Today I want to talk about a terrific 1985 article Godowitch et al,, 1985. Evolution of the Nocturnal Inversion Layer at an urban and nonurban location, J Clim Appl Met 791 ff available online here, which helps put some of the UHI discussions in a more complete perspective.
Instead of just considering UHI from a snail point of view (ie. transects from city center to country along the surface) it considers the entire vertical structure over the course of a night, which turns out to be richly textured and to offer much insight to what’s going on at surface.
This is all obviously old hat to Pielke Sr who seems to have cut his teeth on boundary layers, but is helpful to people like us coming into the middle of the debate. (It might have been useful to Parker as well, who also seems to have come into the middle of the debate arriving from prior study of SST bucket adjustments.)
The concept of the “nocturnal inversion layer” is distinct from but related to UHI, with the description of rural settings being as useful as the urban ones. In rural locations, a very strong nocturnal inversion at surface develops rapidly at sunset; this doesn’t happen in urban settings (where only an attenuated inversion develops at an altitude above the surface). In fancier language from Godowitch et al:
” A surface-based radiation inversion layer is a common feature in temperature profiles at night in natural environments. In contrast the nocturnal thermal structure in the highly inhomogeneous urban environment usually consists of an isothermal to adiabiatic region, the urban boundary layer (UBL) which is capped by an elevated nocturnal inversion layer (NIL).
So think of the “urban heat island” not as an “island”, but as a type of dome, keeping in mind that if you get high enough aloft (about 400 m in 1975 St Louis, the urban effect is attenuated or nonexistent.
Godowitch et al (1985) reported on a program around St Louis MO, in which they obtained vertical temperature profiles by an instrumented helicopter during 31 evening and 46 morning runs over 5-6 week periods in July-August 1974-1976, mostly under fair weather with surface wind speeds of 1-3 m s-1. Eleven flights with broken or overcast conditions were excluded from the reported calculations.
The urban (141) and nonurban (401) locations are shown below. They noted that the nonurban location was not ideal, but still had very strong contrast to the urban site. The urban site was in a commercial and multiple story building district about 3 km west of the downtown center; the nonurban site (401) was about 7 km southeast of the city at Bi-State Park airport, where it was surrounded by forested and agricultural land. (It would be interesting to do a followup comparison today.)
Their next figure shows the nocturnal evolution of inversion in deg C for both urban and nonurban settings. The nonurban inversion reaches 3 deg C, while the urban inversion is only about 1 deg C. The nonurban inversion begins to develop immediately at the end of the day, while the inversion develops later in the urban setting. (A note here – keep in mind that at a relatively low altitude, 400 m or so in this case, the differences between urban and rural settings are strongly attenuated or eliminated.) Maybe someone can comment on whether these nonurban inversions are characteristic in other environments (e.g. oceans, tropics).
Their next Figure, Figure 3 (below) shows night-time evolution of the boundary layer in nonurban (top) and urban (bottom) settings. In the nonurban setting, a slight inversion has already developed by 6 pm; in this example, it reached nearly 4 deg C by 5 am, with the height increasing to slightly over 300 m by dawn. [Note - inversion here is a vertical differential and is related to, but is distinct from the horizontal urban-rural UHI differential.]
In the urban case, at 6 pm, the lapse rate above 200 m is pretty similar to the nonurban case, but there is no inversion. Indeed, at ground level, in the urban situation, no inversion develops; in each case, there is a lapse rate (of varying steepness) for the lowermost 100 m or so. At 6 pm, there is already a surface differential (UHI) of a little over 1 deg C. At 11 pm, the two are similar at 400 m; the nonurban is a little warmer at 200 m with strong inversion to surface, while, in the urban setting, there remains a somewhat adiabatic lapse rate near surface with an attenuated inversion.
From a snail perspective, the horizontal urban-nonurban differential (UHI) has reached about 1.5 deg C. at 11 pm increasing to about 2.2 deg C by morning. This may not be representative as elsewhere in the article the UHI is said to max out earlier in the evening.
In Figure 2 above, the altitude of the nonurban inversion top h_T is shown as about 300 m. Godowitch et al estimate this height by reference to underlying physics using the formula:
with being a roughness parameter which they estimate at about 0.75 . They also observe that the amount of surface inversion S (in deg C) also develops according to a square-root time formula, see graphic below:
In addition to the thermal “dome” structure, they also observed a type of “inversion” structure in wind speeds in which there is a wind speed maximum )defined by height becoming well-defined aloft about an hour after sunset and increasing until midnight. They note that the height of the wind speed maximum was higher in the urban setting.
While their analysis is specific to St Louis, they compare the inversion structure in St Louis to larger and smaller cities as follows:
Statistics for New York City (Bornstein 1968) which is considerably larger than St Louis in horizontal and vertical extens show an average h_u about twice that for this urban site, while the average S was about the same. Thus areal size and the vertical dimension of a city are important factors that determine the maximum heeight of the nocturnal UBL. Consequently a shallower average UBL is expected in smaller cities as observed in Cincinnati Ohilo (Clarke 1969) and Columbus Ohio (McElroy 1973) where h_u was typically 100 m or less in the downtown area.
If you go to Google Scholar and see studies that cite Godowitch 1985, you’ll not see a lot of studies (none by Parker) but in a quick browse they tend to be empirical and interestng. They include studies of Moscow and Melbourne. HEre are a few notes from their abstracts.
M. Shahgedanova, Burt and Davies 1997 report for Moscow (1990) report:
The urban-rural temperature differences ranged mainly between 1° and 3° C, with an absolute maximum of 9°C. In summer, the heat island intensity exceeded 3°C on 29 per cent of all early morning observations, confirming the widely held view that anticyclonic conditions generate strong heat islands. Temperature variations within the city were small, with a notable exception of the urban park; in winter, the lee periphery of the city was often warmer than the urban centre.
Morris and Simmonds (J Appl Met 2001) study Melbourne using as “nonurban” comparanda the three airport sites of Melbourne International Airport, Moorabbin Airport, and Laverton Airport (a; BoM station Nos. 086282, 086077, 087031, respectively).