Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 5:15 PM GMT on October 21, 2013
Hurricane Raymond rapidly intensified precariously close to the western coast of Mexico yesterday afternoon and early today in response to warm waters (check out the analysis of sea-surface temperatures below) and a moderate ocean heat content).
The analysis of sea-surface temperatures (in degrees Celsius) over the eastern Pacific basin on October 21, 2013. Courtesy of the University of Miami.
By way of background, the depths of the isotherms (20 and 26 degrees Celsius) obtained by radar altimetry are blended with sea-surface temperatures to estimate ocean heat content (OHC). I include the most recent analysis below.
The analysis of ocean heat content (in kilojoules per square centimeter) over the eastern Pacific basin on October 21, 2013. Courtesy of the University of Miami.
As readers already know, tropical cyclogenesis depends, in part, on a relatively deep, well-mixed oceanic layer (see the most current analysis below). That's because strong winds associated with a developing or intensifying tropical cyclone increase mixing of water in the uppermost layer of the ocean. This turbulent mixing, in turn, increases the depth of the well-mixed oceanic layer. As a result, cooler water "entrains" into the well-mixed layer from below. In turn, this cooler water gets mixed throughout the the now deeper well-mixed layer, lowering ocean heat content and decreasing sea-surface temperatures. In the case of a slow-moving or stationary tropical cyclone, the stage is set for the storm to weaken because lower sea-surface temperatures go hand in hand with reduced fluxes of heat energy and moisture from the sea to the overlying air. In effect, a stationary tropical cyclone dissipates by its own hand.
The analysis of the depth of the mixed layer (in meters) over the eastern Pacific basin on October 21, 2013. Courtesy of the University of Miami.
As of 15Z this morning (8 A.M. PDT), the minimum central pressure of Major Hurricane Raymond was 954 mb, which is consistent the mean wind in the layer between 850 mb (roughly 1.5 kilometers) and 300 mb (approximately nine kilometers) acting as a steering current. I introduced the concept of a tropical cyclone's steering layer as a function of minimum central pressure in an earlier blog (the graph below summarizes the point of my earlier blog).
The steering layer for tropical cyclones as a function of minimum central pressure. Courtesy of CIMSS.
The 12Z analysis of the mean wind in the layer between 850 mb and 300 mb over Hurricane Raymond (below; larger image) indicates a weak steering flow, which is consistent with the storm's slow, somewhat erratic movement over the short term. As a result of the induced stress on the ocean by hurricane-force winds, the well-mixed layer should deepen in time (see current analysis below), allowing colder water to entrain near the bottom of the deepening well-mixed oceanic layer. In turn, ocean heat content and sea-surface temperatures in the vicinity of Hurricane Raymond should decrease with time, paving the way for Raymond's maximum sustained winds to weaken.
The mean wind direction (white streamlines) and wind speed (pink isotachs expressed in knots) between 850 mb and 300 mb at 12Z on October 22, 2013. Larger image. Courtesy of CIMSS.
Still, there is much uncertainty about the predicted track and intensity of Hurricane Raymond. One's things for sure...the threat of heavy rain to neighboring coastal areas is very, very real.
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