Estimates of Eyewall net latent heat release from airborne radar measurements

Is tropical cyclone (TC) intensity related to net latent heat release (LHR)? What is the relevant region over which one should estimate net LHR? It has been established that in the absence of LHR (no rain from deep convection) TCs neither form nor can they be maintained. In contrast to this axiom is that prodigious amounts of LHR can occur in a tropical region without any TC development. In this latter situation the latent heating of condensation is nearly balanced by adiabatic cooling. Anthes (1982) has pointed out that a small difference between these two very large terms can be responsible for a significant warming of the troposphere and subsequent TC intensification. Prior researchers have applied satellite-borne instruments and airborne radars to determine if there is a relationship between TC intensity and LHR. The expected positive correlation between TC intensity and LHR has been observed by a few investigators (e.g., Adler and Rodgers 1977, Rodgers and Adler 1981) for large areas (e.g., a 4 degree radius around the TC center). In other situations the relationship is more complex (e.g., Marks 1985, Cerveny and Newman 2000). The Dvorak scheme suggests that the organization and the amount of deep convection, the latter considered a proxy for net LHR, have a positive correlation to intensity. We will use the 209 images of the reflectivity field of the eyewall from 37 tropical cyclones acquired with the NOAA WP-3D 5.6 cm radar to estimate LHR. These images, acquired from 1997 to 2012 almost entirely from the Atlantic Basin, are chosen when the aircraft is in the eye and thus are not being seriously compromised by beam filling or attenuation issues. These views of the eyewall are also from a single 360 degree sweep, avoiding the smearing that typically occurs when longer time-composites are used. We use the scheme developed by Barnes (2013 thesis, and also described in an earlier presentation at San Diego) to determine the eye and eyewall and their basic traits such as areal coverage, completeness and roundness. The area of each rain rate is determined for the eyewall and the total net LHR, which is simply the amount of water that has fallen multiplied by the latent heat of condensation, is determined. Typical values are on the order of 1013 W for what we define as the eyewall. The estimates of net LHR are compared to TC intensity and motion characteristics from Best Track, and environmental characteristics from the Statistical Hurricane Intensity Prediction Scheme. The preliminary results show that net LHR and TC intensity are poorly correlated. The very weak positive correlation reveals that net LHR can increase by two orders of magnitude with only a 20 hPa deepening. TC intensity can vary by nearly 100 hPa for the same amount of net LHR. At the same time there is a far better positive relationship between the ratio of the eyewall area to eye area and TC intensity. This leads us to infer that organization of the LHR has a greater role in affecting intensity than simply the magnitude of the net LHR. TCs with nearly complete eyewalls that encircle the eye can more easily resist environmental flows that either dilute or tilt the warm core and reduce the intensity. Smaller eyes are also more easily warmed via radial advection from the eyewall or by subsidence concentrated in that smaller volume. We will argue that estimates of LHR over very large areas surrounding the TC are not the most relevant measurement to make; what is crucial are the estimate of net LHR in the eyewall itself in combination with the environmental flows that may be working against the warming induced by the TC circulation. We will discuss our results in the context of the prior LHR measurements and suggest new sampling techniques that have the potential to reveal more about the factors that impact TC intensity.