Bowen ratio and daily temperature range thresholds: Are they signals of transient seasons?

The winter into spring and the summer into autumn transition seasons can last several weeks. Leaf emergence in midlatitude climates decreases the ratio of sensible (H) and latent heat (LE) fluxes - the Bowen ratio (B).  Because there are many more surface climate stations than flux-measuring sites, researchers seek to link the state variables at standard climate station heights to the leaf development.  Schwartz (1996) found out that, during the midlatitude onset of spring, the DTR trend rapidly increases for several weeks and then levels off.  Adopting an alternate approach, Fitzjarrald et al. (2001) linked changes in H and LE to B to the state variable daily change tendencies. This approach is based on assuming that the surface climate alters as a small fraction of the surface fluxes converge on average into the lower atmosphere.

Schwartz’ approach has the advantage of not requiring information from directly-measured fluxes, but station’s representativeness during the daytime (T_max) greatly exceeds the area that the T_min would describe. What’s more, daytime cloudiness depreses T_max but nocturnal cloudiness enhances T_min. The Fitzjarrald et al. approach requires long-term day-to-day averages to determine the times of the year when the surface state variables identify the consequences of leaf emergence.

            Here we seek to refine methods to relate plant characteristics to surface climate state, with emphasis on the spring transition at Harvard Forest (HF, MA, USA). At HF, J. O’Keefe kept a careful log of significant phenological events (Klosterman et al., 2018). The transition to the ‘growing season’ begins with bud break (mid-April), ending with nearly fully leafed crowns ("95%") in most species by mid-May.

We revisited the HF data and found that DTR, from the start of spring transition until the end of autumn, changes along with daily sensible heat flux changes, particularly during the period from sunrise until the daily maximum air temperature occurs. Since the seasonal course of daily temperature T_d  (°C) follows the latent heat flux trend, we normalized the DTR (DTR/T_d) and found that DTR/T_d ≈ 1 approximately at budbreak and again at "95%”. When the DTR next approaches T_d, the autumn transition is beginning. We use the METAR data to identify cloudy/clear periods and assess the sensitivity of DTR to this effect.

We examined the utility of using DTR/T_d ratio as an indicator of spring and autumn transition, exploring  temperature measurements and phenological observations from the HF and PIS network (Lalic et al., 2020). Preliminary results indicate that this approach can identify significant effects of leaf state on local surface climate without the need for averaging over a decade or longer.

Fitzjarrald et al., 2001, 1175/1520-0442(2001)014<0598:CCOLPI>2.0.CO;2.

Klosterman et al., 2018, 1007/s00484-018-1564-9

Lalic et al., 2020, 1007/978-3-030-37421-1.

Schwartz, 1996, 1175/1520-0442(1996)009<0803:ETSDID>2.0.CO;2