Understanding Surface Energy Fluxes: Sensible Heat, Latent Heat, and Net Radiation

Surface energy fluxes play a crucial role in the Earth’s energy balance and climate system. These fluxes include sensible heat, latent heat, and net radiation. This blog will explain what these terms mean, their typical units and ranges, and how they are interconnected within the surface energy balance equation.

Sensible Heat Flux

Definition: Sensible heat flux is the transfer of heat between the Earth’s surface and the atmosphere due to temperature differences. It is termed “sensible” because it can be directly sensed or felt as a change in temperature.

Units:

• Watts per square meter (W/m²) for the rate of energy transfer.
• Joules per square meter (J/m²) when accumulated over a period.

Typical Range:

• Daily measurements typically range from -50 W/m² to 400 W/m².
• Negative values indicate heat transfer from the atmosphere to the surface (cooling effect).

Equation:

Q_{h} = \rho \times c_{p}\times \frac{\partial T}{\partial z}

${\mathrm{<span\; class="base"><span\; style="font-family:\; Georgia,\; \text{'}Times\; New\; Roman\text{'},\; \text{'}Bitstream\; Charter\text{'},\; Times,\; serif;"></span>\; </span>}}_{}$

Where:

• 
  $Q_h$ 

  Qh​ = Sensible heat flux (W/m²)

• 
  $\rho$ 

  $\rho$ = Air density (kg/m³)

• 
  $c_p$ 

  cp​ = Specific heat capacity of air at constant pressure (J/kg·K)

• \frac{\partial T}{\partial z}  = Temperature gradient (K/m)

Latent Heat Flux

Definition: Latent heat flux is the heat transfer due to phase changes of water (e.g., evaporation, condensation) without a change in temperature. It represents the energy required for water to change from liquid to vapor.

Units:

• Watts per square meter (W/m²) for the rate of energy transfer.
• Joules per square meter (J/m²) when accumulated over a period.

Typical Range:

• Daily measurements typically range from 0 W/m² to 400 W/m².

 

Equation:

$Q_e = \lambda E$

Qe​=λE

Where:

• 
  $Q_e$ 

  Qe​ = Latent heat flux (W/m²)

• 
  $\lambda$ 

  $\lambda$ = Latent heat of vaporization (J/kg)

• 
  $E$ 

  $E$ = Evaporation rate (kg/m²·s)

Latent Heat of Vaporization: The energy required to transform 1 kg of water from liquid to vapor, approximately 2.45 x 10⁶ J/kg.

Net Radiation

Definition: Net radiation is the balance between incoming and outgoing radiation at the Earth’s surface. It includes both shortwave (solar) and longwave (terrestrial) radiation.

Units:

• Watts per square meter (W/m²) for the rate of energy transfer.
• Joules per square meter (J/m²) when accumulated over a period.

Typical Range:

• Can vary widely depending on location and time of year, generally between -200 W/m² to 800 W/m².

Equation:

$Q^* = (K_\downarrow – K_\uparrow) + (L_\downarrow – L_\uparrow)$

Q∗=(K↓​−K↑​)+(L↓​−L↑​) Where:

• 
  $Q^*$ 

  Q∗ = Net radiation (W/m²)

• 
  $K_\downarrow$ 

  K↓​ = Incoming shortwave radiation (W/m²)

• 
  $K_\uparrow$ 

  K↑​ = Reflected shortwave radiation (W/m²)

• 
  $L_\downarrow$ 

  L↓​ = Incoming longwave radiation (W/m²)

• 
  $L_\uparrow$ 

  L↑​ = Outgoing longwave radiation (W/m²)

Surface Energy Balance Equation

The surface energy balance is a fundamental principle stating that the energy input into a surface must equal the energy output plus any changes in stored energy.

Equation:

$Q^* = Q_h + Q_e + Q_g$

Q∗=Qh​+Qe​+Qg​ Where:

• 
  $Q^*$ 

  Q∗ = Net radiation (W/m²)

• 
  $Q_h$ 

  Qh​ = Sensible heat flux (W/m²)

• 
  $Q_e$ 

  Qe​ = Latent heat flux (W/m²)

• 
  $Q_g$ 

  Qg​ = Ground heat flux (W/m²)

Ground Heat Flux: The heat transfer into or out of the ground, typically smaller compared to

$Q_h$

Qh​ and

$Q_e$

Qe​, hence ignored in many research studies. But it is sometimes very applicable in many research papers. 

 

Understanding these fluxes is essential for various applications:

• Climate Modeling: Accurate representation of these fluxes helps in predicting climate patterns and understanding climate change.
• Agriculture: Knowledge of latent heat flux is crucial for irrigation planning and assessing water needs for crops.
• Meteorology: Sensible heat flux influences weather patterns and local climate conditions.
• Hydrology: The energy balance affects evaporation rates, influencing water resource management.

Surface energy fluxes—sensible heat, latent heat, and net radiation—are key components in understanding the Earth’s energy balance. By accurately measuring and modeling these fluxes, scientists can gain insights into weather, climate, and various environmental processes. I hope this knowledge is essential for you and help you to understand these terms better.