Seasonal and diurnal performance of daily forecasts with WRF-NOAHMP V3.8.1 over the United Arab Emirates

Abstract

Abstract. Effective numerical weather forecasting is vital in arid regions like the United Arab Emirates (UAE) where extreme events like heat waves, flash floods and dust storms are severe. Hence, accurate forecasting of quantities like surface temperatures and humidity is very important. To date, there have been few seasonal-to-annual scale verification studies with WRF at high spatial and temporal resolution. This study employs a convection-permitting scale (2.7 km grid scale) simulation with WRF-NOAHMP, in daily forecast mode, from January 01 to November 30 2015. WRF was verified using measurements of 2 m air temperature (T-2m), dew point (TD-2m), and 10 m windspeed (UV-10m) from 48 UAE surface stations. Analysis was made of seasonal and diurnal performance within the desert, marine and mountain regions of the UAE. Results show that WRF represents temperature (T-2m) quite adequately during the daytime with biases ≤ +1 ˚C. There is however a nocturnal cold bias (−1 to −4 ˚C), which increases during hotter months in the desert and mountain regions. The marine region has the lowest T-2m biases (≤−0.75 ˚C). WRF performs well regarding TD-2m, with mean biases mostly ≤ 1 ˚C. TD-2m over the marine region is overestimated though (0.75–1 ˚C), and nocturnal mountain TD-2m is underestimated (~ −2 ˚C). UV-10m performance on land still needs improvement, and biases can occasionally be large (1–2 m s−1). This performance tends to worsen during the hot months, particularly inland with peak biases reaching ~ 3 m s−1. UV-10m are better simulated in the marine region (bias ≤ 1 m s−1). There is an apparent relationship between T-2m bias and UV-10m bias, which may indicate issues in simulation of the daytime sea breeze. TD-2m biases tend to be more independent. Studies such as these are vital for accurate assessment of WRF nowcasting performance and to identify model deficiencies. By combining sensitivity tests, process and observational studies with seasonal verification, we can further improve forecasting systems for the UAE.

Seasonal and diurnal performance of daily forecasts with WRF V3.8.1 over the United Arab Emirates

“…Secondly, the soil representation, such as soil texture classification -and associated parameters like heat capacity, thermal diffusivity, and albedo -may require adjustment. Underestimations of albedo in WRF have recently been observed, particularly for bright desert soils where measurements show typical albedo values of 0.3 to 0.34 (Nelli et al, 2020b). The WRF albedo value in this study is around 0.23 for much of the UAE lowlands, which would likely result in an overly high net radiation and sensible heating, especially on dry soils.…”

“…By combining seasonal verification with sensitivity tests, and process and observational studies, we will move towards improved forecasting systems for the UAE and other arid regions. (Branch et al, 2020c). The verification statistics dataset can be found at https://doi.org/10.5281/zenodo.4004195 (Branch et al, 2020d).…”

“…(Branch et al, 2020c). The verification statistics dataset can be found at https://doi.org/10.5281/zenodo.4004195 (Branch et al, 2020d).…”

“…During summer, subtropical subsidence leads to a strong reduction of precipitation and higher temperatures, and consequently summer precipitation represents only around 20 % of the annual amounts. However, upper-level disturbances from the southern monsoon flows can still transport moisture towards the Arabian Peninsula and the UAE (Böer, 1997;Schwitalla et al, 2020), and convection is initiated sporadically over the mountains of Oman and the UAE in summertime (Branch et al, 2020a).…”

The First Characterization of Fog Microphysics in the United Arab Emirates, an Arid Region on the Arabian Peninsula

“…However, even in these events, the Arabian Gulf is the dominant moisture source for the UAE. Similarly, the fog events are characterized by calm winds (Figures 2b, 2d and 2f), which are typical of early morning conditions for this area and in coastal UAE in general (Branch et al, 2020;Temimi et al, 2020), and in line with the presence of a high-pressure over the region. The quiescent conditions are further evident in the in-situ measurements, with wind speeds not exceeding 3 m s −1 during the fog events (Figure 3).…”