Surface Radiative Heat Fluxes
Meteorologists and climatologists go to considerable effort to figure out from where air masses originate, because of course that has a great deal to do with the weather. A good example pertains to the upcoming winter’s weather. La Niña conditions will likely prevail which tends to result in anomalous flow from the northwest out of the Gulf of Alaska, bringing cooler and wetter conditions, with a healthy mountain snowpack. But clearly that is not the only and even necessarily the most important factor controlling our weather, especially temperatures. Radiative heat fluxes can be crucial, as is obvious in association with the downward shortwave flux (incoming solar radiation from the sun) in summer and the downward longwave flux in winter because of its relationship to surface-based inversions. These fluxes are not measured nearly as much as temperature and precipitation, and time series for them are not as easily available. With that in mind, here we present some sample time series of radiative fluxes, mostly for illustrative purposes. These results are from current work being carried out in development of a real-time system featuring fundamental, but previously unavailable, environmental metrics for Puget Sound, with the support of the Puget Sound Environmental Monitoring Program (PSEMP).
As mentioned above, direct measurements of radiative heat fluxes, especially the longwave component, can be hard to come by. It turns out that our friends just north of the border in Richmond, BC collected a full suite of high-quality observations at Delta Burns Bog from 2014 into 2018 as part of the AmeriFlux network (ameriflux.lbl.gov/sites/siteinfo/CA-DBB). Two excerpts from this data set are shown here: a pair of 13-day time series of the net radiative heat fluxes (balance between the incoming and outgoing radiation) at the surface during the summer of 2016 (Fig. 1) and the following winter (Fig. 2). The summer time series features a distinct diurnal cycle with prominent positive peaks in the middle of the day when the sun is high in the sky, and lesser negative fluxes in the nighttime hours as the surface loses heat by the net emission of longwave radiation. The day of 3 August was evidently quite cloudy as indicated by the much reduced heating; those clouds stuck around through the following night when there was essentially no loss of energy due to the ground and atmosphere apparently being in radiative equilibrium. The sample time series in winter (Fig. 2) also includes a diurnal cycle, but of much reduced amplitude, which is no surprise whatsoever. It is striking how little incoming solar radiation at the surface the Pacific NW can receive in the wintertime, as shown by the paltry maximum of about 25 W/m2 in the net radiative heating on 29 December. The period near the end of 2016 included nights with close to zero net radiative heat fluxes, which occurs with low, thick clouds. The time series also includes 4 sunny days at the beginning of the calendar year with a couple of cold and dry nights, resulting in enhanced loss of heat from the surface. These time series are just examples, of course. But hopefully they are useful in that most of us do not look at this kind of data very often, and show in a quantitative way how clouds tend to result in it being cool in summer and warm in winter.
We are using the Delta Burns Bog data as means of characterizing the downward longwave radiative fluxes for our PSEMP project. There have been a variety of schemes developed to estimate this component based on cloud cover and surface values of temperature and humidity; we have begun calibrating and validating the method suggested by Crawford and Duchon (1999), and the initial results look promising. The cloud cover itself can be characterized using ASOS-based observations or from insolation measurements (solar energy that actually reaches the Earth’s surface), which can then be used to infer cloud fractions based on the differences between actual and clear-sky irradiances. There happen to be quite a few locations around WA state, especially in the agricultural regions on the east side of the Cascades, where the downward solar radiation is directly measured, in large part in association with the network of stations maintained by WSU’s AgWeatherNet program. An example of these data are shown here (Fig. 3), specifically monthly totals of the downward solar energy measured at the “Poulsbo.S” station on the west side of Puget Sound for the period of December 2013 through October 2020. The variability in this component of the radiative heat fluxes is considerably greater during the summer months, naturally. Prominent anomalies in this record include the positive value exceeding 100 MJ/m2 (equivalent to an average rate of about 40 W/m2) for the month of June 2015; the following summer of 2016 featured a negative anomaly of 75 MJ/m2 in the month of July. The summers of 2019 and 2020 were on the cloudy side, at least relative to the mean over this admittedly short record. Back to the original motivation for our inquiry into the subject, the surface radiative fluxes definitely matter to Puget Sound in terms of their impacts on heating rates, with consequences for the marine ecosystem. Presumably they also matter to other things such as forest productivity and crop yields, either directly from a photosynthetic perspective or indirectly through impacts on factors such as soil moisture.
Reference
Crawford, T.M. and C.E. Duchon (1999): An improved parameterization for estimating effective atmospheric emissivity for use in calculating daytime downwelling longwave radiation. J. Appl. Meteor., 38, 474-480.