several features of the storm that could be verified by satellite and aircraft observations.

2) A vertical cross section taken immediately ahead of the storm center at the time of rapid deepening revealed a symmetrically neutral or slightly unstable state in and near the warm frontal zone and a narrow, sloping sheet of rapidly ascending air (w > 50 cm s−1) at the frontal boundary. Low-level convergence exceeded 1.0×10−4 s−1 as the air approached the zone. Vorticity grew from near zero to 6–7 f in only a few hours.

3) Moist processes were essential to the rapid development. Dry simulations produced deepenings of only 13–15 mb in the 24 hour period, implying that roughly half the intensification in the control experiment can be ascribed to dry baroclinicity and half to latent beat release and its interactions with baroclinicity.

4) Surface energy fluxes had no significant impact on the development during the 24 hour period of rapid deepening.

5) An experiment with parameterized convective and nonconvective precipitation yielded essentially the same final pressure as the control experiment. However, the time of most rapid deepening was delayed in the simulation with parameterized convection. The delay was related to differences in the vertical heating profile in the two experiments.

6) Reduction of the grid size from 80 km to 40 km had only a minor effect on the central pressure, suggesting that further reduction would not eliminate the 19 mb error in the predicted central pressure.

7) A considerably weaker cyclone (982 mb vs 971 mb central pressure) resulted when the supplementary data were withheld in an experiment conducted on the 80 km grid.

8) An experiment designed to match most closely the conditions of the LFM forecast yielded the weakest development of all. It is speculated that the absence of development in the LFM forecast stemmed from short-comings of the initial analysis.

9) A possible cause of the failure of the present experiments to fully capture the storm intensity is the deficiency of middle and upper-level observations, and the attendant uncertainties in the upper-level analyses, in the prestorm period.

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