Estimating planetary boundary layer heights over a valley from rawinsonde observations at a nearby airport: A case study for the Page Valley in Virginia, USA

The height of the PBL is required for many applications including air quality studies, weather forecasting, and carbon cycle studies. Long-term PBL height estimates are not easily available and oftentimes only come from twice-daily rawinsonde observations at airports. PBL heights at any given location may be estimated from these nearby sounding stations. Making this estimation requires knowledge of the spatial variability in PBL height which is oftentimes unknown, especially over complex terrain. In the present study, we attempt to estimate afternoon PBL heights in the Page Valley in the Virginia Blue Ridge Mountains by investigating 1) PBL heights at Dulles Airport (IAD) near Washington DC, about 100 km to the northeast of the Page Valley and 2) the spatial variability of PBL heights in this region of the eastern US from North American Regional Reanalysis (NARR), mesoscale modeling, and observations.

We first compare IAD PBL heights with NARR PBL heights. We find that, for the entire 4-year period of interest, there is a poor agreement (r=0.49, p<0.01), with the highest correlations occurring in winter (r=0.71, p<0.01) and the weakest correlations occurring in summer (r=0.22, p<0.01). The correlation between NARR and the IAD soundings improves significantly when filtering days on which the PBL top is poorly defined, e.g. days with no strong elevated inversion.

We then use NARR to quantify spatial differences in PBL height between the Page Valley and IAD. The correlation is strongest in the winter (r=0.92, p<0.01) and weakest in the summer (r=0.86, p<0.01) due to greater spatial variability in PBL heights during the summer. To further quantify spatial variability in PBL height, we compare PBL heights determined from valley rawinsonde observations made during several field experiments with PBL heights from IAD and from NARR. We find that sounding-derived PBL heights on the 6 days with PBL heights measured in the Page Valley agree well with PBL heights at IAD and NARR output (r=0.84, p<0.01).

To better understand the spatial differences in PBL height that occur between the Page Valley and IAD and to characterize the spatial variability in PBL heights within the Page Valley itself, we perform simulations with the Weather Research and Forecast (WRF) model on two days with rawinsonde measurements from the Page Valley for two extreme cases: 1) a day on which the afternoon PBL is very shallow, and 2) a day on which the afternoon PBL is very deep. We find that afternoon PBL heights in the Page Valley and IAD compare well on both days, but the spatial variability of afternoon PBL height is larger on the day with a deep afternoon PBL. From these simulations and our analyses with IAD rawinsonde observations and NARR output, we conclude that NARR-derived and sounding-derived PBL heights on days with clearly defined deep PBL heights can provide an appropriate surrogate for PBL heights in the Page Valley.