Seasonal and Diurnal Cycles of Surface Boundary Layer and Energy Balance in the Central Andes of Perú, Mantaro Valley

Abstract

The present study presents a detailed analysis of the diurnal and monthly cycles the surface boundary layer and of surface energy balance in a sparse natural vegetation canopy on Huancayo observatory (12.04 ∘ S, 75.32 ∘ W, 3313 m ASL), which is located in the central Andes of Perú (Mantaro Valley) during an entire year (May 2018–April 2019). We used a set of meteorological sensors (temperature, relative humidity, wind) installed in a gradient tower 30 m high, a set of radiative sensors to measure all irradiance components, and a set of tensiometers and heat flux plate to measure soil moisture, soil temperatures and soil heat flux. To estimate turbulent energy fluxes (sensible and latent), two flux–gradient methods: the aerodynamic method and the Bowen-ratio energy-balance method were used. The ground heat flux at surface was estimated using a molecular heat transfer equation. The results show minimum mean monthly temperatures and more stable conditions were observed in June and July before sunrise, while maximum mean monthly temperatures in October and November and more unstable conditions in February and March. From May to August inverted water vapor profiles near the surface were observed (more intense in July) at night hours, which indicate a transfer of water vapor as dewfall on the surface. The patterns of wind direction indicate well-defined mountain–valley circulation from south-east to south-west especially in fall–winter months (April–August). The maximum mean monthly sensible heat fluxes were found in June and September while minimum in February and March. Maximum mean monthly latent heat fluxes were found in February and March while minimum in June and July. The surface albedo and the Bowen ratio indicate semi-arid conditions in wet summer months and extreme arid conditions in dry winter months. The comparisons between sensible heat flux ( Q H ) and latent heat flux ( Q E ), estimated by the two methods show a good agreement (R 2 above 0.8). The comparison between available energy and the sum of Q E and Q H fluxes shows a good level of agreement (R 2 = 0.86) with important imbalance contributions after sunrise and around noon, probably by advection processes generated by heterogeneities on the surface around the Huancayo observatory and intensified by the mountain–valley circulation.

Rainfall Characteristics in the Mantaro Basin over Tropical Andes from a Vertically Pointed Profile Rain Radar and In-Situ Field Campaign

“…DJFM and SON have similar trends, however, the mean rain rate is higher during SON (~0.4 mm/h) months compared to DJFM months (~0.35 mm/h). The difference between afternoon and mid-night maxima is higher during SON compared to DJFM and indicates that most of the rainfall maxima occur during the afternoon, and could be related to topographical condition and diurnal variation of the surface temperature rather than the seasonal variation [19,20]. For example, the mean rain rate peaks (has higher rain rate values) at~0.35 mm/h and 0.18 mm/h in the afternoon and early morning during DJFM months respectively, whereas corresponding values are~0.4 mm/h and~0.05 for SON months in afternoon and early morning, respectively.…”

“…The transported moisture in the upslope and easterly flow [10] and the rising moist airflow along the slope, simulate the condensation and raindrops start falling and enhance the local precipitation [74]. The opposite slope below the~4.5 km for rain rate ≤0.2 mm/h, reveals that small-sized of hydrometeors are not able to reach the surface, and, evaporation of small size of hydrometeors are effective for the lowest class of rain rate [19,20,28]. Figure 6c shows the vertical variation of LWC for the different classes of rain rate and shows similar features as observed in average rain rate profiles.…”

“…But this feature is not visible during DJFM months. This could be because of the sufficient moisture supplied by the SA-LLJ and surface flow from the Amazon basin towards the Andes mountain during JJAS months [19][20][21]. Although the total number of smaller drops (D m < 1.0 mm) remain constant within the mixed-phase regimes and increases below the freezing level during both the seasons.…”

“…Francou et al [15] and Salzmann et al [16] showed that a correct estimation of bright band height is essential and can be used in the numerical models to predict the melting of glaciers in the tropical Andes [17,18]. A better understanding of the precipitation is particularly important to interpret the local features on the precipitation in numerical models [19][20][21][22][23][24].…”

“…In 2015, the Instituto Geofísico del Perú (IGP) installed a vertically pointed profile rain radar (VPRR, Figure 1b) namely MIRA35c at Huancayo (HYO, 12 • S, 75.3 • W) to investigate the rainfall characteristics in the mountain valley especially in Mantaro Basin. Various moisture and directional fluxes play a vital role in the precipitation distribution over the tropical Andes [17][18][19][20][21][22][23][24][25][26][27][28]. For example, the rainfall over the tropical Andes is affected by the low-level jet (LLJ) and surface flow (Figure 1a) at eastern and western flank of Andes [10,34,41].…”